Download Reaction of glycolysis

Carbohydrate Catabolism
Figure 24.5
Glycolysis
Glycolysis
Start materials
End products
Arrow type
No. of reactions
Type of reactions
Enzyme name for each reaction
Condition of each reaction
Intermediates name and No. of there
carbon
Functional group of each metabolites
ATP production/ consumption
Glycolysis
Start materials
End products
Arrow type
No. of reactions
Type of reactions
Enzyme name for each reaction
Condition of each reaction
Intermediates name and No. of there carbon
Functional group of each metabolites
ATP production/ consumption
Glycolysis
• What is the overall pathway in glycolysis?
• How is the 6-carbon glucose converted to the 3carbon glyceraldehyde-3-phosphate?
• How is glyceraldehyde-3-phosphate converted to
pyruvate?
• How is pyruvate metabolized anaerobically?
• Where are the control points in the glycolytic
pathway?
• How much energy can be produced by glycolysis?
Glycolysis
• Definition
– Glycolysis is the sequence of reactions that converts glucose
into pyruvate in the presence of oxygen (aerobic) or lactate in
the absence of oxygen (anaerobic) with the production of ATP
• Location
– Glycolysis is the major pathway for the utilization of glucose and
is found in cytosol of all cells
• Reaction of glycolysis
– The breakdown of glucose to pyruvate is brought about by
sequential action of 10 enzymes which can be divided into two
phases
• First phase, energy requiring phase or preparative phase
• Second phase, energy generating phase pay off phase
Glycolysis
• A three-phase pathway in which:
– Glucose is oxidized into pyruvic acid (PA)
• It loses 2 pairs of hydrogens
– NAD+ is reduced to NADH + H+
• It accepts 2 pairs of hydrogens lost by glucose
– ATP is synthesized by substrate-level phosphorylation
• Pyruvic acid: end-product of glycolysis
– Moves on to the Krebs cycle in an aerobic pathway (i.e.
sufficient oxygen available to cell)
– Is reduced to lactic acid in an anaerobic environment
(insufficient O2 available to cell)
– pyruvic acid
lactic aicd
Glycolysis: Phase 1 and 2
• Phase 1: Sugar activation
– Two ATP molecules activate glucose into
fructose-1,6-diphosphate
• The 1 and 6 indicate which carbon atom to which
they are attached.
• Phase 2: Sugar cleavage (splitting)
– Fructose-1,6-bisphosphate (6 C’s) is split into
two 3-carbon compounds:
• Glyceraldehyde 3-phosphate (GAP)
Glycolysis: Phase 3
• Phase 3: Oxidation and ATP formation
– The 3-carbon sugars are oxidized (reducing
NAD+); i.e., 2 H’s + NAD
NADH2
– Inorganic phosphate groups (Pi) are attached to
each oxidized fragment
– The terminal phosphates are cleaved and
captured by ADP to form four ATP molecules
– The final products are:
• Two pyruvic acid molecules
• Two NADH + H+ molecules (reduced NAD+)
• A net gain of two ATP molecules
Stage 1
Glucose
F-1,6-BP
Stage 2
DHAP
GAP
Stage 3
2X
PEP
Pyruvate
Glycolysis
The overall pathway of glycolysis
• Glycolysis is the first stage
of glucose metabolism
• One molecule of glucose
is converted to fructose1,6-bisphosphate, which
gives rise to two
molecules of pyruvate
• Glycolysis plays a key role
in the way organisms
extract energy from
nutrients
• Once pyruvate is formed,
it has one of several fates
The reactions of glycolysis
What are the reaction of
glycolysis?
In glycolysis, glucose is
converted to pyruvate in a
multistep pathway.
Glucose is converted to
pyruvate in a series of 10
reactions, only one of
which is an oxidation
The reactions of glycolysis
(cont’d)
1. Phosphorylation of glucose to
give glucose-6-phosphate
2. Isomerization of glucose-6phosphate to give fructose-6phosphate
3. Phosphorylation of fructose-6phosphate to yield fructose-1,6bisphosphate
4. Cleavage of fructose-1,6,bisphosphate to give
glyceraldehyde-3-phosphate and
dihyroxyacetone phosphate
5. Isomerization of dihyroxyacetone
phosphate to give glyceraldehyde3-phosphate
The reactions of glycolysis
(cont’d)
6. Oxidation of glyceraldehyde-3phosphate to give 1,3bisphosphoglycerate
7. Transfer of a phosphate group
from 1,3-bisphosphoglycerate
to ADP to give 3phosphoglycerate
8. Isomerization of 3phosphoglycerate to give 2phosphoglycerate
9. Dehydration of 2phosphoglycerate to give
phosphoenolpyruvate
10. Transfer of a phosphate group
from phosphoenolpyruvate to
ADP to give pyruvate
Conversion of glucose to glyceraldehyde-3-phosphate
(preparation phase) of glycolysis
In step 1 of glycolysis, glucose is phosphorylated to give
glucose-6-phosphate
G6P is an intermediate in several metabolic pathway
The reaction is endergonic, as it is driven by the free
energy of hydrolysis of ATP
The kinase is ATP dependent enzymes that transfer a
phosphate group from ATP to a substrate
hexokinas
Glucokinase
This reaction is irreversible (control point)
Conversion of glucose to glyceraldehyde3-phosphate (cont,d)
•The second step is the isomerization of glucose-6-phosphate to
fructose-6-phosphate
•The C-1 aldehyde of glucose-6-phosphate is reduced to a
hydroxyl group
•The C-2 hydroxyl group is oxidized to give the ketone group of
fructose-6-phosphate
•There is no net redox reaction
•Isomerase is an enzyme that catalyzes the structural
rearrangement of isomer
Conversion of glucose to glyceraldehyde3-phosphate (cont,d)
•Step 3
•Fructose-6-phosphate is then phosphorylated by ATP
again to generate fructose-1,6-bisphosphate
•This is the second reaction to be coupled to ATP
hydrolysis
•This is the key point reaction (F1,6BP key intermediate)
•This reaction is irreversible (control point)
Conversion of glucose to glyceraldehyde3-phosphate (cont,d)
In step 4 Fructose-1,6-bisphosphate is split into two 3carbon fragments
Reaction catalyzed by aldolase
Side chains of an essential Lys and Cys play key roles in
catalysis the reaction
aldolase
Conversion of glucose to glyceraldehyde3-phosphate (cont,d)
In step 5, dihydroxyacetone phosphate (DHAP) is
converted to glyceraldehyde-3-phosphate
These compounds are trioses
Summary
• In the first stages of glycolysis, glucose is
converted to two molecules of glyceraldehyde-3phosphate
• The key intermediate in this series of reactions is
fructose-1,6-bisphosphate. The reaction that
produces this intermediate is a key control point
of the pathway, and the enzyme that catalyzes it,
phosphofructokinase, is subject to allosteric
control
Glyceraldehyde-3-Phohsphate is Converted to Pyruvate (pay
off phase)
•Step 6
•The first reaction that begins the conversion to pyruvate involves
the oxidation of glyceraldehyde-3-phosphate to 1,3bisphosphoglycerate
•This reaction involves addition of a phosphate group, as well as an
electron transfer reaction
•The oxidizing agent, NAD+, is reduced to NADH
•Dehydrogenase is an enzyme that oxidizes a substrate by
transferring one or more hydrides (H-) to an acceptor, usually
Oxidation and Phosphorylation
Reaction
Oxidation and Phosphorylation
Reaction
Glyceraldehyde-3-Phohsphate is Converted to
Pyruvate (Cont’d)
•1,3-bisphosphoglycerate is converted to 3 phosphoglycerate
•This step (step 7) involves another reaction in which ATP is
produced by phosphorylation of ADP
•1,3-bisphosphoglycerate transfers a phosphate group to ADP.
This is known as substrate-level phosphorylation
•Reaction is catalyzed by phosphoglycerate kinase
•This reaction is the sum of the endergonic phosphorylation
of ADP and the exergonic hydrolysis of the mixed phosphate
anhydride
Glyceraldehyde-3-Phohsphate is Converted
to Pyruvate (Cont’d)
•The next step (step 8) involves the isomerization of 3phosphoglycerate to 2-phosphoglycerate
•The phosphate group is transferred from carbon 3 to
carbon 2 of the glyceric acid backbone
•This reaction is catalyzed by phosphoglyceromutase
•Mutase is an enzyme that catalyzes the intramolecular
shift of a chemical group from one position to another
within the same molecule such as phosphoryl group
Glyceraldehyde-3-Phohsphate is Converted to
Pyruvate (Cont’d)
•Next, (step 9) 2-phosphoglycerate loses one molecule
of water, producing phosphenolpyruvate
•Enolase catalyzes the dehydration reaction (-H2O) and
requires a Mg2+ as a cofactor
•Phosphoenolpyruvate contains a high energy bond
•enolase
Glyceraldehyde-3-Phohsphate is Converted to
Pyruvate (Cont’d)
•Phosphenolpyruvate (PEP) transfers its phosphate
group to ADP, producing ATP and pyruvate
•Reaction is catalyzed by pyruvate kinase
•The double bond shift to the oxygen on carbon 2 and
a hydrogen shifts to carbon 3
• This reaction is irreversible (control point)
Summary
• In the final stages of glycolysis, two molecules of
pyruvate are produced for each molecule of
glucose that entered the pathway
• These reactions involve electron transfer (redox
step 6), and the net production of two ATP for
each glucose
• There are three control points in the glycolytic
pathway
Entering substrates
Glucose and other monosaccharides
Enzyme location
cytosol
Net ATP production
2 ATP / glucose enter the pathway
Coenzyme production
2NADH + 2H+ formed under aerobic
conditions
Final products
under aerobic conditions
Pyruvate
under anaerobic conditions
Lactate
Net reaction
Aerobic:
Anaerobic:
Fates of pyruvate from glycolysis
Anaerobic Metabolism of Pyruvate
• Under anaerobic conditions, the most important
pathway for the regeneration of NAD+ is reduction of
pyruvate to lactate
• Lactate dehydrogenase (LDH) is a tetrameric
isoenzyme consisting of H and M subunits; H4
predominates in heart muscle, and M4 in skeletal
muscle
The conversion of pyruvate to
lactate in muscles
• NAD+ Needs to be
Recycled to Prevent
Decrease in Oxidation
Reactions
• So NAD + will be
present for further
glycolysis to take place
Alcoholic Fermentation
• Two reactions lead to the
production of ethanol:
– Decarboxylation of pyruvate
to acetaldehyde
– Reduction of acetaldehyde
to ethanol
• Pyruvate decarboxylase is
the enzyme that catalyzes
the first reaction
– This enzyme require Mg2+
and the cofactor, thiamine
pyrophosphate (TPP)
• Alcohol dehydrogenase
catalyzes the conversion of
acetaldehyde to ethanol
Alcoholic fermentation
• NAD+ Needs to be
Recycled to Prevent
Decrease in Oxidation
Reactions
• So NAD + will be
present for further
glycolysis to take place
Summary
• Pyruvate is converted to lactate in anaerobic
tissues, such as actively metabolizing muscle.
NAD+ is recycled in the process
• In some organisms, pyruvate is converted to
ethanol in a process requiring thiamine
pyrophosphate as a coenzyme
Control Points in Glycolysis
Three reactions exhibit
particularly large decreases
in free energy; the enzymes
that catalyze these reactions
are sites of allosteric control
– Hexokinase (step 1)
– Phosphofructokinase (step 3)
– Pyruvate kinase (step 10)
Energy yield from glycolysis
• The ATP yield from glycolysis is different in
anaerobic and aerobic conditions
– During aerobic (oxygen plenty) condition, the two
NADH generated in step 6 can enter the
mitochondrial electron transport chain to
complete oxidation
– As each NADH provides 3ATPs, this reaction
provides 3 x 2 = 6 ATPs
– The net gain of energy from glycolysis pathways
is 8 ATPs
Energy yield (number of ATP generated) per molecule
of glucose in the glycolytic pathway, under aerobic
conditions (oxygen is available)
Step Reaction
Enzyme
1
Hexokinase
Source No. of ATPs
gained /glucose
molecule
-1
3
Posphofructokinase
-
-1
6
Glyceraldehyde-3phosphate
dehydrogenase
1,3bisPhosphoglycerate
kinase
Pyruvate kinase
NADH
3X2=6
ATP
1x2=2
ATP
1x2=2
7
10
Total = 10 minus 2 = 8
Energy yield from glycolysis (cont’d)
– But when oxygen is in deficiency (anaerobic
condition)one molecule of glucose is converted to
two molecule of lactate, the net yield of 2
molecules of ATP
• 4 molecules of ATP are synthesised by the 2 substrate
level phosphorylation (steps 7 and 10)
• But 2 ATP molecules are used in the steps 1 and 3, the
net yield is only 2 ATP
Energy yield (number of ATP generated) per molecule
of glucose in the glycolytic pathway, under anaerobic
conditions (oxygen deficiency)
Step
Enzyme
Source
1
Hexokinase
-
No. of ATPs
gained /glucose
molecule
-1
3
Posphofructokinase
-
-1
7
1,3ATP
bisPhosphoglycerate
kinase
1x2=2
10
Pyruvate kinase
1x2=2
ATP
Total = 4 minus 2 = 2
Glucose
1
Hexokinase
ATP
G-6-P
2
F-6-P
3
Glucosephosphate
isomerase
Phosphofructokinase
ATP
F-1,6-BP
4
DHAP
Triosphosphate
isomerase
5
6
1,3BPG
7
ATP
NADH
Aldolase
GA3P
Glyceraldehyde-3-phosphate
dehydrogenase
3-Phosphoglycerate kinase
3-PG
Phosphoglyceromutase
8
2-PG
Enolase
9
PEP
10
ATP
Pyruvate
Pyruvate kinase
What did you learn?
• Answer the following questions regarding
glycolysis and energy.
– What is the overall energy yield in ATP production when
one molecule of glucose is converted to pyruvate?
– Based on free energy yield, predict the reactions that are
irreversible or poorly reversible in the glycolysis pathway.
• Why is the phosphoglycerate kinase reaction reversible?
What about the other kinase reactions?
• Explain why G-6-P is ismerized to F-6-P, which then
phosphorylated again to fructose F1,6-bP utilizing
another ATP
Practice 4
Q 4 During glycolysis, a 6-carbon sugar diphosphate
molecule is split into two 3-carbon sugar
phosphate molecules.
A) True
B) False
Q 5 Under aerobic conditions, the end-product of
glycolysis is further reduced to yield more ATP.
A) True
B) False
Answer to practice 4
Q 4 During glycolysis, a 6-carbon sugar diphosphate
molecule is split into two 3-carbon sugar
phosphate molecules.
A) True
B) False
Q 5 Under aerobic conditions, the end-product of
glycolysis is further reduced to yield more ATP.
A) True
B) False
Practice 1
Q 1 In glycolysis, glucose is converted to
A) CO2 and H2O.
B) pyruvate.
C) citrate.
D) acetyl coA.
E) NAD+ and ADP.
Practice 2
Q 2 The NET result of a single glycolysis run is
the formation of
A) 1 NADH and 1 ATP.
B) 2 NADH and 2 ATP.
C) 2 NADH and 4 ATP.
D) 4 NADH and 2 ATP.
E) 4 NADH and 4 ATP.
Answer to practice 2
Q 2 The NET result of a single glycolysis run is
the formation of
A) 1 NADH and 1 ATP.
B) 2 NADH and 2 ATP.
C) 2 NADH and 4 ATP.
D) 4 NADH and 2 ATP.
E) 4 NADH and 4 ATP.
Practice 3
Q 3 Under anaerobic conditions, the endproduct of glycolysis is converted to
A) CO2 and H2O.
B) amino acids.
C) lactic acid.
D) hydrochloric acid.
E) acetic acid.
Answer to practice 3
Q 3 Under anaerobic conditions, the endproduct of glycolysis is converted to
A) CO2 and H2O.
B) amino acids.
C) lactic acid.
D) hydrochloric acid.
E) acetic acid.