Succinyl Coenzyme A cleavage in TCA

Succinate thiokinase (also called succinyl CoA synthetase, named for the reverse reaction) cleaves the high-energy thioester bond of succinyl CoA ( Fig. 1). This reaction is coupled to phosphorylation of guanosine diphosphate (GDP) to guanosine triphosphate (GTP).

Figure 1:  Formation of malate from α-ketoglutarate. FAD(H2) and NAD(H) =flavin and nicotinamide adenine dinucleotides; GDP and GTP = guanosine diand triphosphates; ~ = high-energy bond; CoA = coenzyme A.

GTP and ATP are energetically interconvertible by the nucleoside diphosphate kinase reaction:

The generation of GTP by succinate thiokinase is another example of substrate-level phosphorylation . [Note: Succinyl CoA is also produced from propionyl CoA derived from the metabolism of fatty acids with an odd number of carbon atoms  and from the metabolism of several amino acids . It can be converted to pyruvate for gluconeogenesis  or used in heme synthesis .]