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Succinyl-CoA synthetase (succinate thiokinase) catalyzes the conversion of energy-rich succinyl-CoA to succinate and free CoA. The free energy of the thioester bond in
succinyl-CoA is conserved by formation of GTP from GDP and inorganic phosphate (Pi). Since the respiratory chain is not involved, this is a substrate-level phosphorylation reaction, like the reactions catalyzed by phosphoglycerate kinase and pyruvate kinase in glycolysis (Chapter 11). GTP is used by enzymes such as phosphoenolpyruvate carboxykinase (PEPCK) in gluconeogenesis (Chapter 12), but is also readily equilibrated with ATP by the enzyme nucleoside diphosphate kinase:
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The next three reactions in the TCA cycle illustrate a common theme in metabolism for introducing a carbonyl group into a molecule:
- introduction of a double bond
- addition of water across the double bond to form an alcohol
- oxidation of the alcohol to a ketone.
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| This same sequence occurs in the form of enzyme-bound intermediates during conversion of citrate to α-ketoglutarate and in the oxidation of fatty acids.
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