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Reactions of TCA cycle (Contd.)

TCA Cycle

Figure-1- Reactions of TCA cycle

For  details of reactions 1-4 click the link below

 5) Formation of Succinate- Succinyl CoA is an energy-rich thioester compound. The ∆for the hydrolysis of succinyl CoA is about -8 kcal mol-1 (-33.5 kJ mol-1), which is comparable to that of ATP (-7.3 kcal mol-1, or -30.5 kJ mol-1). The cleavage of the thioester bond of succinyl CoA is coupled to the phosphorylation of a purine nucleoside diphosphate, usually GDP (Figure-2). This reaction is catalyzed by succinyl CoA synthetase (succinate thiokinase).



 Formation of Succinate

Figure-2- Formation of succinate and GTP

Biological significance

This is the only example in the citric acid cycle of substrate level phosphorylation. Tissues in which gluconeogenesis occurs (the liver and kidney) contain two isoenzymes of succinate thiokinase, one specific for GDP and the other for ADP. The GTP formed is used for the decarboxylation of oxaloacetate to phosphoenolpyruvate in gluconeogenesis, and provides a regulatory link between citric acid cycle activity and the withdrawal of oxaloacetate for gluconeogenesis. Nongluconeogenic tissues have only the isoenzyme that uses ADP.

6) Regeneration of Oxaloacetate

The onward metabolism of succinate, leading to the regeneration of oxaloacetate, is the same sequence of chemical reactions (Figure-4) as occurs in the β-oxidation of fatty acids:

  • Dehydrogenation to form a carbon-carbon double bond,
  • Addition of water to form a hydroxyl group, and
  • A further dehydrogenation to yield the oxo-group of oxaloacetate.

a) The first dehydrogenation reaction, forming fumarate, is catalyzed by succinate dehydrogenase, which is bound to the inner surface of the inner mitochondrial membrane. This is the only enzyme present in the bound form rest all enzymes are present in the mitochondrial matrix. The enzyme contains FAD and iron-sulfur (Fe:S) protein, and directly reduces ubiquinone in the electron transport chain.

 Reaction catalyzed by succinate dehydrogenase

FAD is the hydrogen acceptor in this reaction because the free-energy change is insufficient to reduce NAD+.

Clinical significance- The enzyme succinate dehydrogenase is inhibited by Malonate, a competitive inhibitor of the enzyme.

b)  Addition of water–  Fumarase (fumarate hydratase) catalyzes the addition of water across the double bond of fumarate, yielding malate (Figure-3)


    Formation of malate       

Figure-3-Formation of Malate from Fumarate

c) Second dehydrogenation reaction -Malate is converted to oxaloacetate by malate dehydrogenase, a reaction requiring NAD+.

 Formation of oxaloacetate

Although the equilibrium of this reaction strongly favors malate, the net flux is to oxaloacetate because of the continual removal of oxaloacetate (to form citrate, as a substrate for gluconeogenesis, or to undergo transamination to aspartate) and also the continual reoxidation of NADH.

 Formation of oxaloacetate from succinate

Figure-4- The regeneration of Oxaloacetate from Succinate. Dehydrogenation is followed by hydration to be followed by second dehydrogenation to form oxaloacetate.

 Energy yield per Acetyl co A per turn of cycle

 The net reaction of the citric acid cycle is-

 Net reaction of TCA cycle

As a result of oxidations catalyzed by the dehydrogenases of the citric acid cycle, three molecules of NADH and one of FADH2 are produced for each molecule of acetyl-CoA catabolized in one turn of the cycle. These reducing equivalents are transferred to the respiratory chain, where reoxidation of each NADH results in formation of 3, and 2 ATP of FADH2. Consequently, 11 high-transfer-potential phosphoryl groups are generated when the electron-transport chain oxidizes 3 molecules of NADH and 1 molecule of FADH2,  In addition, 1 ATP (or GTP) is formed by substrate-level phosphorylation catalyzed by succinate thiokinase.

Thus, 1 acetate unit generates approximately 12 molecules of ATP. In dramatic contrast, only 2 molecules of ATP are generated per molecule of glucose (which generates 2 molecules of acetyl CoA) by anaerobic glycolysis.

Molecular oxygen does not participate directly in the citric acid cycle. However, the cycle operates only under aerobic conditions because NAD+ and FAD can be regenerated in the mitochondrion only by the transfer of electrons to molecular oxygen. Glycolysis has both an aerobic and an anaerobic mode, whereas the citric acid cycle is strictly aerobic. Glycolysis can proceed under anaerobic conditions because NAD+ is regenerated in the conversion of pyruvate into lactate.

ATP Formation in the Catabolism of Glucose

Oxidation of Glucose yields up to 38 Mol of ATP under aerobic conditions, but only 2 Mol when O2 is absent

When 1 mol of glucose is combusted in a calorimeter to CO2 and water, approximately 2870 kJ are liberated as heat. When oxidation occurs in the tissues, approximately 38 mol of ATP are generated per molecule of glucose oxidized to CO2 and water. In vivo, ∆G for the ATP synthase reaction has been calculated as approximately 51.6 kJ. It follows that the total energy captured in ATP per mole of glucose oxidized is 1761 kJ, or approximately 68% of the energy of combustion. Most of the ATP is formed by oxidative phosphorylation resulting from the reoxidation of reduced coenzymes by the respiratory chain. The remainder is formed by substrate level phosphorylation.

Pathway Reaction Catalyzed by Method of ATP Formation ATP per Mol of Glucose
Glycolysis Glyceraldehyde 3-phosphate dehydrogenase Respiratory chain oxidation of 2 NADH 6*
Phosphoglycerate kinase Substrate level phosphorylation 2
Pyruvate kinase Substrate level phosphorylation 2
                                                                   Total 10
Consumption of ATP for reactions of hexokinase and phosphofructokinase –2
                                                                   Net 8
Pyruvate dehydrogenase complex Pyruvate dehydrogenase Respiratory chain oxidation of 2 NADH 6
Citric acid cycle Isocitrate dehydrogenase Respiratory chain oxidation of 2 NADH 6
α-Ketoglutarate dehydrogenase Respiratory chain oxidation of 2 NADH 6
Succinate thiokinase Substrate level phosphorylation 2
Succinate dehydrogenase Respiratory chain oxidation of 2 FADH2
Malate dehydrogenase Respiratory chain oxidation of 2 NADH 6
                                                                   Net 30
Total per mol of glucose under aerobic conditions 38
Total per mol of glucose under anaerobic conditions 2
  •  NADH formed in glycolysis is transported into mitochondria by the malate shuttle . If the Glycerophosphate shuttle is used, then only 2 ATP will be formed per mol of NADH. As in brain and skeletal muscle due to transport of reducing equivalents through glycerophosphate shuttle the net yield of ATP molecules per glucose oxidation is 36  in comparison to 38 ATP in other cells of the body  .
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