Download Bis2A 07.1 Glycolysis

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Bis2A 07.1 Glycolysis
Mitch Singer
Based on Glycolysis† by
OpenStax College
This work is produced by OpenStax-CNX and licensed under the
Creative Commons Attribution License 4.0‡
Abstract
By the end of this section, you will be able to:
• Describe the overall result in terms of molecules produced in the breakdown of glucose by glycolysis
• Compare the output of glycolysis in terms of ATP molecules and NADH molecules produced
1 Intoduction to Glycolysis
You are about to begin a series of modules that focus on the oxidation of carbon compounds. This process
serves two distinct purposes for any cell. The rst is the generation of
metabolic substrates, small carbon
monomers
based molecules that all cells need in order to "build" or synthesize larger complexes such as
which lead to the formation of macromolecules or polymers, such as proteins, or polysaccharides. All cells
need twelve (12) basic building blocks or metabolic substrates. In the next few modules we will learn where
these metabolic substrates come from and how cells synthesize them. The second purpose is the generation
This can be in the form of ATP (or ATP equivalents) or the formation of reducing
power. This is primarily in the form of NADH, NADPH or FADH2 .
A note from the instructor as to what is expected of you to know from the reading and lecture
of cellular energy.
There is a lot of material.
I do not expect you to memorize specic names of compounds or enzymes.
However, I will give you those names for completeness.
For exams I will always provide you with the
pathways we discuss in class and in the BioStax Biology text modules.
What you need to be able to do
is understand what is going on in each reaction. We will go over in lecture, problems that will be similar
to those I will ask of you on exams.
Do not be overwhelmed with specic enzyme names and specic
structures. What you should know are the general types of enzymes used and the types of structures found.
For example you do not need to know that that the enzyme that converts glyceraldhyde-3-phosphate to
1,3-bisphosphoglycerate is called glyceraldehyde-3-phosphate dehydrogenase. You should know what type of
dehydrogenase catalyzes and while you do not need to memorize the structures of glyceraldhydealdehyde (is says so in the name)
other is an organic acid (the term "ate" denotes an acid). That is the level of understanding I
reaction a
3-phosphate and 1,3-bisphosphoglycerate; you should know that one is an
and the
expect. If you have any questions please ask.
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‡ http://creativecommons.org/licenses/by/4.0/
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2 Glycolysis: an overview
So what is glycolysis: it is the process of oxidizing 1 molecule of glucose to 2 pyruvate molecules and the
generation of 2 NADH molecules and 2 ATP molecules. Cells can generate cellular energy from the process,
2 ATP molecules are obtained for every molecule of glucose entering the pathway as well as 2 molecules of
NADH are generated.
In many organism, the oxidation of glucose ends with the generation of pyruvate.
For these organisms, for every 1 molecule of glucose oxidized, cells generate only 2 ATP molecules. In other
words, these organisms only utilize or extract a small amount of the total potential energy within the glucose
molecule. However, for many other organisms, including us humans, the end product pyruvate can be further
oxidized by a series of additional reactions, which will be discussed later. In general, these organisms rst
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oxidize pyruvate to acetate or acetyl∼CoA, and then the acetyl∼CoA is completely oxidized to CO
Tricarboxylic acid cycle or TCA cycle.
by the
In addition to the 2 molecules of ATP and NADH the cells gain a series of small intermediates or
monomers which in turn are used to build polyGlucose-6-phosphate, Fructose-6-phosphate, Triose-phosphate, 3Phosphoglycerate, Phosphoenolpyruvate, and Pyruvate . These substrates are the building blocks
precursors that are necessary for the construction of
mers. These precursors include:
to form monomers that lead to a variety of biopolymers including including proteins (monomer: amino acids)
and polysaccharides.
The net result of glycolysis: 2 pyruvates, 2 NADH and 2 ATP. the other important point is that this is
anaerobic process. There is no requirement for molecular oxygen in glycolysis. This process occurs in
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the cytosol or cytoplasm of cells. For a short (3 minute) overview YouTube video of glycolysis click here
an
.
3 Glycolysis: the oxidation of glucose to pyruvate
Glycolysis
is the metabolic process of breaking down or oxidizing hexoses, or six carbon sugars, to two
molecules of pyruvate, a three carbon keto acid.
The importance of glycolysis is really two fold, rst
is to generate small carbon compounds that the cell can use as building blocks construct other cellular
components.
Secondly, many cells, including mammals generate energy in form of ATP from glycolysis.
Whhile not all cells generate energy from glycolysis, nearly all living organisms carry out glycolysis as part
of their metabolism. The process does not use oxygen and is therefore
in the cytoplasm of bacteral, archeal and eukaryotic cells.
anaerobic.
Glycolysis takes place
Remember that most biological processes are
freely reversible, depending upon the needs of the cell. The reverse set of reactions to glycolysis, that is,
the process of taking two molecules of pyruvate and reducing them to form one molecule of glucose is called
gluconeogenesis.
The balance between these two process keeps the ux of carbon (hexoses) sensitive to
the needs of the cell.
While glycolysis begins with glucose being activated with the addition of a phosphate from ATP, many
dierent types of hexoses (six carbon sugars) and polysaccharides (polymers of sugars) can feed into glycolysis
at the point of glucose or glucose-6-phosphate. Many dierent hexoses, such as Galactose or Mannose, can
be converted to glucose by a
hexose isomerase, an enzyme that can rearrange the hydroxyl groups on the
hexose and form glucose. Disaccharides (such as lactose, maltose or sucrose), trisaccharides (such as maltose
triose) and polysaccharides (longer sugar polymers such as starch or glycogen) can be degraded by hydrolysis
reactions to the monomers which can then be converted to glucose and enter glycolysis. The importance
of glycolysis and gluconeogenesis, and along with the TCA cycle is central to all cells for the production of
compounds necessary to build the monomers for biopolymers. As a result, these pathways has been given
Central Metabolism".
the common name of "
Glycolysis begins with the six carbon ring-shaped structure of a single glucose molecule and ends with
two molecules of a three-carbon sugar called
pyruvate.
Glycolysis consists of two distinct phases. The rst
part of the glycolysis pathway traps the glucose molecule in the cell and uses energy to modify it so that
the six-carbon sugar molecule can be split evenly into the two three-carbon molecules. The second part of
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glycolysis extracts energy from the molecules and stores it in the form of ATP and NADH, the reduced form
+
Detailed step by step video of glycolysis
2
YouTube presents glycolysis
of NAD .
4 First Half of Glycolysis (Energy-Requiring Steps)
Step 1. The rst step in glycolysis (Figure 1) is catalyzed by hexokinase, an enzyme with broad specicity
that catalyzes the phosphorylation of six-carbon sugars. Hexokinase phosphorylates glucose using ATP as
the source of the phosphate, producing glucose-6-phosphate, a more reactive form of glucose. This reaction
prevents the phosphorylated glucose molecule from continuing to interact with the GLUT proteins, and it can
no longer leave the cell because the negatively charged phosphate will not allow it to cross the hydrophobic
interior of the plasma membrane.
Step 2. In the second step of glycolysis, an isomerase converts glucose-6-phosphate into one of its isomers,
fructose-6-phosphate. An
isomerase is an enzyme that catalyzes the conversion of a molecule into one of
its isomers. (This change from phosphoglucose to phosphofructose allows the eventual split of the sugar into
two three-carbon molecules.).
Step 3. The third step is the phosphorylation of fructose-6-phosphate, catalyzed by the enzyme phosphofructokinase. A second ATP molecule donates a high-energy phosphate to fructose-6-phosphate, producing
fructose-1,6-bisphosphate. In this pathway, phosphofructokinase is a rate-limiting enzyme. It is active when
the concentration of ADP is high; it is less active when ADP levels are low and the concentration of ATP is
high. Thus, if there is sucient ATP in the system, the pathway slows down. This is a type of end product
inhibition, since ATP is the end product of glucose catabolism.
Step 4.
The newly added high-energy phosphates further destabilize fructose-1,6-bisphosphate.
The
fourth step in glycolysis employs an enzyme, aldolase, to cleave 1,6-bisphosphate into two three-carbon
isomers: dihydroxyacetone-phosphate and glyceraldehyde-3-phosphate.
Step 5.
In the fth step, an isomerase transforms the dihydroxyacetone-phosphate into its isomer,
glyceraldehyde-3-phosphate.
Thus, the pathway will continue with two molecules of a single isomer.
At
this point in the pathway, there is a net investment of energy from two ATP molecules in the breakdown of
one glucose molecule.
Figure 1: The rst half of glycolysis uses two ATP molecules in the phosphorylation of glucose, which
is then split into two three-carbon molecules.
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Exercise 1: Reading chemical reactions in glycolysis
(Solution on p. 8.)
In the rst reaction in gure 1. What are the reactants and what are the products?
a. Reactants: glucose
b. Products: glucose-6-phosphate
c. Reactants:glucose and ATP
d. Products: ADP and glucose-6-phosphate
e. a and b
f. c and d
Exercise 2
(Solution on p. 8.)
The phosphorlyation of glucose to glucose 6-phosphate:
a. Occurs without a catalyst
b. Is so favorable that the source of phosphate is not important
c. Is so favorable that it can be used to synthesize ATP
d. Requires energy from ATP to occur.
5 Second Half of Glycolysis (Energy-Releasing Steps)
So far, glycolysis has cost the cell two ATP molecules and produced two small, three-carbon sugar molecules.
Both of these molecules will proceed through the second half of the pathway, and sucient energy will be
extracted to pay back the two ATP molecules used as an initial investment and produce a prot for the cell
of two additional ATP molecules and two even higher-energy NADH molecules.
Step 6. The sixth step in glycolysis (Figure 2) oxidizes the sugar (glyceraldehyde-3-phosphate), extracting
+
high-energy electrons, which are picked up by the electron carrier NAD , producing NADH. The sugar is
then phosphorylated by the addition of a second phosphate group, producing 1,3-bisphosphoglycerate. Note
that the second phosphate group does not require another ATP molecule.
Figure 2: The second half of glycolysis involves phosphorylation without ATP investment (step 6) and
produces two NADH and four ATP molecules per glucose.
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Exercise 3
(Solution on p. 8.)
Which of the following characteristics apply to reaction 6 in gure 2 above?
a. This reaction is a redox reaction
b. The reactants are NAD, P and G3P
c. The products are NADH, H, 13BPG
d. Reaction 6 is actually two dierent, unconnected reactions so it should have two dierent lists
of reactants and two dierent lists of products.
e. a, b, c
f. a and d
Exercise 4
(Solution on p. 8.)
The energy released by glucose oxidation is captured on______ and_____.
a. ATP, NADH
b. NADH, proton gradient
c. NAD+, ADP
d. ATP, FADH2
Here again is a potential limiting factor for this pathway. The continuation of the reaction depends upon the
+
+
If NAD is not available, the second half of glycolysis slows
availability of the oxidized form of the electron carrier, NAD . Thus, NADH must be continuously oxidized
+
back into NAD in order to keep this step going.
down or stops. If oxygen is available in the system, the NADH will be oxidized readily, though indirectly,
and the high-energy electrons from the hydrogen released in this process will be used to produce ATP. In
an environment without oxygen, an alternate pathway (fermentation) can provide the oxidation of NADH
+
to NAD .
Step 7.
In the seventh step, catalyzed by phosphoglycerate kinase (an enzyme named for the reverse
reaction), 1,3-bisphosphoglycerate donates a high-energy phosphate to ADP, forming one molecule of ATP.
(This is an example of substrate-level phosphorylation.) A carbonyl group on the 1,3-bisphosphoglycerate
is oxidized to a carboxyl group, and 3-phosphoglycerate is formed.
Step 8. In the eighth step, the remaining phosphate group in 3-phosphoglycerate moves from the third
carbon to the second carbon, producing 2-phosphoglycerate (an isomer of 3-phosphoglycerate). The enzyme
catalyzing this step is a mutase (isomerase).
Step 9.
Enolase catalyzes the ninth step.
This enzyme causes 2-phosphoglycerate to lose water from
its structure; this is a dehydration reaction, resulting in the formation of a double bond that increases the
potential energy in the remaining phosphate bond and produces phosphoenolpyruvate (PEP).
Step 10.
The last step in glycolysis is catalyzed by the enzyme pyruvate kinase (the enzyme in this
case is named for the reverse reaction of pyruvate's conversion into PEP) and results in the production of a
second ATP molecule by substrate-level phosphorylation and the compound pyruvic acid (or its salt form,
pyruvate). Many enzymes in enzymatic pathways are named for the reverse reactions, since the enzyme can
catalyze both forward and reverse reactions (these may have been described initially by the reverse reaction
that takes place in vitro, under non-physiological conditions).
:
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Gain a better understanding of the breakdown of glucose by glycol-
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ysis by visiting this site
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to see the process in action.
6 Outcomes of Glycolysis
+
Unfortunately, glycolysis by itself can leave the cell with a problem; how to regenerate NAD
molecules of NADH produced. If the
+
NAD
from the 2
is not regenerated all of the cell's NAD will be transformed
+
into NADH. This would then cause glycolysis to come to a halt. So how do cells regenerate NAD , they
oxidize the NADH completing the cycle by reducing another compound, usually a small metabolite, such
as pyruvate. This process is called
fermentation. The reduction of pyruvate or another small metabolite
+ and the production of a fermentation product such as lactate,
leads to the reoxidation of NADH to NAD
acetate, ethanol or some other product. We will discuss fermentation reactions shortly.
The last step in glycolysis is the production of pyruvate. Pyruvate, has a variety of cell fates. In cells that
lack an electron transport chain, pyruvate or a breakdown product can act as a terminal electron acceptor
in a fermentation reaction necessary to regenerate NAD
+
from the NADH produced during glycolysis.
Alternatively, Pyruvate can be oxidized to acetyl∼CoA which can then be used as the starting point for
the
Tricarboxcylic Acid Cycle (TCA Cycle)
or
Krebs Cycle.
This will generate additional ATP
(equivalents) NADH and additional precursors necessary for the building of monomers and bio-polymers.
Exercise 5
(Solution on p. 8.)
The ow of carbon through glycolysis can be described as:
a. Oxidation of a six carbon sugar.
b. Oxidation of a six carbon sugar, followed by cleavage into two three carbon molecules.
c. Cleavage of a six carbon sugar into two three carbon molecules, followed by their oxidation.
d. Conversion of glucose into carbon dioxide.
Exercise 6
(Solution on p. 8.)
Glycolysis
a. Does not require oxygen to generate energy.
b. Requires oxygen to generate energy
c. Is inhibited by oxygen.
d. Rate is increased in the presence of oxygen
7 Section Summary
Glycolysis is the rst pathway used in the breakdown of glucose to extract energy. It was probably one of the
earliest metabolic pathways to evolve and is used by nearly all of the organisms on earth. Glycolysis consists
of two parts: The rst part prepares the six-carbon ring of glucose for cleavage into two three-carbon sugars.
ATP is invested in the process during this half to energize the separation.
The second half of glycolysis
+
extracts ATP and high-energy electrons from hydrogen atoms and attaches them to NAD .
Two ATP
molecules are invested in the rst half and four ATP molecules are formed by substrate phosphorylation
during the second half. This produces a net gain of two ATP and two NADH molecules for the cell.
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8 Review Questions
Exercise 7
(Solution on p. 8.)
During the second half of glycolysis, what occurs?
a. ATP is used up.
b. Fructose is split in two.
c. ATP is made.
d. Glucose becomes fructose.
9 Free Response
Exercise 8
(Solution on p. 8.)
Nearly all organisms on earth carry out some form of glycolysis. How does that fact support or
not support the assertion that glycolysis is one of the oldest metabolic pathways?
Exercise 9
(Solution on p. 8.)
Red blood cells do not perform aerobic respiration, but they do perform glycolysis. Why do all
cells need an energy source, and what would happen if glycolysis were blocked in a red blood cell?
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Solutions to Exercises in this Module
Solution to Exercise (p. 3)
f
Solution to Exercise (p. 4)
d
Solution to Exercise (p. 5)
e
Solution to Exercise (p. 5)
a
Solution to Exercise (p. 6)
b
Solution to Exercise (p. 6)
Insert Solution Text Here
to Exercise (p. 7)
C
to Exercise (p. 7)
If glycolysis evolved relatively late, it likely would not be as universal in organisms as it is. It probably
evolved in very primitive organisms and persisted, with the addition of other pathways of carbohydrate
metabolism that evolved later.
to Exercise (p. 7)
All cells must consume energy to carry out basic functions, such as pumping ions across membranes. A red
blood cell would lose its membrane potential if glycolysis were blocked, and it would eventually die.
Glossary
Denition 1: aerobic respiration
process in which organisms convert energy in the presence of oxygen
Denition 2: anaerobic
process that does not use oxygen
Denition 3: glycolysis
process of breaking glucose into two three-carbon molecules with the production of ATP and NADH
Denition 4: isomerase
enzyme that converts a molecule into its isomer
Denition 5: pyruvate
three-carbon sugar that can be decarboxylated and oxidized to make acetyl CoA, which enters the
citric acid cycle under aerobic conditions; the end product of glycolysis
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