Download Glycolysis - WordPress.com

Adenosine Triphosphate (ATP) - The Energy Source for
Muscle Contraction
Before discussing the various systems by which your body can provide energy to your muscles,
we first need to define what muscle "energy" actually is. We know that your muscle cells need
an energy source to be able to contract during exercise. At the highest level, the energy source
for muscle contractions is the food you eat. A complex chemical process within your cells, called
cellular respiration, ultimately converts the energy stored in the foods you eat into a form that is
optimized for use at the cellular level of your muscles. Once food energy has been converted by
cellular respiration it exists at the cellular level in the form of a molecule called adenosine
triphosphate (ATP).
The composition of an ATP molecule can be inferred from its name. It is composed of three (or
"tri") phosphate groups attached to an adenine (or "adenosine") nucleotide. The energy that is
stored within an ATP molecule is released for your muscles to use when the bond between the
second and third phosphate groups is broken. Breaking this bond releases the third phosphate
group on its own and thus reduces the ATP molecule to adenosine diphosphate (ADP). The ADP
molecule can be restored back to its ATP form by replenishing the missing phosphate group (this
is called rephosphorylization).
Three Exercise Energy Systems
The cellular respiration process that converts your food energy into ATP is in large part
dependent on the availability of oxygen. When you exercise, the supply and demand of oxygen
available to your muscle cells is affected by the duration and intensity of your exercise and by
your cardiorespiratory fitness level. Luckily, you have three exercise energy systems that can be
selectively recruited, depending on how much oxygen is available, as part of the cellular
respiration process to generate the ATP energy for your muscles. They are summarized below.
The Alactic Anaerobic Energy System
This energy system is the first one recruited for exercise and it is the dominant source of muscle
energy for high intensity explosive exercise that lasts for 10 seconds or less. For example, the
alactic anaerobic energy system would be the main energy source for a 100 m sprint, or a short
set of a weightlifting exercise. It can provide energy immediately, it does not require any oxygen
(that's what "anaerobic" means), and it does not produce any lactic acid (that's what "alactic"
means). It is also referred to as the ATP-PCr energy system or the phosphagen energy system.
The alactic anaerobic energy system provides its ATP energy through a combination of ATP
already stored in the muscles (about 1 or 2 seconds worth from prior cellular respiration during
rest) and its subsequent rephosphorylization (about 8 or 9 seconds worth) after use by another
molecule called phosphocreatine (PCr). Essentially, PCr is a molecule that carries back-up
phosphate groups ready to be donated to the already used ADP molecules to rephosphorylize
them back into utilizable ATP. Once the PCr stored in your muscles runs out the alactic
anaerobic energy system will not provide further ATP energy until your muscles have rested and
been able to regenerate their PCr levels. Creatine supplementation is a method used to extend the
duration of effectiveness of the alactic anaerobic energy system for a few seconds by increasing
the amount of PCr stored within your muscles.
The Lactic Anaerobic Energy System
This system is the dominant source of muscle energy for high intensity exercise activities that
last up to approximately 90 seconds. For example, it would be the main energy contributor in an
800 m sprint, or a single shift in ice hockey. Essentially, this system is dominant when your
alactic anaerobic energy system is depleted but you continue to exercise at an intensity that is too
demanding for your aerobic energy system to handle. Like the alactic anaerobic energy system,
this system is also anaerobic and so it does not require any oxygen. However, unlike the alactic
anaerobic energy system, this system is lactic and so it does produce lactic acid. It is also
referred to as the lactic acid system or the anaerobic glycolytic system.
In contrast to the alactic anaerobic energy system, which uses ATP stored from previous cellular
respiration in combination with a PCr phosphate buffer, the lactic anaerobic energy system must
directly recruit the active cellular respiration process to provide ATP energy. The cellular
respiration process consists of a very complex series of chemical reactions, but the short
summary of it is that it ultimately converts food energy (from carbohydrates, fats, and proteins)
into ATP energy. When oxygen is not available for cellular respiration, as is the case for the
lactic anaerobic energy system, lactic acid is produced as a byproduct.
The Aerobic Energy System
During continuous aerobic exercise your intensity level, relative to the high intensity levels that
recruit your alactic anaerobic and lactic anaerobic energy systems, must be reduced so that the
energy demand placed on your muscles equals the energy supply (compare this to the alactic
anaerobic and lactic anaerobic systems, where demand usually exceeds supply and energy stores
are quickly depleted). The energy supply at this lower intensity level, in contrast to the alactic
anaerobic and lactic anaerobic systems, which do not require oxygen, now becomes dependent
on how efficiently oxygen can be delivered to, and processed by, your muscles. A continuous
supply of oxygen allows you to maintain a reduced intensity level for a long period of time. If
you are able to extend an exercise activity beyond approximately two minutes in length it will be
due to the fact that you are working at an exercise intensity level that can be accommodated by
your aerobic energy system. By five minutes of exercise duration the aerobic energy system will
have become your dominant energy source. As an example, the aerobic energy system would be
the main energy contributor to a marathon runner. The aerobic energy system does not produce
lactic acid, but unlike the other two energy systems, it does require oxygen.
Just like the lactic anaerobic energy system, the aerobic energy system must directly recruit the
active cellular respiration process to provide ATP energy. Food energy is converted into ATP by
your muscle cells through a very complex series of reactions. The difference, relative to the
lactic anaerobic energy system, however, is that since oxygen is now available to your muscles
no lactic acid will be produced as a byproduct. The generation of ATP energy by the aerobic
energy system can be continued as long as oxygen is available to your muscles and your food
energy supplies don't run out.
Exercise Energy Systems - Conclusion
Now you have a basic understanding of the three exercise energy systems that keep you active.
As a final note, it's important to understand that, although one of the systems will be the
dominant source of your energy during a particular type of exercise, all of the exercise energy
systems are active at all times. It is simply the relative amount of energy that each system is
providing that will change with varying exercise intensity and duration. Therefore, you will
never be receiving your energy exclusively from one energy system while you are exercising, but
from all three to different degrees.
There are three sources of Adenosine triphosphate (ATP), the body's main energy source on the
cellular level.

ATP-PC System (Phosphogen System) - This system is used only for very short
durations of up to 10 seconds. The ATP-PC system neither uses oxygen nor produces
lactic acid if oxygen is unavailable and is thus said to be alactic anaerobic. This is the
primary system behind very short, powerful movements like a golf swing or a 100 m
sprint.

Anaerobic System (Lactic Acid System) - Predominates in supplying energy for
exercises lasting less than 2 minutes. Also known as the Glycolytic System. An example
of an activity of the intensity and duration that this system works under would be a 400 m
sprint.

Aerobic System - This is the long duration energy system. By 5 minutes of exercise the
O2 system is clearly the dominant system. In a 1 km run, this system is already providing
approximately half the energy; in a marathon run it provides 98% or more.[1]
Contents
[hide]






1 ATP-PC System
2 Anaerobic System
3 Aerobic System
4 How they work
5 References
6 See also
ATP-PC System
The creatine phosphate or ATP-PC system is unrivalled in our bodies for instant production of
energy; it works by reforming ATP by breaking down a chemical compound called creatine
phosphate which creates and provides for some ADP to reform into ATP. This is the first energy
pathway that is used by our bodies to resynthesise ATP (Adenosine Tri >PO3). Instead of oxygen
it uses another chemical known as CreatinePhosphate found in the muscle cells. This is not used
for muscle contraction, but is mainly used for resynthesising ATP and to maintain a constant
supply of energy. These reactions occur very rapidly and only last up to high intensity (this only
lasts for a short period of time). The ATP-PC system is for short bursts of energy but is burnt out
in 10 seconds. As the ATP-PC only lasts for around 10 seconds it is optimal for sports that
require fast bursts of energy, e.g 100m sprinting.
Anaerobic System
The lactic acid or anaerobic glycolysis system converts glycogen to glucose. Then, with
enzymes, glucose is broken down anaerobically to produce lactic acid; this process creates
enough energy to reform ATP molecules, but due to the detrimental effects of lactic acid and H+
ions building up and causing the pH of the blood to become more acidic, this system cannot be
relied on for extended periods.
Aerobic System



Glycolysis
The Krebs Cycle
Oxidative Phosphorylation
Glycolysis - The first stage is known as glycolysis, which produces 2 ATP molecules, a reduced
molecule of NAD (NADH), and 2 pyruvate molecules which move on to the next stage - the
Krebs cycle. Glycolysis takes place in the cytoplasm of normal body cells, or the sarcoplasm of
muscle cells.
The Krebs Cycle - This is the second stage, and the products of this stage of the aerobic system
are a net production of 1 ATP, 1 carbon dioxide Molecule, three reduced NAD molecules, 1
reduced FAD molecule (The molecules of NAD and FAD mentioned here are electron carriers,
and if they are said to be reduced, this means that they have had a H+ ion added to them). The
things produced here are for each turn of the Krebs Cycle. The Krebs cycle turns twice for each
molecule of glucose that passes through the aerobic system - as 2 pyruvate molecules enter the
Krebs Cycle. In order for the Pyruvate molecules to enter the Krebs cycle they must be converted
to Acetyl Coenzyme A. During this link reaction, for each molecule of pyruvate that gets
converted to Acetyl Coenzyme A, an NAD is also reduced. This stage of the aerobic system
takes place in the matrix of the cells' mitochondria.
Oxidative Phosphorylation - This is the last stage of the aerobic system and produces the
largest yield of ATP out of all the stages - a total of 34 ATP molecules. It is called 'Oxidative
Phosphorylation' because oxygen is the final acceptor of the electrons and hydrogen ions that
leave this stage of aerobic respiration (hence oxidative) and ADP gets phosphorylated (an extra
phosphate gets added) to form ATP (hence phosphorylation).
This stage of the aerobic system occurs on the cristae (infoldings on the membrane of the
mitochondria). The NADH+ from glycolysis and the Krebs cycle, and the FADH+ from the
Krebs cycle pass down electron carriers which are at decreasing energy levels, in which energy is
released to reform ATP. Each NADH+ that passes down this electron transport chain provides
enough energy for 3 molecules of ATP and each molecule, and each molecule of FADH+
provides enough energy for 2 molecules of ATP. If you do your math this means that 10 total
NADH+ molecules allow the rejuvenation of 30 ATP, and 2 FADH+ molecules allow for 4 ATP
molecules to be rejuvenated (The total being 34 from oxidative phosphorylation, plus the 4 from
the previous 2 stages meaning a total of 38 ATP being produced during the aerobic system). The
NADH+ and FADH+ get oxidized to allow the NAD and FAD to return to be used in the aerobic
system again, and electrons and hydrogen ions are accepted by oxygen to produce water, a
harmless by-product.
Glycolysis, an overview
Glycolysis (a sweet splitting process) is a central pathway for the
catabolism of carbohydrates in which the six-carbon sugars are split to
three-carbon compounds with subsequent release of energy used to
transform ADP to ATP. Glycolysis can proceed under anaerobic
(without oxygen) and aerobic conditions.
Glycolysis, an overall equation
Glycolysis is a 10-step pathway which converts glucose to 2 pyruvate
molecules. The overall Glycolysis step can be written as a net equation:
Glucose + 2xADP + 2xNAD+ -> 2xPyruvate + 2xATP + 2xNADH
Glycolysis consists from two main phases.
 First phase, energy investment. During this step 2xATP are
converted to 2xADP molecules.
 Second phase, energy generation. During this step 4xADP are
converted to 2xATP molecules and 2xNAD+ are converted to
2xNADH molecules.
Diagram of Glycolysis pathway
Glycolysis: Energy investment phase
Glycolysis step 1:
Glucose phosphorylation
catalysed by Hexokinase:
α-D-Glucose + ATP -> α-D-Glucose6-phosphate + ADP + H+
Glycolysis step 2:
Isomerization of glucose-6phosphate catalysed by
Phosphoglucoisomerase:
α-D-Glucose-6-phosphate <=> D-
Fructose-6-phosphate
Glycolysis step 3:
Second phosphorylation
catalysed by
Phosphofructokinase:
D-Fructose-6-phosphate + ATP ->
D-Fructose-1,6-bisphosphate +
ADP + H+
Glycolysis step 4:
Cleavage to two Triose
phosphates catalysed by
Aldolase:
D-Fructose-1,6-bisphosphate <=>
Dihydroxyacetone phosphate +
D-glyceroaldehyde-3-phosphate
Glycolysis step 5:
Isomerization of
dihydroxyacetone phosphate
catalysed by Triose phosphate
isomerase:
Dihydroxyacetone phosphate <=>
D-glyceroaldehyde-3-phosphate
Glycolysis: Energy generation phase
Glycolysis step 6:
Generation of 1,3-Bisphosphoglycerate catalysed by Glyceraldehyde-3phosphate dehydrogenase:
D-glyceroaldehyde-3-phosphate + NAD+ +Pi <=> 1,3-Bisphosphoglycerate
+ NADH + H+
Glycolysis step 7:
Substrate-level phosphorylation, 3-Phosphoglycerate catalysed by
Phosphoglycerate kinase:
1,3-Bisphosphoglycerate + ADP <=> 3-Phosphoglycerate + ATP
Glycolysis step 8:
Phosphate transfer to 2-Phosphoglycerate catalysed by
Phosphoglycerate mutase:
3-Phosphoglycerate <=> 2-Phosphoglycerate
Glycolysis step 9:
Synthesis of Phosphoenolpyruvate catalysed by Enolase:
2-Phosphoglycerate <=> Phosphoenolpyruvate + H2O
Glycolysis step 10:
Substrate-level phosphorylation. Pyruvate synthesis catalysed by
Pyruvate kinase:
Phosphoenolpyruvate + H+ + ADP -> Pyruvate + ATP
Anaerobic Glycolysis pathway
For anaerobic Glycolysis pathway there are two major fermentation
processes exists.
Lactic acid fermentation. This pathway is common for animal cells and
lactic acid bacteria. In animals the anaerobic glycolysis take place in
many tissues. Red blood cells take most of the energy from anaerobic
metabolism. Skeletal muscle take energy from glycolysis and from
respiration. The lactate produced utilise through diffusion from the
tissues to bloodstream and then to aerobic tissues, such as liver and
heart. In these aerobic tissues lactate can be catabolized further or can
be converted back through gluconeogenesis.
One step conversion of Pyruvate to Lactate catalysed by Lactate
dehydrogenase.
Alcoholic fermentation. This two-step pathway is common for yeast.
Pyruvate -> Acetaldehyde + CO2 catalysed by Pyruvate decarboxylase.
This reaction requires thiamine pyrophosphate, derived from vitamin B1
as a coenzyme.
Conversion of Acetaldehyde to Ethanol by Alcohol dehydrogenase.
Aerobic Glycolysis pathway
With the present of oxygen in cells pyruvate is oxidized to acetyl-CoA,
which then enters the citric acid cycle. The NADH molecules are
reoxidized through the mitochondrial electron transport chain with
electrons transferred to the O2 molecules. The aerobic Glycolysis consists
from two major steps:
Glucose + 2xADP + 2xPi + 2xNAD+ =>
2xPyruvate + 2xATP + 2xNADH + 2H+ + 2xH2O
NADH oxidation pathway which generaly take place in the
mitochohdrion:
2xNADH + 8xH+ + O2 + 6xADP + 6xPi =>
2xNAD+ + 8H2O + 6xATP
The final net equation for aerobic Glycolysis:
Glucose + 8xADP + 8xPi + 8xH+ + O2 => 2xPyruvate + 8xATP + 10xH2O
Aerobic and anaerobic glycolysis. Overview
The metabolism of glucose trough aerobics or anaerobic pathways is a
nonoxidative process. Both types of glycolysis release a small fraction of
potential energy stored in the glucose molecules. During the first 10 steps
of glycolysis, only a small part of all glucose energy is released and the
rest of the potential energy is released during the last steps after
glycolysis. For this reason aerobic degradation is much more efficient
than anaerobic metabolism. That is why the aerobic mechanism is now
much more spread within living organisms, but nevertheless anaerobic
pathways still take place even in animals under certain physiological
circumstances.
Within the glycolytic sequence pyruvate kinase is responsible for net
ATP production. In contrast to mitochondrial respiration energy
production within the pyruvate kinase reaction is not dependet on
oxygen supply and thereby allows the survival of the organs in the
absence of oxygen. Therefore, all tissues are equiped with high pyruvate
kinase activities.
Glycolysis Summary
Introduction to Glycolysis:
Glucose is transported into cells as needed and once inside of the cells, the
energy producing series of reactions commences. The three major
carbohydrate energy producing reactions are glycolysis, the citric acid
cycle, and the electron transport chain.
The overall reaction of glycolysis which occurs in the cytoplasm is
represented simply as:
C6H12O6 + 2 NAD+ + 2 ADP + 2 P -----> 2 pyruvic acid, (CH3(C=O)COOH + 2
ATP + 2 NADH + 2 H+
The major steps of glycolysis are outlined in the graphic on the left.
There are a variety of starting points for glycolysis; although, the most
usual ones start with glucose or glycogen to produce glucose-6-phosphate.
The starting points for other monosaccharides, galactose and fructose,
are also shown.
Glycolysis - with white background for printing
Link to: Great Animation of entire Glycolysis - John Kyrk
Click for larger image
Important Facts about Glycolysis:
The major steps of glycolysis are outlined in the graphic on the left.
There are a variety of starting points for glycolysis; although, the most
usual ones start with glucose or glycogen to produce glucose-6-phosphate.
The starting points for other monosaccharides, galactose and fructose,
are also shown.
Glycolysis - with white background for printing
There are five major important facts about glycolysis which are
illustrated in the graphic.
1) Glucose Produces Two Pyruvic Acid Molecules:
Glucose with 6 carbons is split into two molecules of 3 carbons each at
Step 4. As a result, Steps 5 through 10 are carried out twice per glucose
molecule. Two pyruvic acid molecules are the end product of glycolysis
per mono- saccharide molecule.
2) ATP Is Initially Required:
ATP is required at Steps 1 and 3. The hydrolysis of ATP to ADP is
coupled with these reactions to transfer phosphate to the molecules at
Steps 1 and 3. These reactions evidently require energy as well. You may
consider that this is a little strange if the overall objective of glycolysis is
to produce energy. This energy is used in the same way that it initially
takes heat to ignite the burning of paper or other fuels - you need to
expand some energy to get it started.
3) ATP is Produced:
Reactions 6 and 9 are coupled with the formation of ATP. To be exact, 2
ATP are produced at step 6 (remember that the reaction occurs twice)
and 2 more ATP are produced at Step 9. The net production of "visible"
ATP is: 4 ATP.
Steps 1 and 3 = - 2ATP
Steps 6 and 9 = + 4 ATP
Net "visible" ATP produced = 2.
Click for larger image
Important Facts about Glycolysis (cont.):
4) Fate of NADH + H+:
Reaction 5 is an oxidation where NAD+ removes 2 hydrogens and 2
electrons to produce NADH and H+. Since this reaction occurs twice, 2
NAD+ coenzymes are used.
If the cell is operating under aerobic conditions (presence of oxygen),
then NADH must be reoxidized to NAD+ by the electron transport
chain. This presents a problem since glycolysis occurs in the cytoplasm
while the respiratory chain is in the mitochondria which has membrane
that is not permeable to NADH. This problem is solved by using glycerol
phosphate as a "shuttle." - see graphic on the left. The hydrogens and
electrons are transferred from NADH to glycerol phosphate which can
diffuse through the membrane into the mitochondria. Inside the
mitochondria, glycerol phosphate reacts with FAD coenzyme in enzyme
complex 2 in the electron transport chain to make dihydroxyacetone
phosphate which in turn diffuses back to the cytoplasm to complete the
cycle.
As a result of the the indirect connection to the electron transport at
FAD, only 2 ATP are made per NAD used in step 5. If step 5 is used
twice per glucose, then a total of 4 ATP are made in this manner.
If the cell is anaerobic (absence of oxygen), the NADH product of
reaction 5 is used as a reducing agent to reduce pyruvic acid to lactic
acid at step 10. This results in the regeneration of NAD+ which returns
for use in reaction 5.
Electron Transport Diagram
Click for larger image
ATP Summary for Glycolysis:
Starting with glucose (six carbons) how many ATP are made using
aerobic glycolysis? E.T.C = electron transport chain
Step
ATP (used -) (produced +)
1
-1
3
-1
5 - NADH to E.T.C to FAD = 2
step 5 used twice
2 x 2 = +4
6 used twice
1x2=+2
9 used twice
1x2=+2
NET
6 ATP
Starting with glucose (six carbons) how many ATP are made using
anaerobic glycolysis? E.T.C = electron transport chain
Step
ATP (used -) (produced +)
1
-1
3
-1
5 - NADH to pyruvic acid to lactic
acid. E.T.C. not used
0
6 used twice
1x2=+2
9 used twice
1x2=+2
NET
2 ATP
Quiz: Starting with glycogen to
make glucose-6-phosphate, how
many ATP are made using aerobic
glycolysis?
Starting with glycogen to make
glucose-6-phosphate, how many
ATP are made using anaerobic
glycolysis?
Answer
Glycolysis






first step of glycolysis is glucose transport (facilitated diffusion)
across the sarcolemma that is accomplished by a specific protein on
the plasma membrane-requires insulin during resting states but
less insulin is required during exercise
after glucose enters the muscle, it is immediately phosphorylated in
a non-reversible reaction by hexokinase
primary rate-limiting step of glycolysis is the phosphofructokinase
(PFK) reaction where fructose-6-phosphate is phosphorylated to
fructose-1,6-bisphosphate
notice that by this point, 2 ATP have been utilized by the pathway
the 6-carbon molecule is next split into two 3-carbon molecules,
each having a phosphate group
from an isomerization reaction, DHAP is also formed into PGAL
(glyceraldehyde-3-phosphate) so that there are now two identical
PGALs
Note: Keep in mind that from this point, all steps are occurring twice
with the metabolism of one glucose.



in the next step, NAD+ is reduced to NADH, and, two ADP are
rephosphorylated to ATP in two subsequent steps
final step of glycolysis is formation of pyruvate. At this step,
pyruvate forms either into lactate (lactate dehydrogenase, LDH)
or enters the mitochondria and the Kreb's cycle
production of lactate requires the oxidation of NADH to NAD+
Muscle Glycogen
Glycogen is basically glucose units linked together into small hard
granules stored in sarcoplasm to provide readily-available fuel that can
be rapidly metabolized
Glycogenolysis


a glycosyl (glucose) molecule is split off the glycogen, a Pi is added,
and glucose-6-phosphate is formed. Note that this does not require
any energy (ATP) input
during high-intensity exercise, glycogen supplies most of the
glucose units



breakdown of muscle glycogen is controlled by phosphorylase a
through a series of events initially started by Ca2+ and increased
[Pi]
[Pi] is an important stimulator of glycogenolysis, however, other
unknown modulators are involved
EPI activates adenylate cyclase which stimulates cAMP
production and glycogenolysis
2+
, ADP, AMP, IMP, Pi
phosphorylase b (inactive) <======> phosphorylase a (active)
-6-P, H+

activation of phosphorylase a inactivates glycogen synthase
Glycogenesis




glucose enters the muscle and is phosphorylated (requires an ATP)
forming G-6-P
the phosphate group is removed and the glucose unit linked to the
glycogen molecule
glycogenesis is stimulated by activation of glycogen synthase which
is stimulated by insulin and inhibited by Ca2+ and cAMP
generally, glycogen synthase is activated when phosphorylase is
inactive and vice versa
Regulation of Glycolysis
adenylate nucleotide energy charge

Adenine nucleotide energy charge = [ADP] + 2 [ATP] _ X 0.5
[AMP] + [ADP] + [ATP]

if all adenine is in the form of ATP, the energy charge is 1.0; if all
adenine is in the form of AMP, the energy charge is 0.0; energy
charge is usually ~0.8
phosphofructokinase (PFK)



primary rate-limiting enzyme of glycolysis
changes in [ADP], [AMP], and [Pi] are likely responsible for
stimulating PFK with onset of high-intensity exercise
other modulators take longer to influence PFK because their
intracellular changes take longer to occur
Primary Stimulators
Primary Inhibitors
ADP
ATP
Pi
PCr
AMP
H+
NH4+
citrate
increased temperature
lactate dehydrogenase (LDH)



LDH is in competition with the mitochondria for pyruvate,
however, some lactate is always formed regardless of whether
exercising or not
LDH inhibited by high energy charge
naerobic glycolysis is the transformation of glucose to pyruvate
when limited amounts of oxygen (O2) are available. This is only an
effective means of energy production during short, intense
exercise, providing energy for a period ranging from 10 seconds to
2 minutes. The anaerobic glycolysis (lactic acid) system is dominant
from about 10–30 seconds during a maximal effort. It replenishes
very quickly over this period and produces 2 ATP molecules per
glucose molecule, or about 5% of glucose's energy potential (38 ATP
molecules). The speed at which ATP is produced is about 100 times
that of oxidative phosphorylation. The pH in the cytoplasm quickly




drops when hydrogen ions accumulates in the muscle, eventually
inhibiting enzymes involved in glycolosis.
The burning sensation in muscles during hard exercise can be
attributed to the production of hydrogen ions during a shift to
anaerobic glycolysis as oxygen is converted to carbon dioxide by
aerobic glycolysis faster than the body can replenish it. These
hydrogen ions form a part of lactic acid along with lactate. The
body falls back on this less efficient but faster method of producing
ATP under low oxygen conditions. This is thought to have been the
primary means of energy production in earlier organisms before
oxygen was at high concentration in the atmosphere and thus
would represent a more ancient form of energy production in cells.
The liver later gets rid of this excess lactate by transforming it
back into an important glycolysis intermediate called pyruvate.
Aerobic glycolysis is a method employed by muscle cells for the
production of lower-intensity energy over a longer period of time.
The process of converting the excess lactate back into pyruvate is
known as the Cori cycle, and occurs in the Liver.
Many anaerobic microorganisms carry out Anaerobic Glycolysis
through Fermentation.
All living organisms need energy to perform various functions. This
energy is obtained by a process known as glycolysis, which produces
energy in the form of ATP. In the process of glycolysis, glucose gets
oxidized to either lactate or pyruvate. There are two different pathways
by which the glycolysis process takes place- aerobic glycolysis and
anaerobic glycolysis. Let us learn about the anaerobic glycolysis and the
products of glycolysis in detail.
Anaerobic Glycolysis Definition
A metabolic pathway, involving transformation of glucose to pyruvate,
and the further conversion of pyruvate to lactate, in the absence of
oxygen is known as anaerobic glycolysis.
The anaerobic glycolysis system is also known as the lactic acid system,
as the end product of anaerobic glycolysis is lactate, which is the
conjugate base of lactic acid. The conversion of pyruvate into lactate is
brought about with the help of an enzyme lactate dehydrogenase (LDH).
The energy produced by this pathway ranges from 10 seconds to 20
minutes. Anaerobic glycolysis generally takes place when instant energy
is required in complete absence of oxygen or in limited supply of oxygen.
Anaerobic Glycolysis System
Anaerobic glycolysis is the main source of energy for some plants and
organisms. It is an important source of ATP during vigorous exercise,
when there is not enough oxygen supply. Anaerobic glycolysis is active
in bacteria involved in souring milk and formation of curds. This
pathway also exists in yeasts, where pyruvate is first converted to
acetaldehyde and carbon dioxide and then to ethanol in the absence of
oxygen.
There are two types of anaerobic fermentation process that can occur in
the absence of oxygen. They are as follows- lactic acid fermentation and
alcoholic fermentation. Let us get some more information about these
processes.
Lactic Acid Fermentation
Lactic acid fermentation pathway is commonly seen in animal cells and
in lactic acid bacteria. Animal tissues produce energy by using this type
of anaerobic glycolysis. During anaerobic glycolysis, breakdown of
glucose takes place in the absence of oxygen. Carbohydrate break down
takes place in the cells and results in the formation of pyruvic acid and
hydronium ions. The pyruvate further undergoes oxidation, forming
lactic acid that dissociates into lactate and H+. NADH gets oxidised in
this whole process which is the source of energy to the cells. The reaction
involved in the conversion of pyruvate into lactate can be represented
as follows:
Puruvate + NADH + H+ → Lactate + NAD+
The lactate produced diffuses out of the cell and passes into the liver. It is
then transformed into glucose which is capable of passing back into the
peripheral cells to re-enter glycolysis and forms a continuous cycle. The
red blood cells take most of their energy through this process of
anaerobic glycolysis. However, excess lactic acid production can lead to
lactic acidosis.
Glycolysis is the systematic breakdown of glucose and other sugars to
power the process of cellular respiration. It is a universal biochemical
reaction that occurs in every living unicellular or multicellular
organism which respires aerobically and anaerobically. There are many
metabolic pathways through which glycolysis occurs. The glycolysis steps
that I present here refer to the particular glycolysis pathway called
Embden-Meyerhof-Parnus pathway. The glycolysis process is a small
part of the cellular respiration cycle and overall body metabolism,
directed towards creating ATP (Adenosine Triphosphate) which is the
energy currency of the body. Read more on why is ATP an important
molecule in metabolism.
The process of glycolysis occurs in the cytoplasm of eukaryotic cells and
is controlled by various enzymes. Glycolysis steps involve a series of
chemical reactions that occur one after another with a series of subtle
energy changes. Below you will find a brief overview of the glycolysis
steps involved in Embden-Meyerhof-Parnus pathway.
What are the Steps of Glycolysis?
Glycolysis literally means glucose breakdown or decomposition. Through
the process of glycolysis, one glucose molecule is completely broken down
to yield two molecules of pyruvic acid, two molecules of ATP and two
NADH (Reduced Nicotinamide Adenine Dinucleotide) radicals carrying
electrons are produced. It took years of painstaking research in
biochemistry which revealed the glycolysis steps that make cellular
respiration possible. Here is are the various glycolysis steps presented in
the order of occurrence beginning with glucose as the main raw
material. The whole process of glycolysis involves ten steps with a
product forming at every stage and every stage regulated by a different
enzyme. The production of various compounds at every step offer
different entry points into the process of glycolysis. That means, a
glycolysis process may directly start from an intermediate stage if the
compound that is the reactant at that stage is directly made available.
So let us begin with the first glycolysis step.
Step 1: Phosphorylation of Glucose
The first of glycolysis steps is the phosphorylation of glucose (adding of a
phosphate group). This reaction is made possible by the enzyme
hexokinase, which separates one phosphate group out of ATP (Adenosine
Triphsophate) and adds it to glucose, transforming it to glucose 6phosphate. In the process one ATP molecule, which is the energy
currency of the body, is used up and gets transformed to ADP
(Adenosine Diphosphate), due to the separation of one phosphate group.
The entire reaction can be summarized as follows:
Glucose (C6H12O6) + ATP + Hexokinase → Glucose 6-Phosphate (C6H11O6P1) +
ADP
Step 2: Production of Fructose-6 Phosphate
The second of glycolysis steps is the production of fructose 6-phosphate. It
is made possible by the action of the enzyme phosphoglucoisomerase. It
acts on the product of the earlier step, glucose 6-phosphate and
transforms it into fructose 6-phosphate which is its isomer (Isomers are
different molecules with the same molecular formula but different
arrangement of atoms). The entire reaction is summarized as follows:
Glucose 6-Phosphate (C6H11O6P1) + Phosphoglucoisomerase (Enzyme) →
Fructose 6-Phosphate (C6H11O6P1)
Step 3: Production of Fructose 1, 6-Diphosphate
In the next step of glycolysis the isomer Fructose 6-Phosphate is
converted to fructose 1, 6-diphosphate by the addition of another
phosphate group. This conversion is made possible by the enzyme
phosphofructokinase which utilizes one more ATP molecule in the
process. The reaction is summarized as follows:
Fructose 6-Phosphate (C6H11O6P1) + phosphofructokinase (Enzyme) + ATP
→ Fructose 1, 6-diphosphate (C6H10O6P2)
Step 4: Splitting of Fructose 1, 6-Diphosphate
In the fourth of glycolysis steps, the enzyme adolase brings about the
splitting of Fructose 1, 6-diphosphate
into two different sugar molecules that are both isomers of each other.
The two sugars formed are glyceraldehyde phosphate and
dihydroxyacetone phosphate. The reaction goes as follows:
Fructose 1, 6-diphosphate (C6H10O6P2) + Aldolase (Enzyme) →
Glyceraldehyde Phosphate (C3H5O3P1) + Dihydroxyacetone phosphate
(C3H5O3P1)
Step 5: Interconversion of the Two Sugars
Dihydroxyacetone phosphate is a short lived molecule. As soon as it is
created, it gets converted into Glyceraldehyde phosphate by the enzyme
called triose phosphate. So in totality, the fourth and fifth steps of
glycolysis yield two molecules of Glyceraldehyde phosphate.
Dihydroxyacetone phosphate (C3H5O3P1) + Triose Phosphate →
Glyceraldehyde phosphate (C3H5O3P1)
Step 6: Formation of NADH & 1,3-Diphoshoglyceric acid
The sixth of glycolysis steps involves two important reactions. First is
the formation of NADH from NAD+ (nicotinamide adenine
dinucleotide) by the use of enzyme triose phosphate dehydrogenase and
second is the creation of 1,3-diphoshoglyceric acid from the two
glyceraldehyde phosphate molecules produced in the earlier step. The
two reaction are as follows:
Triose phosphate dehydrogenase (Enzyme) + 2 NAD+ + 2 H- → 2NADH
(Reduced Nicotinamide Adenine Dinucleotide) + 2 H+
Triose phosphate dehydrogenase + 2 Glyceraldehyde phosphate (C3H5O3P1)
+ 2P (from cytoplasm) → 2 molecules of 1,3-diphoshoglyceric acid
(C3H4O4P2)
Step 7: Production of ATP & 3-Phosphoglyceric Acid
The seventh of glycolysis steps involves the creation of 2 ATP molecules
along with two molecules of 3-phosphoglyceric acid from reaction of
phosphoglycerokinase on the two product molecules of 1,3diphoshoglyceric acid, yielded from the previous step.
2 molecules of 1,3-diphoshoglyceric acid (C3H4O4P2) + 2ADP +
phosphoglycerokinase → 2 molecules of 3-Phosphoglyceric acid (C3H5O4P1)
+ 2ATP (Adenosine Triphosphate)
Step 8: Relocation of Phosphorus Atom
Step eight is a very subtle rearrangement reaction in which involves the
relocation of the Phosphorus atom in in 3-phosphoglyceric acid from the
third carbon in the chain to the second carbon and creates 2phosphoglyceric acid. The entire reaction is summarized as follows:
2 molecules of 3-Phosphoglyceric acid (C3H5O4P1) + phosphoglyceromutase
(enzyme) → 2 molecules of 2-Phosphoglyceric acid (C3H5O4P1)
Step 9: Removal of Water
In the second last of glycolysis steps, the enzyme enolase comes into play
and removes a water molecule from 2-phosphoglyceric acid to form
another acid called phosphoenolpyruvic acid (PEP). This reaction
converts both the molecules of 2-Phosphoglyceric acid that form in the
previous step.
2 molecules of 2-Phosphoglyceric acid (C3H5O4P1) + Enolase (Enzyme) –> 2
molecules of phosphoenolpyruvic acid (PEP) (C3H3O3P1) + 2 H2O
Step 10: Creation of Pyruvic Acid & ATP
The last of glycolysis steps involves the creation of two ATP molecules
along with two molecules of pyruvic acid from the action of the enzyme
pyruvate kinase on the two molecules of phosphoenolpyruvic acid
produced in the previous step. This is made possible by the transfer of a
Phosphorus atom from phosphoenolpyruvic acid (PEP) to ADP
(Adenosine triphosphate).
2 molecules of phosphoenolpyruvic acid (PEP) (C3H3O3P1) + 2ADP +
Pyruvate kinase (Enzyme) → 2ATP + 2 molecules of pyruvic acid.
As you can see, glycolysis steps mostly involve the manipulation of the
phosphate group and then the phosphorus atom which is made possible
by the various enzymes in the cytoplasm. Enzymes are like catalysts
which make a reaction possible and then disengage.
Summary of Glycolysis Reaction
Let me summarize all the glycolysis steps in the end in a concise form.
The whole process involves the breakdown of one glucose molecule and it
yields 2 molecules of NADH, 2 molecules of ATP, 2 molecules of water of
water and 2 molecules of pyruvic acid. The products of glycolysis are
further used further in the citric acid cycle or Krebs cycle which is a
part of cellular respiration.
Glucose (C6H12O6) + 2 [NAD]+ + 2[ADP (Adenosine Diphosphate)] + 2 [P]i --> 2 [C3H3O3]-(Pyruvate) + 2 [NADH] (Reduced Nicotinamide Adenine
Dinucleotide) + 2H+ + 2 [ATP] (Adenosine Triphosphate) + 2 H2O
Read more on:



Anaerobic Glycolysis
Cellular Respiration Formula
Aerobic and Anaerobic Respiration
Each of the glycolysis steps are subtle energy changes made possible by
the various enzymes present in the cytoplasm which work in
coordination. The precision with which each of these reactions go ahead
in a synchronized fashion is simply amazing. As you go deeper and
deeper into biochemistry, you can increasingly appreciate the miracle
that life is!
Glycolysis Process: Overview
Before we discuss the products of glycolysis, let us see what is glycolysis
and why does it play such an important role in metabolism. Glycolysis is
a series of chemical reactions, that are a part of the cellular respiratory
cycle of aerobic and anaerobic type. The entire glycolysis process takes
place in the cytoplasm of eukaryotic cells. The entire process of glycolysis
is a sequence of ten reactions, with many intermediate compounds being
created. Every one of the glycolysis steps involves the creation of
intermediate compounds. There are two important types of Glycolysis
pathways. One is the Embden-Meyerhof pathway, while the other one is
the Entner-Doudoroff pathway. The glycolysis reaction which I talk
about here is the Embden-Meyerhof pathway. Glycolysis products and
reactants are a part of many metabolic processes.
The entire process of glycolysis can be summarized as follows:
Glucose (C6H12O6) + 2 [NAD]+ + 2[ADP (Adenosine Diphosphate)] + 2 [P]i ---
> 2 [C3H3O3]-(Pyruvate) + 2 [NADH] (Reduced Nicotinamide Adenine
Dinucleotide) + 2H+ + 2 [ATP] (Adenosine Triphosphate) + 2 H2O
Though this reaction looks simple enough, it is actually very complex
and this is just its summarized version. In anaerobic organisms too,
glycolysis is the process that forms an important part of sugar
fermentation. Organisms like yeast utilize glycolysis to produce alcohol
which is found in beer. In aerobic respiration, glycolysis plays the
important part of producing pyruvate which plays a major role in
metabolic cycles and is used in the production of ATP molecules.
Important Products of Glycolysis in Cellular Respiration
Every reaction like glycolysis plays a small part in the overall
biochemical machinery of the body. The products created by one
reaction are the raw materials for another one. All of these reactions are
controlled by the blueprint that exists in the DNA of every cell. Let us
discuss the role played by products of glycolysis and see what makes
them important. Following are the major products of glycolysis in
cellular respiration:
Pyruvate ([C3H3O3]-)
Pyruvate is the carboxylate ion part of pyruvic acid. The chemical
formula for pyruvate is CH3COCOO-. It is a key ion which is used in many
metabolic pathways. Pyruvate is used to supply energy to the cells
during the citric acid cycle. Pyruvate can also be converted back to
carbohydrates via a process called as 'Gluconeogenesis'. It is also
converted in to either fatty acids or energy through the Acetyl-CoA
molecule. It is also used in the creation of an amino acid called alanine.
Ethanol can also be created from pyruvate.
Reduced Nicotinamide Adenine Dinucleotide (NADH)
NAD+, that is Nicotinamide Adenine Dinucleotide is a type of coenzyme that carries out redox reactions in various biochemical
reactions as an oxidizing agent. Reduced Nicotinamide Adenine
Dinucleotide or NADH is the reduced form of NAD and acts as a
reducing agent in many reactions. An oxidizing agent accepts electrons
and becomes reduced, while a reducing agent gives electrons to become
oxidized. The NADH produced in the cytoplasm through glycolysis is
transferred to the mitochondria by the mitochondrial shuttles. It is used
to reduce the mitochondrial NAD+ into NADH. This NADH further
plays a role in oxidative photophosphorylation.
Adenosine Triphosphate (ATP)
Adenosine Triphosphate (ATP) is a nucleotide, that is used in various
biochemical reactions as a coenzyme. It is the energy currency of the
cell, as it is used for intracellular energy transfer. It also acts as a
signaling molecule in various biochemical reactions.
What is Metabolism
Food is the most important instrument that will help you either
lose/gain weight, considering that metabolism needs the energy from
what you eat. Chemical reactions take place within the body's cells that
convert fuel from the consumed food, ultimately into energy to do
everyday functions like thinking, growth, household/work activities and
so on. Without metabolism, cells wouldn't be kept healthy and
functioning.
Enzymes break down proteins from food present in the digestive system.
These are then converted into amino acids, fatty acids, carbohydrates
and sugar. These are then absorbed into the blood, namely amino and
fatty acids, making their way into the cells. Other enzymes then actively
control the chemical reactions thus metabolizing the compounds that are
the result of the process.
Metabolism Kinds
There are two kinds of metabolism, namely catabolism and anabolism.
Catabolism
Also known as destructive metabolism, catabolism produces energy to
make the cells active. Carbohydrates and fats are broken down to
produce energy. This energy is then released to provide fuel for
anabolism (read below). This in turn increases temperature within the
body, making muscles contract in being able make body parts move.
Complex chemical units are converted into simple matter - like waste
through the skin, lungs and kidneys.
Anabolism
Also known as constructive metabolism, anabolism caters to storing and
building. New cells are then formed and energy is stored for later use.
This is then converted to large molecules of protein, carbohydrates and
fats.
Now you must have understood what is metabolism and why is it
important. There are many reasons that stunt one's ability to lose weight
like genetics, problems like hyperthyroidism, type 1 and 2 diabetes and so
on. Make a point to check your family history, to gage whether you have
problems that hinder the way your body functions. Consult a doctor now
if you're unsure of why multiple diet plans and workouts fail to bear
fruit for you.
Glycogenesis, Glycogenolysis,
and Gluconeogenesis
Biosynthesis of Glycogen:
The goal of glycolysis, glycogenolysis, and the citric acid cycle is to
conserve energy as ATP from the catabolism of carbohydrates. If the
cells have sufficient supplies of ATP, then these pathways and cycles are
inhibited. Under these conditions of excess ATP, the liver will attempt to
convert a variety of excess molecules into glucose and/or glycogen.
Glycogenesis:
Glycogenesis is the formation of glycogen from glucose. Glycogen is
synthesized depending on the demand for glucose and ATP (energy). If
both are present in relatively high amounts, then the excess of insulin
promotes the glucose conversion into glycogen for storage in liver and
muscle cells.
In the synthesis of glycogen, one ATP is required per glucose
incorporated into the polymeric branched structure of glycogen.
actually, glucose-6-phosphate is the cross-roads compound. Glucose-6phosphate is synthesized directly from glucose or as the end product of
gluconeogenesis.
Link to: Interactive Glycogenesis (move cursor over arrows)
Jim Hardy, Professor of Chemistry, The University of Akron.
Click for larger image
Glycogenolysis:
In glycogenolysis, glycogen stored in the liver and muscles, is converted
first to glucose-1- phosphate and then into glucose-6-phosphate. Two
hormones which control glycogenolysis are a peptide, glucagon from the
pancreas and epinephrine from the adrenal glands.
Glucagon is released from the pancreas in response to low blood glucose
and epinephrine is released in response to a threat or stress. Both
hormones act upon enzymes to stimulate glycogen phosphorylase to
begin glycogenolysis and inhibit glycogen synthetase (to stop
glycogenesis).
Glycogen is a highly branched polymeric structure containing glucose as
the basic monomer. First individual glucose molecules are hydrolyzed
from the chain, followed by the addition of a phosphate group at C-1. In
the next step the phosphate is moved to the C-6 position to give glucose 6phosphate, a cross road compound.
Glucose-6-phosphate is the first step of the glycolysis pathway if glycogen
is the carbohydrate source and further energy is needed. If energy is not
immediately needed, the glucose-6-phosphate is converted to glucose for
distribution in the blood to various cells such as brain cells.
Click for larger image
Biosynthesis of Glucose:
Gluconeogenesis:
Gluconeogenesis is the process of synthesizing glucose from noncarbohydrate sources. The starting point of gluconeogenesis is pyruvic
acid, although oxaloacetic acid and dihydroxyacetone phosphate also
provide entry points. Lactic acid, some amino acids from protein and
glycerol from fat can be converted into glucose. Gluconeogenesis is
similar but not the exact reverse of glycolysis, some of the steps are the
identical in reverse direction and three of them are new ones. Without
going into detail, the general gluconeogenesis sequence is given in the
graphic on the left.
Notice that oxaloacetic acid is synthesized from pyruvic acid in the first
step. Oxaloacetic acid is also the first compound to react with acetyl CoA
in the citric acid cycle. The concentration of acetyl CoA and ATP
determines the fate of oxaloacetic acid. If the concentration of acetyl
CoA is low and concentration of ATP is high then gluconeogenesis
proceeds. Also notice that ATP is required for a biosynthesis sequence of
gluconeogenesis.
Gluconeogenesis occurs mainly in the liver with a small amount also
occurring in the cortex of the kidney. Very little gluconeogenesis occurs
in the brain, skeletal muscles, heart muscles or other body tissue. In fact,
these organs have a high demand for glucose. Therefore, gluconeogenesis
is constantly occurring in the liver to maintain the glucose level in the
blood to meet these demands.
Overview of Carbohydrate Metabolism
Introduction:
Carbohydrate metabolism begins with digestion in the small intestine
where monosaccharides are absorbed into the blood stream. Blood sugar
concentrations are controlled by three hormones: insulin, glucagon, and
epinephrine. If the concentration of glucose in the blood is too high,
insulin is secreted by the pancreas. Insulin stimulates the transfer of
glucose into the cells, especially in the liver and muscles, although other
organs are also able to metabolize glucose.
In the liver and muscles, most of the glucose is changed into glycogen by
the process of glycogenesis (anabolism). Glycogen is stored in the liver
and muscles until needed at some later time when glucose levels are low.
If blood glucose levels are low, then eqinephrine and glucogon hormones
are secreted to stimulate the conversion of glycogen to glucose. This
process is called glycogenolysis (catabolism).
If glucose is needed immediately upon entering the cells to supply
energy, it begins the metabolic process called glycoysis (catabolism). The
end products of glycolysis are pyruvic acid and ATP.
Since glycolysis releases relatively little ATP, further reactions continue
to convert pyruvic acid to acetyl CoA and then citric acid in the citric
acid cycle. The majority of the ATP is made from oxidations in the citric
acid cycle in connection with the electron transport chain.
During strenuous muscular activity, pyruvic acid is converted into
lactic acid rather thatn acetyl CoA. Durlng the resting period, the lactic
acid is converted back to pyruvic acid. The pyruvic acid in turn is
converted back to glucose by the process called gluconeogenesis
(anabolism). If the glucose is not needed at that moment, it is converted
into glycogen by glycogenesis. You can remember those terms if you
think of "genesis" as the formation-beginning.
These processes are summarized in the Metaboism Summary in the
graphic on the left. Each of these processes will be developed in greater
detail various pages of this module.
Glycolysis is an anaerobic process through which ATP is synthesized
during
the conversion of the six-carbon sugar glucose to two molecules of the
three-carbon compound pyruvate. It has two phases: an energy
investment
phase, where ATP is consumed, and an energy generation phase, where
ATP is produced.
A total of ten reactions are involved, each of which is
catalyzed by a different enzyme. Factors affecting the rate of glycolysis
do so by inhibiting or activating one or more of the enzymes
involved. Some of these factors include:
*the presence of iodoacetate or
heavy metals, which inhibit glyceraldehyde-3-phosphate dehydrogenase
(******is this why people get sick from mercury poisoning????****)
*the presence of carbohydrate, which stimulates the production of
pyruvate
kinase in the liver, increasing rate of glycolysis
*the presence of oxygen, which inhibits substrate flow through
phosphofructokinase.
Inhibition of a certain enzyme can be detected by measuring the
amounts of
reaction intermediates after addition of a particular
inhibitor/activator. For example, it was determined that oxygen
inhibits
phosphofructokinase (which catalyzes the synthesis of fructose
1-6-bisphosphate) because levels of all intermediates past (&
including) fructose 1-6-bisphosphate decreased upon addition of O2.
Homeostasis must be maintained during glycolysis. This is acheived by
reoxidizing the 2 mol NADH (that are produced during glycolysis) back
to
NAD+. In aerobic glycolysis, these electrons are used to reduce oxygen;
in
anaerobic glycolysis, they drive the reduction of pyruvate to lactate.
Glycolysis is not completely efficient, releasing only a small amount of
the energy stored in the glucose molecule. Coupling glycolysis to the
citric acid cycle increases the amount of energy converted into ATP to
40%, through the synthesis of 38 additional mol ATP.
Glycolytic pathways exist for obtaining energy from sugars other than
glucose as well, including lactose, sucrose, and mannose. Energy is
obtained from polysaccharides such as glycogen and amylose through a
hormonally regulated metabolic cascade.
I found it interesting that glycolysis was an ancient metabolic pathway
used
by the earliest known bacteria. I also did not know that lactic acid
fermentation was vital to the production of cheese. I did not before know
the
definition of fermentation as an energy yielding metabolic pathway that
does
not have a net oxidation state change. Do oxidation states change but
cancle
eachother out? I'm not quite sure. I found the fairly in-depth sections on
the energy investment and energy generation phases of glycolysis to be
somewhat tedious. I will definitly have to re-read these sections to
understand them. The reason why pyruvate is converted into lactate in
both
aerobic and anaerobic cells makes sense. I was surprised to read that red
blood cells derive most of their energy from anaerobic metabloism. I did
not
know that skeletal muscle attains much of its energy when exerted from
glycolysis. I was also not aware that the products made in anaerobic
respiration such as lactate will then move through the body to parts of
the
body which are heavily involved in aerobic respiration to be catabolized.
I
was amazed that i had never before heard that glycolysis not only
generates
ATP and pyruvate, but that it produces intermediates which are used to
make
lipids and amino acids.
This chapter is mostly devoted to glycolysis and compounds associated
with
glycolysis. Glycolysis was the first metabolic pathway understood, is
universal in most cells and the regulation of glycolysis is well
understood. This metabolic pathway consists of 10 steps, 5 energy
investment phases and 5 pay off phases. The book goes into great details
about the 10 steps including products, side products, enzymes andeven
strucutre. How detailed should our study of this process be? Also, the
analysis of key enzymes and products are also very detailed and
seemingly
relevant, but some clue about the level of comprehension we need would
be
nice.
Chapter 13 begins the detailed study on metabolici pathways with
anaerobic
and aerobic glycolysis. The initial and a universal process, glycolysis
was widely studied in yeasts, whose genetic sequence was the subject of
our last lab. Glycolysis is divided into two phases: the energy
investment phases, which expences two ATP's, and the energy
generation
phase which generates 4 ATP and 2 NADH anaerobically or 10 ATP
aerobically. Although glycolysis only releases a small fraction of energy
available from glucose, the energy is needed as fuel for aerobic
energy-generating pathways. The Pasteur effect is the inhibition of
glycolysis by oxygen. Other glycolytic controls are known and will be
further studied in later chapters.
Chapter 13 covered glycolysis. Glycolysis is a central part of the
metabolic processes of all cells. It is the first part of the pathway
to break down glucose and other sugars to produce ATP in aerobic
organisms, and the only source of ATP in anaerobic organisms. There
are
ten steps in glycolysis, each of which is catalyzed by a different
enzyme. There are various control mechanisms, including a
feedback-controlled system, and oxygen inhibition. Glycolysis fits into
many different metabolic pathways, including digestion and conversion
of
other sugars, monosaccharides and polysaccharides, into usable energy
for cells (ATP). The breakdown and use of glycogen, which we studied
earlier, in the chapter about carbohydrates, also involves glycolysis as
a central pathway.
Aerobic Respiration
Aerobic respiration is the process that takes place in presence of oxygen.
Aerobic respiration is the metabolic process that involves break down of
fuel molecules to obtain bio-chemical energy and has oxygen as the
terminal electron acceptor. Fuel molecules commonly used by cells in
aerobic respiration are glucose, amino acids and fatty acids.. The process
of obtaining energy in aerobic respiration can be represented in the
following equation:
Glucose + Oxygen →Energy + Carbon dioxide + Water
The aerobic respiration is a high energy yielding process. During the
process of aerobic respiration as many as 38 molecules of ATP are
produced for every molecule of glucose that is utilized. Thus aerobic
respiration process breaks down a single glucose molecule to yield 38
units of the energy storing ATP molecules.
Anaerobic respiration
The term anaerobic means without air and hence anaerobic respiration
refers to the special type of respiration, which takes place without
oxygen. Anaerobic respiration is the process of oxidation of molecules in
the absence of oxygen, which results in production of energy in the form
of ATP or adenosine tri-phosphate. Anaerobic respiration is synonymous
with fermentation especially when the glycolytic pathway of energy
production is functional in a particular cell. The process of anaerobic
respiration for production of energy can occur in either of the ways
represented below:
Glucose (Broken down to) →Energy (ATP) + Ethanol + Carbon dioxide
(CO2)
Glucose (Broken down to) →Energy (ATP) + Lactic acid
The process of anaerobic respiration is relatively less energy yielding as
compared to the aerobic respiration process. During the alcoholic
fermentation or the anaerobic respiration (represented in the first
equation) two molecules of ATP (energy) are produced. for every
molecule of glucose used in the reaction. Similarly for the lactate
fermentation (represented in the second equation) 2 molecules of ATP
are produced for every molecule of glucose used. Thus anaerobic
respiration breaks down one glucose molecule to obtain two units of the
energy storing ATP molecules.
Glycolysis Reactions
Introduction to Glycolysis:
The overall reaction of glycolysis which occurs in the cytoplasm is
represented simply as:
C6H12O6 + 2 NAD+ + 2 ADP + 2 P -----> 2 pyruvic acid, (CH3(C=O)COOH + 2
ATP + 2 NADH + 2 H+
At this time, concentrate on the fact that glucose with six carbons is
converted into two pyruvic acid molecules with three carbons each. Only
a net "visible" 2 ATP are produced from glycolysis. The 2 NADH will be
considered separately later.
The major steps of glycolysis are outlined in the graphhic on the left.
There are a variety of starting points for glycolysis; although, the most
usual ones start with glucose or glycogen to produce glucose-6-phosphate.
The starting points for other monosaccharides, galactose and fructose,
are also shown.
Glycolysis - with white background for printing
Overview of Metabolism
Link to: Great Animation of entire Glycolysis - John Kyrk
Link to: Interactive Glycolysis (move cursor over arrows)
Jim Hardy, Professor of Chemistry, The University of Akron.
Link to Glycolysis Aninmation 1
Link to Glycolysis Aninmation 2
Reaction 1: Phosphate Ester Synthesis
Phosphate is added to the glucose at the C-6 position. The reaction is a
phosphate ester synthesis using the alcohol on the glucose and a
phosphate from ATP.
This first reaction is endothermic and thus requires energy from a
coupled reaction with ATP. ATP is used by being hydrolyzed to ADP
and phosphate giving off energy and the phosphate for reaction with the
glucose for a net loss of ATP in the overall glycolysis pathway.
Hydrolysis: ATP + H2O --> ADP + P + energy
P = PO4-3; ATP = adenine triphosphate;ADP = adenine diphosphate
This reaction is catalyzed by hexokinase.
Off-site chime link: Boyer Tutorial - Hexokinase
Reaction 1 - Chime in new window
Reaction 2: Isomerization
The glucose-6-phosphate is changed into an isomer, fructose-6-phosphate.
This means that the number of atoms is unchanged, but their positions
have changed.
This works because the ring forms may open to the chain form, and then
the aldehyde group on glucose is transformed to the keone group on
fructose. The ring then closes to form the fructose-6-phosphate.
This reaction is catalyzed by phosphoglucoisomerase.
Off-site chime link: Phosphoglucoisomerase
Reaction 2 - Chime in new window
Reaction 3: Phosphate ester synthesis
This reaction is virtually identical to reaction 1 The fructosee-6phosphate has an alcohol group on C-1 that is reacted with phosphate
from ATP to make the phosphate ester on C-1.
Again this reaction is endothermic and thus requires energy from a
coupled reaction with ATP. ATP is used by being hydrolyzed to ADP
and phosphate giving off energy and the phosphate for reaction with the
glucose for a net loss of ATP in the overall glycolysis pathway.
Hydrolysis: ATP + H2O --> ADP + P + energy
This reaction is catalyzed by phosphofructokinase.
Off-site chime link: Phosphofructokinase
Link to: Rodney Boyer Animation of Phosphofructokinase
Reaction 3 - Chime in new window
Reaction 4: Split Molecule in half
The six carbon fructose diphophate is spit into two three-carbon
compounds, an aldehyde and a ketone.
The slit is made between the C-3 and C-4 of the fructose. The ring also
opens at the anomeric carbon. The product on the right is the
glyceraldehyde.
Technically this is called a reverse aldol condensation.
This reaction is catalyzed by aldolase.
Off-site chime link: Aldolase
Reaction 4 - Dihydroxyacetonephosphate Chime in new window
Reaction 4 - Glyceraldehyde-3-phosphate Chime in new window
Reaction 4A: Isomerization
The dihydroxyacetone phosphate must be converted to glyceraldehyde-3phosphate to continue the glycolysis reactions. This reaction is an
isomerization between the keone group and an aldehyde group.
As a result of this reaction, all of the remaining glycolysis reactions are
carried out a second time. The first series of reactions occurs with the
first glyceraldehyde molecule from the orginal split. Then the second
series of reactions occurs after the isomerization of the
dihydroxyacetone into the glyceraldehyde.
This reaction is catalyzed by triose phosphate isomerase.
Off-site chime link: Triose Phosphate Isomerase (TIM)
Reaction 4A - Isomerization Chime in new window
Reaction 5: Oxidation/Phosphate Ester Synthesis
This reaction is first an oxidation involving the coenzyme NAD+. The
aldehyde is oxidized to an acid as an intermediate through the
conversion of NAD+ to NADH + H+. Then an inorganic phosphate is
added in a phosphate esteer synthesis.
This and all remaining reactions occur twice for each glucose-6phosphate (six carbons), since there are now two molecules of 3-carbons
each.
This reaction is catalyzed by glyceraldehyde-3-phosphate.
Off-site chime link: G3P Dehydrogenase
Reaction 5 - 1,3-diphosphoglycerate Chime in new window
Reaction 6: Hydrolysis of Phosphate;
Synthesis of ATP
One of the phosphate groups undergoes hydrolysis to form the acid and a
phosphate ion, giving off energy. This first energy producing reaction is
coupled with the next endothermic reaction making ATP. The phosphate
is transferred directly to an ADP to make ATP.
This reaction is catalyzed by phosphoglycerokinase.
Off-site chime link: Phosphoglycerate Kinase
Reaction 6 - 3-phosphoglycerate Chime in new window
Reaction 7: Isomerization
In this reaction the phosphate group moves from the 3 position to the 2
position in an isomerization reaction.
This reaction is catalyzed by phosphoglycerate mutase.
Off-site chime link: Phosphoglycerate Mutase
Reaction 7 - 2-phosphoglycerate Chime in new window
Reaction 8: Alcohol Dehydration
In this reaction, which is the dehydration of an alcohol, the -OH on C-3
and the -H on C-2 are removed to make a water molecule. At the same
time a double bond forms between C-2 and C-3. This change makes the
compound somewhat unstable, but energy for the final step of glycolysis.
This reaction is catalyzed by enolase.
Off-site chime link: Enolase
Reaction 8 - phosphoenol pyruvic acid Chime in new window
Reaction 9:Phosphate Ester Hydrolysis;
Synthesis of ATP
This is the final reaction in glycolysis. Again one of the phosphate groups
undergoes hydrolysis to form the acid and a phosphate ion, giving off
energy. This first energy producing reaction is coupled with the next
endothermic reaction making ATP. The phosphate is transferred directly
to an ADP to make ATP.
This reaction is catalyzed by pyruvic kinase.
Off-site chime link: Pyruvate Kinase
Reaction 9 - pyruvic acid Chime in new window
Conclusion:
Starting with glucose-6-phosphate with 6 carbons, the final result of the
glycolysis reactions is two molecules of pyruvic acid, since reaction 5-9
are each carried out twice.
Lactic Acid
The expression "lactic acid" is used most commonly by athletes to
describe the intense pain felt during exhaustive exercise, especially in
events like the 400 metres and 800 metres. When energy is required to
perform exercise, it is supplied from the breakdown of Adenosine
Triphosphate (ATP). The body has a limited store of about 85 grms of
ATP and would use it up very quickly if we did not have ways of
resynthesising it. There are three systems that produce energy to
resynthesise ATP: ATP-PC, lactic acid and aerobic.
The lactic acid system is capable of releasing energy to resynthesise ATP
without the involvement of oxygen and is called anaerobic glycolysis.
Glycolysis (breakdown of carbohydrates) results in the formation of
pyruvic acid and hydronium ions (H+). The pyruvic acid molecules
undergo oxidation in the mitochondrion and the Krebs cycle begins. A
build up of H+ will make the muscle cells acidic and interfere with their
operation so carrier molecules, called nicotinamide adenine dinucleotide
(NAD+), remove the H+. The NAD+ is reduced to NADH that deposit
the H+ at the electron transport gate (ETC) in the mitrochondria to be
combined with oxygen to form water (H2O).
If there is insufficient oxygen then NADH cannot release the H+ and
they build up in the cell. To prevent the rise in acidity pyruvic acid
accepts H+ forming lactic acid that then dissociates into lactate and H+.
Some of the lactate diffuses into the blood stream and takes some H+
with it as a way of reducing the H+ concentration in the muscle cell. The
normal pH of the muscle cell is 7.1 but if the build up of H+ continues
and pH is reduced to around 6.5 then muscle contraction may be
impaired and the low pH will stimulate the free nerve endings in the
muscle resulting in the perception of pain (the burn). This point is often
measured as the lactic threshold or anaerobic threshold (AT) or onset of
blood lactate accumulation (OBLA).
The process of lactic acid removal takes approximately one hour, but
this can be accelerated by undertaking an appropriate cool down that
ensures a rapid and continuous supply of oxygen to the muscles.
The normal amount of lactic acid circulating in the blood is about 1 to 2
millimoles/litre of blood. The onset of blood lactate accumulation (OBLA)
occurs between 2 and 4 millimoles/litre of blood. In non athletes this
point is about 50% to 60% VO2 max and in trained athletes around 70%
to 80% VO2 max.
Krebs Cycle
The Krebs cycle is a series of reactions which occurs in the mitochondria
and results in the formation of ATP. The pyruvic acid molecules from
glycolysis undergo oxidation in the mitochondrion to produce acetyl
coenzyme A and then the Krebs cycle begins.
Three major events occur during the Krebs cycle. One guanosine
triphosphate (GTP) is produced which donates a phosphate group to ADP
to form one ATP; three molecules of Nicotinamide adenine dinucleotide
(NAD) and one molecule of flavin adenine dinucleotide (FAD) are
reduced. Although one molecule of GTP leads to the production of one
ATP, the production of the reduced NAD and FAD are far more
significant in the cell's energy generating process because they donate
their electrons to an electron transport system that generates large
amounts ATP.
Cori Cycle
The Cori cycle refers to the metabolic pathway in which lactate
produced by anaerobic glycolysis in the muscles moves via the blood
stream to the liver where it it is converted to blood glucose and glycogen.
hydronium ions
The breakdown of glucose or glycogen produces lactate and hydronium
ions - for each lactate molecule, one hydrogen ion is formed. The presence
of hydronium ions, not lactate, makes the muscle acidic that will
eventually halt muscle function. As hydrogen ion concentrations
increase the blood and muscle become acidic. This acidic environment
will slow down enzyme activity and ultimately the breakdown of glucose
itself. Acidic muscles will aggravate associated nerve endings causing
pain and increase irritation of the central nervous system. The athlete
may become disorientated and feel nauseous.
Aerobic Capacity
Given that high levels of lactate/hydronium ions will be detrimental to
performance, one of the key reasons for endurance training is to enable
the body to perform at a greater pace with a minimal amount of lactate.
This can be done by long steady runs, which will develop the aerobic
capacity by means of capillarisation (formation of more small blood
vessels, thus enhancing oxygen transport to the muscles) and by creating
greater efficiency in the heart and lungs. If the aerobic capacity is
greater, it means there will be more oxygen available to the working
muscles and this should delay the onset of lactic acid at a given work
intensity.
Anaerobic Threshold
Lactic acid starts to accumulate in the muscles once you start operating
above your anaerobic threshold. This is normally somewhere between
80% and 90% of your maximum heart rate (MHR) in trained athletes.
Anaerobic glycolysis is the transformation of glucose to pyruvate when
limited amounts of oxygen (O2) are available. This is only an effective
means of energy production during short, intense exercise, providing
energy for a period ranging from 10 seconds to 2 minutes
Digestion of Dietary Carbohydrates
Dietary carbohydrate from which humans gain energy enter the body in
complex forms, such as disaccharides and the polymers starch (amylose
and amylopectin) and glycogen. The polymer cellulose is also consumed
but not digested. The first step in the metabolism of digestible
carbohydrate is the conversion of the higher polymers to simpler, soluble
forms that can be transported across the intestinal wall and delivered to
the tissues. The breakdown of polymeric sugars begins in the mouth.
Saliva has a slightly acidic pH of 6.8 and contains lingual amylase that
begins the digestion of carbohydrates. The action of lingual amylase is
limited to the area of the mouth and the esophagus; it is virtually
inactivated by the much stronger acid pH of the stomach. Once the food
has arrived in the stomach, acid hydrolysis contributes to its
degradation; specific gastric proteases and lipases aid this process for
proteins and fats, respectively. The mixture of gastric secretions, saliva,
and food, known collectively as chyme, moves to the small intestine.
The main polymeric-carbohydrate digesting enzyme of the small
intestine is α-amylase. This enzyme is secreted by the pancreas and has
the same activity as salivary amylase, producing disaccharides and
trisaccharides. The latter are converted to monosaccharides by intestinal
saccharidases, including maltases that hydrolyze di- and trisaccharides,
and the more specific disaccharidases, sucrase, lactase, and trehalase.
The net result is the almost complete conversion of digestible
carbohydrate to its constituent monosaccharides. The resultant glucose
and other simple carbohydrates are transported across the intestinal
wall to the hepatic portal vein and then to liver parenchymal cells and
other tissues. There they are converted to fatty acids, amino acids, and
glycogen, or else oxidized by the various catabolic pathways of cells.
Oxidation of glucose is known as glycolysis.Glucose is oxidized to either
lactate or pyruvate. Under aerobic conditions, the dominant product in
most tissues is pyruvate and the pathway is known as aerobic glycolysis.
When oxygen is depleted, as for instance during prolonged vigorous
exercise, the dominant glycolytic product in many tissues is lactate and
the process is known as anaerobic glycolysis.
back to the top
The Energy Derived from Glucose Oxidation
Aerobic glycolysis of glucose to pyruvate, requires two equivalents of
ATP to activate the process, with the subsequent production of four
equivalents of ATP and two equivalents of NADH. Thus, conversion of
one mole of glucose to two moles of pyruvate is accompanied by the net
production of two moles each of ATP and NADH.
Glucose + 2 ADP + 2 NAD+ + 2 Pi ——> 2 Pyruvate + 2 ATP + 2 NADH +
2 H+
The NADH generated during glycolysis is used to fuel mitochondrial
ATP synthesis via oxidative phosphorylation, producing either two or
three equivalents of ATP depending upon whether the glycerol
phosphate shuttle or the malate-aspartate shuttle is used to transport the
electrons from cytoplasmic NADH into the mitochondria.
The malate-aspartate shuttle is the principal mechanism for the
movement of reducing equivalents (in the form of NADH) from the
cytoplasm to the mitochondria. The glycolytic pathway is a primary
source of NADH. Within the mitochodria the electrons of NADH can be
coupled to ATP production during the process of oxidative
phosphorylation. The electrons are "carried" into the mitochondria in the
form of malate. Cytoplasmic malate dehydrogenase (MDH) reduces
oxaloacetate (OAA) to malate while oxidizing NADH to NAD+. Malate
then enters the mitochondria where the reverse reaction is carried out
by mitochondrial MDH. Movement of mitochondrial OAA to the
cytoplasm to maintain this cycle requires it be transaminated to
aspartate (Asp, D) with the amino group being donated by glutamate
(Glu, E). The Asp then leaves the mitochondria and enters the cytoplasm.
The deamination of glutamate generates α-ketoglutarate (α-KG) which
leaves the mitochondria for the cytoplasm. All the participants in the
cycle are present in the proper cellular compartment for the shuttle to
function due to concentration dependent movement. When the energy
level of the cell rises the rate of mitochondrial oxidation of NADH to
NAD+ declines and therefore, the shuttle slows. G3PDH is
glyceraldehyde-3-phosphate dehydrogenase.
The glycerol phosphate shuttle is a secondary mechanism for the
transport of electrons from cytosolic NADH to mitochondrial carriers of
the oxidative phosphorylation pathway. The primary cytoplasmic
NADH electron shuttle is the malate-aspartate shuttle. Two enzymes are
involved in this shuttle. One is the cytosolic version of the enzyme
glycerol-3-phosphate dehydrogenase (glycerol-3-PDH) which has as one
substrate, NADH. The second is is the mitochondrial form of the enzyme
which has as one of its' substrates, FAD+. The net result is that there is a
continual conversion of the glycolytic intermediate, DHAP and glycerol3-phosphate with the concomitant transfer of the electrons from reduced
cytosolic NADH to mitochondrial oxidized FAD+. Since the electrons
from mitochondrial FADH2 feed into the oxidative phosphorylation
pathway at coenzyme Q (as opposed to NADH-ubiquinone
oxidoreductase [complex I]) only 2 moles of ATP will be generated from
glycolysis. G3PDH is glyceraldehyde-3-phoshate dehydrogenase.
The net yield from the oxidation of 1 mole of glucose to 2 moles of
pyruvate is, therefore, either 6 or 8 moles of ATP. Complete oxidation of
the 2 moles of pyruvate, through the TCA cycle, yields an additional 30
moles of ATP; the total yield, therefore being either 36 or 38 moles of
ATP from the complete oxidation of 1 mole of glucose to CO2 and H2O.
back to the top
The Individual Reactions of Glycolysis
The pathway of glycolysis can be seen as consisting of 2 separate phases.
The first is the chemical priming phase requiring energy in the form of
ATP, and the second is considered the energy-yielding phase. In the first
phase, 2 equivalents of ATP are used to convert glucose to fructose 1,6bisphosphate (F1,6BP). In the second phase F1,6BP is degraded to
pyruvate, with the production of 4 equivalents of ATP and 2 equivalents
of NADH.
Pathway of glycolysis from glucose to pyruvate. Substrates and products
are in blue, enzymes are in green. The two high energy intermediates
whose oxidations are coupled to ATP synthesis are shown in red (1,3bisphosphoglycerate and phosphoenolpyruvate). Place mouse over
intermediate names to see chemical structures.
The Hexokinase Reaction:
The ATP-dependent phosphorylation of glucose to form glucose 6phosphate (G6P)is the first reaction of glycolysis, and is catalyzed by
tissue-specific isoenzymes known as hexokinases. The phosphorylation
accomplishes two goals: First, the hexokinase reaction converts nonionic
glucose into an anion that is trapped in the cell, since cells lack transport
systems for phosphorylated sugars. Second, the otherwise biologically
inert glucose becomes activated into a labile form capable of being
further metabolized.
Four mammalian isozymes of hexokinase are known (Types I–IV), with
the Type IV isozyme often referred to as glucokinase. Glucokinase is the
form of the enzyme found in hepatocytes and pancreatic β-cells. The high
Km of glucokinase for glucose means that this enzyme is saturated only
at very high concentrations of substrate.
Comparison of the activities of hexokinase and glucokinase. The Km for
hexokinase is significantly lower (0.1mM) than that of glucokinase
(10mM). This difference ensures that non-hepatic tissues (which contain
hexokinase) rapidly and efficiently trap blood glucose within their cells
by converting it to glucose-6-phosphate. One major function of the liver is
to deliver glucose to the blood and this in ensured by having a glucose
phosphorylating enzyme (glucokinase) whose Km for glucose is
sufficiently higher that the normal circulating concentration of glucose
(5mM).
This feature of hepatic glucokinase allows the liver to buffer blood
glucose. After meals, when postprandial blood glucose levels are high,
liver glucokinase is significantly active, which causes the liver
preferentially to trap and to store circulating glucose. When blood
glucose falls to very low levels, tissues such as liver and kidney, which
contain glucokinases but are not highly dependent on glucose, do not
continue to use the meager glucose supplies that remain available. At the
same time, tissues such as the brain, which are critically dependent on
glucose, continue to scavenge blood glucose using their low Km
hexokinases, and as a consequence their viability is protected. Under
various conditions of glucose deficiency, such as long periods between
meals, the liver is stimulated to supply the blood with glucose through
the pathway of gluconeogenesis. The levels of glucose produced during
gluconeogenesis are insufficient to activate glucokinase, allowing the
glucose to pass out of hepatocytes and into the blood.
The regulation of hexokinase and glucokinase activities is also different.
Hexokinases I, II, and III are allosterically inhibited by product (G6P)
accumulation, whereas glucokinases are not. The latter further insures
liver accumulation of glucose stores during times of glucose excess, while
favoring peripheral glucose utilization when glucose is required to
supply energy to peripheral tissues.
Phosphohexose Isomerase:
The second reaction of glycolysis is an isomerization, in which G6P is
converted to fructose 6-phosphate (F6P). The enzyme catalyzing this
reaction is phosphohexose isomerase (also known as phosphoglucose
isomerase). The reaction is freely reversible at normal cellular
concentrations of the two hexose phosphates and thus catalyzes this
interconversion during glycolytic carbon flow and during
gluconeogenesis.
6-Phosphofructo-1-Kinase (Phosphofructokinase-1, PFK-1):
The next reaction of glycolysis involves the utilization of a second ATP
to convert F6P to fructose 1,6-bisphosphate (F1,6BP). This reaction is
catalyzed by 6-phosphofructo-1-kinase, better known as
phosphofructokinase-1 or PFK-1. This reaction is not readily reversible
because of its large positive free energy (ΔG0' = +5.4 kcal/mol) in the
reverse direction. Nevertheless, fructose units readily flow in the reverse
(gluconeogenic) direction because of the ubiquitous presence of the
hydrolytic enzyme, fructose-1,6-bisphosphatase (F-1,6-BPase).
The presence of these two enzymes in the same cell compartment
provides an example of a metabolic futile cycle, which if unregulated
would rapidly deplete cell energy stores. However, the activity of these
two enzymes is so highly regulated that PFK-1 is considered to be the
rate-limiting enzyme of glycolysis and F-1,6-BPase is considered to be the
rate-limiting enzyme in gluconeogenesis.
Aldolase:
Aldolase catalyses the hydrolysis of F1,6BP into two 3-carbon products:
dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate
(G3P). The aldolase reaction proceeds readily in the reverse direction,
being utilized for both glycolysis and gluconeogenesis.
Triose Phosphate Isomerase:
The two products of the aldolase reaction equilibrate readily in a
reaction catalyzed by triose phosphate isomerase. Succeeding reactions
of glycolysis utilize G3P as a substrate; thus, the aldolase reaction is
pulled in the glycolytic direction by mass action principals.
Glyceraldehyde-3-Phosphate Dehydrogenase:
The second phase of glucose catabolism features the energy-yielding
glycolytic reactions that produce ATP and NADH. In the first of these
reactions, glyceraldehyde-3-P dehydrogenase (G3PDH) catalyzes the
NAD+-dependent oxidation of G3P to 1,3-bisphosphoglycerate (1,3BPG)
and NADH. The G3PDH reaction is reversible, and the same enzyme
catalyzes the reverse reaction during gluconeogenesis.
Phosphoglycerate Kinase:
The high-energy phosphate of 1,3-BPG is used to form ATP and 3phosphoglycerate (3PG) by the enzyme phosphoglycerate kinase. Note
that this is the only reaction of glycolysis or gluconeogenesis that
involves ATP and yet is reversible under normal cell conditions.
Associated with the phosphoglycerate kinase pathway is an important
reaction of erythrocytes, the formation of 2,3-bisphosphoglycerate,
2,3BPG (see Figure below) by the enzyme bisphosphoglycerate mutase.
2,3BPG is an important regulator of hemoglobin's affinity for oxygen.
Note that 2,3-bisphosphoglycerate phosphatase degrades 2,3BPG to 3phosphoglycerate, a normal intermediate of glycolysis. The 2,3BPG shunt
thus operates with the expenditure of 1 equivalent of ATP per triose
passed through the shunt. The process is not reversible under
physiological conditions.
The pathway for 2,3-bisphosphoglycerate (2,3-BPG) synthesis within
erythrocytes. Synthesis of 2,3-BPG represents a major reaction pathway
for the consumption of glucose in erythrocytes. The synthesis of 2,3-BPG
in erythrocytes is critical for controlling hemoglobin affinity for oxygen.
Note that when glucose is oxidized by this pathway the erythrocyte loses
the ability to gain 2 moles of ATP from glycolytic oxidation of 1,3-BPG to
3-phosphoglycerate via the phosphoglycerate kinase reaction.
Phosphoglycerate Mutase and Enolase:
The remaining reactions of glycolysis are aimed at converting the
relatively low energy phosphoacyl-ester of 3PG to a high-energy form
and harvesting the phosphate as ATP. The 3PG is first converted to 2PG
by phosphoglycerate mutase and the 2PG conversion to
phosphoenoylpyruvate (PEP) is catalyzed by enolase.
Pyruvate Kinase:
The final reaction of aerobic glycolysis is catalyzed by the highly
regulated enzyme pyruvate kinase (PK). In this strongly exergonic
reaction, the high-energy phosphate of PEP is conserved as ATP. The loss
of phosphate by PEP leads to the production of pyruvate in an unstable
enol form, which spontaneously tautomerizes to the more stable, keto
form of pyruvate. This reaction contributes a large proportion of the free
energy of hydrolysis of PEP.
There are two distinct genes encoding PK activity. One is located on
chromosome 1 and encodes the liver and erythrocyte PK proteins
(identified as the PKLR gene) and the other is located on chromosome 15
and encodes the muscle PK proteins (identified as the PKM gene). The
muscle PKM gene directs the synthesis of two isoforms of muscle PK
termed PK-M1 and PK-M2. Deficiencies in the PKLR gene are the cause
of the most common form of inherited non-spherocytic anemia.
back to the top
Anaerobic Glycolysis
Under aerobic conditions, pyruvate in most cells is further metabolized
via the TCA cycle. Under anaerobic conditions and in erythrocytes
under aerobic conditions, pyruvate is converted to lactate by the
enzyme lactate dehydrogenase (LDH), and the lactate is transported out
of the cell into the circulation. The conversion of pyruvate to lactate,
under anaerobic conditions, provides the cell with a mechanism for the
oxidation of NADH (produced during the G3PDH reaction) to NAD+
which occurs during the LDH catalyzed reaction. This reduction is
required since NAD+ is a necessary substrate for G3PDH, without which
glycolysis will cease. Normally, during aerobic glycolysis the electrons of
cytoplasmic NADH are transferred to mitochondrial carriers of the
oxidative phosphorylation pathway generating a continuous pool of
cytoplasmic NAD+.
Aerobic glycolysis generates substantially more ATP per mole of glucose
oxidized than does anaerobic glycolysis. The utility of anaerobic
glycolysis, to a muscle cell when it needs large amounts of energy, stems
from the fact that the rate of ATP production from glycolysis is
approximately 100X faster than from oxidative phosphorylation. During
exertion muscle cells do not need to energize anabolic reaction
pathways. The requirement is to generate the maximum amount of ATP,
for muscle contraction, in the shortest time frame. This is why muscle
cells derive almost all of the ATP consumed during exertion from
anaerobic glycolysis.
back to the top
Regulation of Glycolysis
The reactions catalyzed by hexokinase, PFK-1 and PK all proceed with a
relatively large free energy decrease. These non-equilibrium reactions of
glycolysis would be ideal candidates for regulation of the flux through
glycolysis. Indeed, in vitro studies have shown all three enzymes to be
allosterically controlled.
Regulation of hexokinase, however, is not the major control point in
glycolysis. This is due to the fact that large amounts of G6P are derived
from the breakdown of glycogen (the predominant mechanism of
carbohydrate entry into glycolysis in skeletal muscle) and, therefore, the
hexokinase reaction is not necessary. Regulation of PK is important for
reversing glycolysis when ATP is high in order to activate
gluconeogenesis. As such this enzyme catalyzed reaction is not a major
control point in glycolysis. The rate limiting step in glycolysis is the
reaction catalyzed by PFK-1.
PFK-1 is a tetrameric enzyme that exist in two conformational states
termed R and T that are in equilibrium. ATP is both a substrate and an
allosteric inhibitor of PFK-1. Each subunit has two ATP binding sites, a
substrate site and an inhibitor site. The substrate site binds ATP equally
well when the tetramer is in either conformation. The inhibitor site binds
ATP essentially only when the enzyme is in the T state. F6P is the other
substrate for PFK-1 and it also binds preferentially to the R state
enzyme. At high concentrations of ATP, the inhibitor site becomes
occupied and shifting the equilibrium of PFK-1 conformation to that of
the T state decreasing PFK-1's ability to bind F6P. The inhibition of PFK1 by ATP is overcome by AMP which binds to the R state of the enzyme
and, therefore, stabilizes the conformation of the enzyme capable of
binding F6P. The most important allosteric regulator of both glycolysis
and gluconeogenesis is fructose 2,6-bisphosphate, F2,6BP, which is not an
intermediate in glycolysis or in gluconeogenesis.
Regulation of glycolysis and gluconeogenesis by fructose 2,6-bisphosphate
(F2,6BP). The major sites for regulation of glycolysis and gluconeogenesis
are the phosphofructokinase-1 (PFK-1) and fructose-1,6-bisphosphatase (F1,6-BPase) catalyzed reactions. PFK-2 is the kinase activity and F-2,6BPase is the phosphatase activity of the bi-functional regulatory
enzyme, phosphofructokinase-2/fructose-2,6-bisphosphatase. PKA is
cAMP-dependent protein kinase which phosphorylates PFK-2/F-2,6-
BPase turning on the phosphatase activity. (+ve) and (-ve) refer to
positive and negative activities, respectively.
The synthesis of F2,6BP is catalyzed by the bifunctional enzyme
phosphofructokinase-2/fructose-2,6-bisphosphatase (PFK-2/F-2,6-BPase).
In the nonphosphorylated form the enzyme is known as PFK-2 and
serves to catalyze the synthesis of F2,6BP by phosphorylating fructose 6phosphate. The result is that the activity of PFK-1 is greatly stimulated
and the activity of F-1,6-BPase is greatly inhibited.
Under conditions where PFK-2 is active, fructose flow through the PFK1/F-1,6-BPase reactions takes place in the glycolytic direction, with a net
production of F1,6BP. When the bifunctional enzyme is phosphorylated it
no longer exhibits kinase activity, but a new active site hydrolyzes
F2,6BP to F6P and inorganic phosphate. The metabolic result of the
phosphorylation of the bifunctional enzyme is that allosteric stimulation
of PFK-1 ceases, allosteric inhibition of F-1,6-BPase is eliminated, and net
flow of fructose through these two enzymes is gluconeogenic, producing
F6P and eventually glucose.
The interconversion of the bifunctional enzyme is catalyzed by cAMPdependent protein kinase (PKA), which in turn is regulated by
circulating peptide hormones. When blood glucose levels drop, pancreatic
insulin production falls, glucagon secretion is stimulated, and circulating
glucagon is highly increased. Hormones such as glucagon bind to plasma
membrane receptors on liver cells, activating membrane-localized
adenylate cyclase leading to an increase in the conversion of ATP to
cAMP (see diagram below). cAMP binds to the regulatory subunits of
PKA, leading to release and activation of the catalytic subunits. PKA
phosphorylates numerous enzymes, including the bifunctional PFK-2/F2,6-BPase. Under these conditions the liver stops consuming glucose and
becomes metabolically gluconeogenic, producing glucose to reestablish
normoglycemia.
Representative pathway for the activation of cAMP-dependent protein
kinase (PKA). In this example glucagon binds to its' cell-surface receptor,
thereby activating the receptor. Activation of the receptor is coupled to
the activation of a receptor-coupled G-protein (GTP-binding and
hydrolyzing protein) composed of 3 subunits. Upon activation the alpha
subunit dissociates and binds to and activates adenylate cyclase.
Adenylate cylcase then converts ATP to cyclic-AMP (cAMP). The cAMP
thus produced then binds to the regulatory subunits of PKA leading to
dissociation of the associated catalytic subunits. The catalytic subunits
are inactive until dissociated from the regulatory subunits. Once
released the catalytic subunits of PKA phosphorylate numerous
substrate using ATP as the phosphate donor.
Regulation of glycolysis also occurs at the step catalyzed by pyruvate
kinase, (PK). The liver enzyme has been most studied in vitro. This
enzyme is inhibited by ATP and acetyl-CoA and is activated by F1,6BP.
The inhibition of PK by ATP is similar to the effect of ATP on PFK-1.
The binding of ATP to the inhibitor site reduces its affinity for PEP. The
liver enzyme is also controlled at the level of synthesis. Increased
carbohydrate ingestion induces the synthesis of PK resulting in elevated
cellular levels of the enzyme.
A number of PK isozymes have been described. The liver isozyme (Ltype), characteristic of a gluconeogenic tissue, is regulated via
phosphorylation by PKA, whereas the M-type isozyme found in brain,
muscle, and other glucose requiring tissue is unaffected by PKA. As a
consequence of these differences, blood glucose levels and associated
hormones can regulate the balance of liver gluconeogenesis and
glycolysis while muscle metabolism remains unaffected.
In erythrocytes, the fetal PK isozyme has much greater activity than
the adult isozyme; as a result, fetal erythrocytes have comparatively low
concentrations of glycolytic intermediates. Because of the low steadystate concentration of fetal 1,3BPG, the 2,3BPG shunt (see diagram above)
is greatly reduced in fetal cells and little 2,3BPG is formed. Since 2,3BPG
is a negative effector of hemoglobin affinity for oxygen, fetal
erythrocytes have a higher oxygen affinity than maternal erythrocytes.
Therefore, transfer of oxygen from maternal hemoglobin to fetal
hemoglobin is favored, assuring the fetal oxygen supply. In the newborn,
an erythrocyte isozyme of the M-type with comparatively low PK
activity displaces the fetal type, resulting in an accumulation of
glycolytic intermediates. The increased 1,3BPG levels activate the 2,3BPG
shunt, producing 2,3BPG needed to regulate oxygen binding to
hemoglobin.
Genetic diseases of adult erythrocyte PK are known in which the kinase
is virtually inactive. The erythrocytes of affected individuals have a
greatly reduced capacity to make ATP and thus do not have sufficient
ATP to perform activities such as ion pumping and maintaining osmotic
balance. These erythrocytes have a short half-life, lyse readily, and are
responsible for some cases of hereditary hemolytic anemia.
The liver PK isozyme is regulated by phosphorylation, allosteric
effectors, and modulation of gene expression. The major allosteric
effectors are F1,6BP, which stimulates PK activity by decreasing its Km
for PEP, and for the negative effector, ATP. Expression of the liver PK
gene is strongly influenced by the quantity of carbohydrate in the diet,
with high-carbohydrate diets inducing up to 10-fold increases in PK
concentration as compared to low carbohydrate diets. Liver PK is
phosphorylated and inhibited by PKA, and thus it is under hormonal
control similar to that described earlier for PFK-2.
Muscle PK (M-type) is not regulated by the same mechanisms as the
liver enzyme. Extracellular conditions that lead to the phosphorylation
and inhibition of liver PK, such as low blood glucose and high levels of
circulating glucagon, do not inhibit the muscle enzyme. The result of this
differential regulation is that hormones such as glucagon and
epinephrine favor liver gluconeogenesis by inhibiting liver glycolysis,
while at the same time, muscle glycolysis can proceed in accord with
needs directed by intracellular conditions.
back to the top
Metabolic Fates of Pyruvate
Pyruvate is the branch point molecule of glycolysis. The ultimate fate of
pyruvate depends on the oxidation state of the cell. In the reaction
catalyzed by G3PDH a molecule of NAD+ is reduced to NADH. In order
to maintain the re-dox state of the cell, this NADH must be re-oxidized
to NAD+. During aerobic glycolysis this occurs in the mitochondrial
electron transport chain generating ATP. Thus, during aerobic glycolysis
ATP is generated from oxidation of glucose directly at the PGK and PK
reactions as well as indirectly by re-oxidation of NADH in the oxidative
phosphorylation pathway. Additional NADH molecules are generated
during the complete aerobic oxidation of pyruvate in the TCA cycle.
Pyruvate enters the TCA cycle in the form of acetyl-CoA which is the
product of the pyruvate dehydrogenase reaction. The fate of pyruvate
during anaerobic glycolysis is reduction to lactate.
back to the top
Lactate Metabolism
During anaerobic glycolysis, that period of time when glycolysis is
proceeding at a high rate (or in anaerobic organisms), the oxidation of
NADH occurs through the reduction of an organic substrate.
Erythrocytes and skeletal muscle (under conditions of exertion) derive
all of their ATP needs through anaerobic glycolysis. The large quantity
of NADH produced is oxidized by reducing pyruvate to lactate. This
reaction is carried out by lactate dehydrogenase, (LDH). The lactate
produced during anaerobic glycolysis diffuses from the tissues and is
transported to highly aerobic tissues such as cardiac muscle and liver.
The lactate is then oxidized to pyruvate in these cells by LDH and the
pyruvate is further oxidized in the TCA cycle. If the energy level in
these cells is high the carbons of pyruvate will be diverted back to
glucose via the gluconeogenesis pathway.
Mammalian cells contain two distinct types of LDH subunits, termed M
and H. Combinations of these different subunits generates LDH isozymes
with different characteristics. The H type subunit predominates in
aerobic tissues such as heart muscle (as the H4 tetramer) while the M
subunit predominates in anaerobic tissues such as skeletal muscle as the
M4 tetramer). H4 LDH has a low Km for pyruvate and also is inhibited
by high levels of pyruvate. The M4 LDH enzyme has a high Km for
pyruvate and is not inhibited by pyruvate. This suggests that the H-type
LDH is utilized for oxidizing lactate to pyruvate and the M-type the
reverse.
back to the top
Ethanol Metabolism
Animal cells (primarily hepatocytes) contain the cytosolic enzyme
alcohol dehydrogenase (ADH) which oxidizes ethanol to acetaldehyde.
Acetaldehyde then enters the mitochondria where it is oxidized to
acetate by acetaldehyde dehydrogenase (AcDH).
Acetaldehyde forms adducts with proteins, nucleic acids and other
compounds, the results of which are the toxic side effects (the hangover)
that are associated with alcohol consumption. The ADH and AcDH
catalyzed reactions also leads to the reduction of NAD+ to NADH. The
metabolic effects of ethanol intoxication stem from the actions of ADH
and AcDH and the resultant cellular imbalance in the NADH/NAD+.
The NADH produced in the cytosol by ADH must be reduced back to
NAD+ via either the malate-aspartate shuttle or the glycerol-phosphate
shuttle (see above for pathways). Thus, the ability of an individual to
metabolize ethanol is dependent upon the capacity of hepatocytes to
carry out either of these 2 shuttles, which in turn is affected by the rate
of the TCA cycle in the mitochondria whose rate of function is being
impacted by the NADH produced by the AcDH reaction. The reduction
in NAD+ impairs the flux of glucose through glycolysis at the
glyceraldehyde-3-phosphate dehydrogenase reaction, thereby limiting
energy production. Additionally, there is an increased rate of hepatic
lactate production due to the effect of increased NADH on direction of
the hepatic lactate dehydrogenase (LDH) reaction. This reversal of the
LDH reaction in hepatocytes diverts pyruvate from gluconeogenesis
leading to a reduction in the capacity of the liver to deliver glucose to
the blood.
In addition to the negative effects of the altered NADH/NAD+ ratio on
hepatic gluconeogenesis, fatty acid oxidation is also reduced as this
process requires NAD+ as a cofactor. In fact the opposite is true, fatty
acid synthesis is increased and there is an increase in triacylglyceride
production by the liver. In the mitochondria, the production of acetate
from acetaldehyde leads to increased levels of acetyl-CoA. Since the
increased generation of NADH also reduces the activity of the TCA
cycle, the acetyl-CoA is diverted to fatty acid synthesis. The reduction in
cytosolic NAD+ leads to reduced activity of glycerol-3-phosphate
dehydrogenase (in the glycerol 3-phosphate to DHAP direction) resulting
in increased levels of glycerol 3-phosphate which is the backbone for the
synthesis of the triacylglycerides. Both of these two events lead to fatty
acid deposition in the liver leading to fatty liver syndrome.
back to the top
Regulation of Blood Glucose Levels
If for no other reason, it is because of the demands of the brain for
oxidizable glucose that the human body exquisitely regulates the level of
glucose circulating in the blood. This level is maintained in the range of
5mM.
Nearly all carbohydrates ingested in the diet are converted to glucose
following transport to the liver. Catabolism of dietary or cellular
proteins generates carbon atoms that can be utilized for glucose
synthesis via gluconeogenesis. Additionally, other tissues besides the liver
that incompletely oxidize glucose (predominantly skeletal muscle and
erythrocytes) provide lactate that can be converted to glucose via
gluconeogenesis.
Maintenance of blood glucose homeostasis is of paramount importance to
the survival of the human organism. The predominant tissue responding
to signals that indicate reduced or elevated blood glucose levels is the
liver. Indeed, one of the most important functions of the liver is to
produce glucose for the circulation. Both elevated and reduced levels of
blood glucose trigger hormonal responses to initiate pathways designed
to restore glucose homeostasis. Low blood glucose triggers release of
glucagon from pancreatic α-cells. High blood glucose triggers release of
insulin from pancreatic β-cells. Additional signals, ACTH and growth
hormone, released from the pituitary act to increase blood glucose by
inhibiting uptake by extrahepatic tissues. Glucocorticoids also act to
increase blood glucose levels by inhibiting glucose uptake. Cortisol, the
major glucocorticoid released from the adrenal cortex, is secreted in
response to the increase in circulating ACTH. The adrenal medullary
hormone, epinephrine, stimulates production of glucose by activating
glycogenolysis in response to stressful stimuli.
Glucagon binding to its' receptors on the surface of liver cells triggers an
increase in cAMP production leading to an increased rate of
glycogenolysis by activating glycogen phosphorylase via the PKAmediated cascade. This is the same response hepatocytes have to
epinephrine release. The resultant increased levels of G6P in hepatocytes
is hydrolyzed to free glucose, by glucose-6-phosphatase, which then
diffuses to the blood. The glucose enters extrahepatic cells where it is rephosphorylated by hexokinase. Since muscle and brain cells lack glucose6-phosphatase, the glucose-6-phosphate product of hexokinase is retained
and oxidized by these tissues.
In opposition to the cellular responses to glucagon (and epinephrine on
hepatocytes), insulin stimulates extrahepatic uptake of glucose from the
blood and inhibits glycogenolysis in extrahepatic cells and conversely
stimulates glycogen synthesis. As the glucose enters hepatocytes it binds
to and inhibits glycogen phosphorylase activity. The binding of free
glucose stimulates the de-phosphorylation of phosphorylase thereby,
inactivating it. Why is it that the glucose that enters hepatocytes is not
immediately phosphorylated and oxidized? Liver cells contain an
isoform of hexokinase called glucokinase. Glucokinase has a much lower
affinity for glucose than does hexokinase. Therefore, it is not fully active
at the physiological ranges of blood glucose. Additionally, glucokinase is
not inhibited by its product G6P, whereas, hexokinase is inhibited by
G6P.
Hepatocytes, unlike most other cells, are freely permeable to glucose and
are, therefore, essentially unaffected by the action of insulin at the level
of increased glucose uptake. When blood glucose levels are low, the liver
does not compete with other tissues for glucose since the extrahepatic
uptake of glucose is stimulated in response to insulin. Conversely, when
blood glucose levels are high extrahepatic needs are satisfied and the
liver takes up glucose for conversion into glycogen for future needs.
Under conditions of high blood glucose, liver glucose levels will be high
and the activity of glucokinase will be elevated. The G6P produced by
glucokinase is rapidly converted to G1P by phosphoglucomutase, where it
can then be incorporated into glycogen.
back to the top
Role of the Kidney in Blood Glucose Control
Although the liver is the major site of glucose homeostasis, the kidney
plays a vital role in the overall process of regulating the level of blood
glucose. The kidney carries out gluconeogenesis primarily using the
carbon skeleton of glutamine and while so doing allows for the
elimination of waste nitrogen and maintaining plasma pH balance. For
the role of the kidneys in gluconeogenesis please visit that section of the
Gluconeogenesis page. In addition to carrying out gluconeogenesis, the
kidney regulates blood glucose levels via its ability to excrete glucose via
glomerular filtration as well as to reabsorb the filtered glucose in the
proximal convoluted tubules. The average adult will kidneys will filter
around 180gm of glucose per day. Of this amount less than 1% is excreted
in the urine due to efficient reabsorption. This reabsorption process is
critical for maintaining blood glucose homeostasis and for retaining
important calories for energy production.
Transport of glucose from the tubule into the tubular epithelial cells is
carried out by specialized transport proteins termed sodium-glucose cotransporters (SGLTs). The SGLTs represent a family of transporters that
are involved in the transport of glucose, amino acids, vitamins, and ions
and other osmolytes across the brush-border membranes of kidney tubule
cells and intestinal epithelial cells. There are two SGLTs in the kidney
involved in glucose reabsorption. SGLT1 is found primarily in the distal
S2/S3 segment of the proximal tubule and SGLT2 is expressed exclusively
in the S1 segment (see the Figure below). The location of SGLT2 in the
proximal tubule means that it is primarily responsible for glucose
reabsorption. SGLT2 is a high-capacity low-affinity transporter that due
to its expression location is responsible for approximately 90% of the
glucose reabsorption activity of the kidney.
Diagrammatic representation of the re-uptake of glucose in the S1
segment of the proximal tubule of the kidney by the Na+-glucose cotransporter SGLT2. Following re-uptake the glucose is transported back
into the blood via the action of GLUT2 transporters. The Na+ that is
reabsorbed with the glucose is transported into the blood via a (Na+-K+)ATPase.
As would be expected from the name of the renal glucose transporters,
SGLT1 and SGLT2 catalyze the active transport of glucose against a
concentration gradient across the lumenal (apical) membrane of the
tubule cell and couple this transport to sodium uptake. The inward
sodium uptake is maintained by ATP-driven active transport of the
sodium across the basolateral (anti-lumenal) membrane into the blood
(coupled to inward uptake of potassium). The reabsorbed glucose
passively diffuses out of the tubule cell into the blood via the basolateral
membrane associated GLUT2. Under normal conditions saturation of the
ability of SGLT2 (and SGLT1) to reabsorb glucose is never saturated. The
kidney can filter and reabsorb approximately 375mg of glucose per
minute. The plasma concentration of glucose required to exceed this
capacity is well above that considered normal and is only observed
situations of renal dysfunction/disease or most importantly in type 2
diabetes. Because of the importance of SGLT2 in renal reabsorption of
glucose this transporter has become the target for therapeutic
intervention of the hyperglycemia associated with type 2 diabetes. By
specifically inhibiting SGLT2 there will be increased glucose excretion in
the urine and thus a lowering of plasma glucose levels. Several SGLT2specific inhibitors are currently in clinical trials with a few reaching
phase III status. For information on the SGLT2 inhibitors visit the
Diabetes page.