Succinyl-CoA synthetase functions in the citric acid cycle (TCA), coupling the hydrolysis of succinyl-CoA to the synthesis of either ATP or GTP and thus represents the only step of substrate-level phosphorylation in the TCA. The alpha subunit of the enzyme binds the substrates coenzyme A and phosphate, while succinate binding and specificity for either ATP or GTP is provided by different beta subunits. CoA + GTP + succinate = GDP + phosphate + succinyl-CoA ATP + CoA + succinate = ADP + phosphate + succinyl-CoA Carbohydrate metabolism; tricarboxylic acid cycle; succinate from succinyl-CoA (ligase route): step 1/1. Heterodimer of an alpha and a beta subunit. Different beta subunits determine nucleotide specificity. Together with an ATP-specific beta subunit, forms an ADP-forming succinyl-CoA synthetase (A-SCS). Together with a GTP-specific beta subunit forms a GDP-forming succinyl-CoA synthetase (G-SCS). Phosphorylated; stimulation by low concentration of GDP. Belongs to the succinate/malate CoA ligase alpha subunit family.