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CHAPTER 19
Harvesting Electrons
from the
Citric Acid Cycle
Harvesting electrons from the citric acid cycle
- Enzymes of the cycle are in the mitochondria of eukaryotes
- Energy of the oxidation reactions is largely conserved as reducing power
(stored electrons)
- Coenzymes reduced:
NAD+
FAD
-
NADH
FADH2
The net effect of the cyclic pathway is the complete oxidation of an acetyl
group to 2 CO2 and the transfer of electrons to the above coenzymes.
Figure 19.1: An overview of the citric acid cycle
8 electrons are stored
2 carbon units are converted to CO2 and 1 ATP is made in the cycle
Figure 19.2: Harvested electrons will be used to drive a Proton Pump
1. Citrate Synthase
- Citrate is formed from the acetyl CoA and oxaloacetate
- Methyl carbon from acetyl group reacts with
carbonyl of oxaloacetate
- This is the only cycle reaction with C-C bond formation
1. Citrate Synthase
The induced fit model revisited:
A homodimeric protein
Oxaloacetate caused a major rearrangement that creates a binding
site for acetyl CoA reduces the possibility of side reactions
2. Aconitase
-
Aconitase catalyzes a near equilibrium reaction
Citrate is not a chiral molecule
Elimination of H2O from citrate to form C=C bond
of cis-aconitate
A stereospecific addition of H2O to cis-aconitate forms
2R,3S-isocitrate
3. Isocitrate Dehydrogenase
-
-
Oxidative decarboxylation of isocitrate to a-ketoglutarate
(a metabolically irreversible reaction)
One of four oxidation-reduction reactions of the cycle
Hydride ion from the C-2 of isocitrate is transferred to NAD+ to
form NADH.
(unstable)
4. The a-Ketoglutarate Dehydrogenase Complex
-
Complex is very large and is similar to the pyruvate
dehydrogenase complex
- E1 – a-ketoglutarate dehydrogenase (with TPP)
- E2 – succinyltransferase (with a flexible lipoamide prosthetic group)
- E3 – dihydrolipoamide dehydrogenase (with FAD)
2nd reducing
equivalence
5. Succinyl-CoA Synthetase
The beginning of regenerating oxaloacetate
- The high free energy stored in the thioester bond of succinyl CoA is conserved
as GTP (ATP equivalence):
5. Succinyl-CoA Synthetase
The beginning of regenerating oxaloacetate
- The high free energy stored in the thioester bond of succinyl CoA is conserved
as GTP (or ATP)
- The reaction takes place in four steps:
Figure 19.5
High energy thioester bond
6. The Succinate Dehydrogenase (SDH) Complex
-
The active site is made of two different subunits
- One subunit contains 4 Fe-S cluster
- The other contains covalently bound FAD
-
The SDH dimer is attached to the inner mitochondrial membrane via
two membrane polypeptides
- This is in contrast to other enzymes of the CA cycle which
are dissolved in the mitochondrial matrix
-
The transfer of two electrons occurs from succinate  FADH2  ubiquinone (Q),
a lipid soluble mobile carrier of electrons
- The reduced ubiquinone (QH2) is released as a mobile product
Q
QH + FAD
2
7. Fumarase
- A near equilibrium trans addition of water
to the double bond of fumarate
8. Malate Dehydrogenase
-
A near equilibrium inter-conversion that is dependent on NAD+
-
Oxidation of malate to oxaloacetate, with transfer of electrons
to NAD+ to form NADH.
Figure 19.6
The two carbons that
enter the cycle are NOT
the same to leave as CO2
FAD + QH2
Q
The overall net reaction can be simplified as follows:
- The OH- is ultimately donated by a phosphate group
- From the 8 electrons and 7 H+:
- 6 electrons are transferred to 3 NAD+ along with 3 H+
- 2 electrons are transferred to Q along with 2 H+
- 2 H+ are released per cycle.
The two CO2 molecules do NOT come directly from the acetyl group added to CoA
Reduced Coenzymes Fuel the Production of ATP
- Each acetyl CoA entering the cycle nets:
1. 3 NADH
2. 1 QH2
3. 1 GTP (or 1 ATP)
- Oxidation of each NADH yields 2.5 ATP
- Oxidation of each QH2 yields 1.5 ATP
Oxidative
phosphorylation
- Complete oxidation of 1 acetyl CoA = 10 ATP
Glucose degradation via glycolysis, citric acid cycle
and oxidative phosphorylation
Regulation of the citric acid cycle
Figure 19.7
Regulation of the citric acid cycle occurs
mainly in two reactions:
1. isocitrate dehydrogenase
(isocitrate  a-ketoglutarate)
2. a-ketogluterate dehydrogenase complex
(a-ketoglutarate  succinyl CoA)
Figure 19.8 Routes leading to and from the CA cycle
Figure 19.9: Pyruvate carboxylase supplies
oxaloacetate for the citric acid cycle
When energy demands are high
oxaloacetate is diverted to gluconeogenesis
When energy demands are low
oxaloacetate is shuttled into
citric acid cycle
The Glyoxylate Cycle
- Pathway for the formation of glucose from noncarbohydrate precursors
in plants, bacteria and yeast (few animals)
-
Glyoxylate cycle leads from 2-carbon compounds to glucose
-- Glyoxylate (a 2 C compound) combines with the acetyl group
in acetyl CoA to generate malate leads to glucose formation
-
In most animals, acetyl CoA is NOT a carbon source for the net formation
of glucose (2 carbons of acetyl CoA enter CA cycle, 2 carbons are
released as 2 CO2
-
Glyoxylate cycle provides an anabolic alternative for acetyl CoA
metabolism (Allows for the formation of glucose from acetyl CoA)
-
This cycle is active in oily seed plants (Stored seed oils are converted to
carbohydrates during germination)
Figure 19.10
The glyoxylate cycle bypasses
the two decarboxylation steps
of the CA cycle, conserving the
two carbon atoms as glyoxylate
for synthesis of glucose
Germinating seeds use this
Pathway to synthesize sugar
(glucose) from oil (triacylglycerols)
Assignment
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