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CHAPTER 6.3. OXIDATIVE
DEGRADATION OF GLUCOSE TO CO2
OXIDATIVE DEGRADATION OF
GLUCOSE TO CO2
• Oxidative degradation of glucose to CO2 and energy
production takes place via two important pathways.
• 1. Glycolysis or Embden- Meyerhoff pathway is the
major pathway for the utilization of glucose for the
production of energy and is found in the cytosol of all
cells.
• Glycolysis can function under aerobic and anaerobic
conditions.
• Two molecules of pyruvate are produced. Pyruvate is
then converted to Acetyl CoA.
• 2. In the second pathway, Citric acid cycle, acetyl CoA
is further oxidized to CO2 and H2O in mitochondria
1. GLYCOLYSIS 0R EMBDENMEYERHOFF PATHWAY
Glucose is converted into two molecules of pyruvate, chemical energy in the
form of ATP is produced, and NADH- reduced coenzymes are produced.
This metabolic pathway takes place in almost all cells. All of the enzymes of
the glycolysis pathway are found in the extramitochondrial soluble fraction of
the cell, the cytosol.
They catalyze the reactions under aerobic and anaerobic conditions.
The over all reactions of glycolytic reactions are presented in below.
Glycolysis
Anaerobic condition
Glucose → Pyruvate → Lactic acid
Aerobic condition
Pyruvate → CO2 + H2O
Glycolysis is a ten step process in which every step is enzyme catalyzed.
Details of individual steps with in the glycolytic pathway are now considered.
Step1: Phosphorylation of Glucose
• Glucose enters into the glycolytic pathway by
phosphorylation to glucose 6- phosphate,
accomplished by the enzyme hexokinase.
•
However, in liver parenchyma cells and in
pancreatic islet cells, this function is carried out by
glucokinase.
•
ATP is required as phosphate donor, and it reacts as
the Mg-ATP complex.
Step 2: Converstion of Glucose 6
Phosphate to Fructose 6-Phosphate
• Glucose 6 phosphate is converted to fructose 6phosphate by phosphoglucose isomerase, which
involves an aldose-ketose isomerization.
Step 3: Phosphorlyation of Fructose 6Phosphate to Fructose 1, 6-Bisphosphate
• This reaction is catalyzed by the enzyme
phosphofructokinase to produce fructose 1, 6bisphosphate from Fructose 6 phosphate.
• The phosphofructokinase reaction is irreversible under
physiologic conditions.
Step 4: Cleavage of Fructose 1,6Bisphosphate
• Fructose 1, 6-bisphosphate is split by aldolase
(fructose1, 6-bisphosphate aldolase) into two triose
phosphates,
glyceraldehyde
-3-phosphate
and
dihydroxyacetone phosphate.
Step 5: Inter conversion of triose
phosphates
• Glyceraldehyde-3-phosphate and dihydroxy acetone
phosphate are interconverted by the enzyme
phosphotriose isomerase.
• At this stage 2 molecules glyceraldehyde 3-phosphate
are formed.
•
Dihydroxyacetone phosphate is also formed from
glycerol of fat, which is phosphorylated to glycerol 3
phosphate and then to dihydroxy acetone phosphate
Step 6: Oxidation of Glyceraldehyde 3Phosphate to 1, 3 Bisphosphoglycerate
• Glyceraldehyde-3-phosphate is converted to 1,3
bisphosphglycerate by glyceraldehyde 3-phosphate
dehydrogenase using NAD+ as the coenzyme.
• Finally, by phosphorolysis, inorganic phosphate (pi) is
added, forming1, 3 bisphosphoglycerate, and the free
enzyme.
• Energy released during the oxidation is conserved by the
formation of a high-energy sulfur group that becomes,
after phosphorolysis, a high- energy phosphate group in
position 1 of 1, 3 bisphosphoglycerate.
Step 7: Transfer of phosphate group from
1, 3 bisphosphoglycerate
• 1, 3-bisphosphoglycerate is oxidized to
phosphoglycerate by phosphoglycerate kinase.
3-
• This high- energy phosphate is captured as ATP in a
further reaction with ADP.
• Since two molecules of triose phosphate are formed per
molecule of glucose undergoing glycolysis, two
molecules of ATP are generated at this stage per
molecule of glucose.
Step 8: Conversion of 3Phosphoglycerate to 2-Phosphoglycerate
• In the next reaction 3-phosphoglycerate is converted to
2-phosphoglycerate by the enzyme phosphoglycerate
mutase
Step 9: Dehydration of 2Phosphoglycerate
to Phosphoenolpyruvate
• The subsequent step is catalyzed by enolase that
promotes the reversible removal of a water molecule
from
2-Phosphoglycerate
to
form
phosphoenolpyruvate
Step 10: Conversion of Phosphoenol
pyruvate to pyruvate
• The high-energy phosphate of phosphoenol pyruvate
is transferred to ADP by the enzyme pyruvate kinase to
generate, at this stage, two molecules of ATP per
molecule of glucose oxidized and enolpyruvate is
formed.
• Enolpyruvate formed is converted spontaneousny to the
keto form pyruvate. This is an irreversible step
Energy production
• a. Under aerobic condition
• Under aerobic condition, pyruvate is taken up into
mitochondria, and after conversion to acety-CoA is
oxidized to CO2 by the citric acid cycle.
•
The reducing equivalents from the NADH+H+ formed in
glycolysis are taken up into mitochondria for oxidation.
• Two triosephosphates are produced from each molecule
of hexose metabolized.
Isomerase
Dihydroxyacetone phosphate←→ glyceraldehyde 3 phosphate
•
glyceraldehyde 3 phosphate dehydrogenase
↓ + NAD+Pi
1,3 diphosphoglyceric acid +NADH+H+
↓
Phosphoenol pyruvate +ATP
↓
Enol pyruvate + ATP
Since two molecules of triose phosphates are undergoing the
above reaction totally four ATP molecules of produced.
• The NADH formed by the dehydrogenation of glyceraldehyde 3
phosphate is then rexoidised to NAD+ by O2 via Electron transport
chain of mitochondria and produce 3ATP molecules.
•
• Since two molecules of glyceraldehyde 3 phosphate are involved in
the above reaction six ATP molecules are produced.
• However, two ATP molecules are used up in the
production of glucose-6-phosphate from glucose and
fructose-1, 6-disphosphate from fructose-6-phosphate.
Hence under aerobic glycolysis, the total number of ATP
molecules produced is 10.
• Out of this 2 ATP molecules are used during the initial
reactions.
• The net ATP production is 8.
b. Under anaerobic condition
•
Pyruvate is reduced by the NADH+H+ to lactate, the reaction being catalyzed by
lactate dehydrogenase.
•
The reoxidation of NADH via lactate formation allows glycolysis to proceed in the
absence of oxygen by regenerating sufficient NAD+ for another cycle of the reaction
catalyzed by glyceraldehyde-3 phosphate dehydrogenase.
•
•
•
Lactate dehydrogenase
Pyruvate + NADH + H+ ↔ NAD+ + Lactate
During fermentation, pyruvate is reduced by NADH to ethyl alcohol being catalysed
by alcohol dehydrogenase.
•
•
Alcohol dehydrogenase
Pyruvate + NADH + H+ ↔ NAD+ + Ethyl alcohol
•
The conversion of two triose phosphates to lactic acid (or ethanol) yields four
molecules of ATP. However, two ATP molecules are used up in the production of
glucose-6-phosphate from glucose and fructose-1, 6-disphosphate from fructose-6phosphate.
•
The net production of ATP is thus only two ATP molecules/ mole of glucose
Fates of Pyruvate
Three common fates for pyruvate are of prime
importance: conversion into acteyl CoA, lactate, and
ethanol.
Oxidation of Pyruvate to Acetyl CoA
Pruvate derived from glucose by glycolysis, is
dehydrogenated to yield acetyl-CoA and CO2 by pyruvate
dehydrogenase, located in the mitochondria.
CH3COCOOH+NAD++CoA-SH→CH3S~CoA+NADH+ CO2
Pyruvate
Acetyl CoA
Thiamine pyrophosphate, coenzyme A, lipoic acid and
NAD+ are coenzymes needed for the reaction.
The acetyl CoA is then fed into citric acid cycle and
oxidized to CO2 and water.
2) CITRIC ACID CYCLE
• The reactions of citric acid cycle the
following steps
1. Condensation of acety1- CoA with
oxaloacetate to form citrate
• The initial condensation of acety1- CoA with
oxaloacetate to form citrate is catalyzed by condensing
enzyme, citrate synthase, which effects synthesis of a
carbon to carbon bond between the methyl carbon of
acety1-CoA and the carbony1 carbon of oxaloacetate.
2. Conversion of citrate to isocirtrate via
cis-aconitate
• Citrate is converted to isocitrate by the enzyme
aconitase (aconitate hydratase), which contains iron in
the Fe2+ state in the form of an iron- sulfur protein
(Fe:S)
• This conversion takes place in two steps: dehydration to
cis-aconitate, some of which remains bound to the
enzyme, and rehydration to iocitrate.
3. Dehydrogenation of isocitrate to
oxalosuccinate
• Isocitrate undergoes dehydrogenation in the presence
of isocitrate dehydrogenase to form oxalosuccinate.
•
The linked oxidation of isocitrate proceeds almost
completely through the NAD+ dependent enzyme
isocitrate dehydrogenase.
4. Decarboxylation of oxalosuccinate
to α -ketoglutarate
There follows decarboxylation of oxalosuccinate to α ketoglutarate,
also
catalyzed
by
isocitrate
dehydrogenase.
A CO2 molecule is liberated.
Mn2+(or Mg2+) is an important component of the
decarboxylation .
5. Decarboxylation of α-ketoglutarate to
succiny1-CoA
Next,
α-ketoglutarate
undergoes
decarboxylation to form succinyl CoA
oxidative
The reaction is catalyzed by an α-ketoglutarate
dehydrogenase complex, which requires cofactors
thiamin pyrophosphote, lipoate, NAD+, FAD and CoA
results in the formation of succiny1-CoA, a high- energy
thioester and NADH.
6. Conversion of succinyl-CoA to
succinate
• Succinyl-CoA is converted to succinate by the enzyme
succinate thiokinase (succiny1CoA synthetase).
• High-energy phosphate (ADP+Pi →ATP) is synthesized
at the substrate level because the release of free energy
from the oxidative decarboxylation of α -ketoglutarate is
sufficient to generate a high- energy phosphate..
7. Dehydrogenation of succinate to
fumarate
Succinate is metabolized further by undergoing a
dehydrogenation
catalyzed
by
succinate
dehydrogenase to form fumerate.
It is the only dehydrogenation in the citric acid cycle that
involves the direct transfer of hydrogen from the
substrate to a flavorprotein without the participation of
NAD+.
8. Addition of water to furmarate to give
malate
• Furmarase (furmarate hydratase) catalyzes the addition
of water to furmarate to give malate.
• In addition to being specific for the L-isomer of malate,
furmarase catalyzes the addition of the elements of
water to the double bond of furmarate in the tans
configuration.
9. Dehydrogenation of malate to form
oxaloacetate
• Malate is converted to oxaloacetate by dehydrogenation
catalysed by the enzyme malate dehydrogenase, a reaction
requiring NAD+.
• Thus the citric acid cycle is completed. An acetyl group,
containing two carbon atoms, is fed into the cycle by
combining it with oxaloacetate.
• At the end of the cycle a molecule of oxaloacetate is
generated.
• These reducing equivalents NADH+H+ and FADH2 are
transferred to the respiratory chain in the inner mitochondrial
membrane for reoxidation.
Respiratory chain in the inner
mitochondrial membrane
Pyruvate
Isocitrate
ADP+Pi→ATP ADP+Pi→ATP
ADP+Pi→ATP
↑
↑
↑
α-ketoglutarate →NAD→FMN→CoQ→Cytb→CytC1→CytC→Cyta→ Cyta3
Malate
ATP production in mitochondria
Name of enzyme
Reaction Catalyzed
Isocitrate
dehydrogenase
Respiratory chain oxidation of 3
NADH +H+
α-Ketoglutrate
dehydrogenase
Respiratory chain
oxidation of NADH +H+
Succinate thiokinase
Phosphorylation at substrate 1
level
Succinate
dehydrogenase
Resiphorylation chain oxidation 2
of FADH2
Malate dehydrogenase
Respiratory chain oxidation of 3
NADH +H+
Net
ATP molecules
formed
3
12
ATP production from glucose
• Glycolysis
• One mole. of glucose → 2 moles of pyruvate
• ATP produced = 8
• Pyruvate → Acetyl CoA
• ATP produced = 2x3=6
• Citric acid cycle
• Acetyl CoA →CO2 + H2O
• ATP produced = 12 x2=24
• Net production =30+8=38