Johnson et al. describe two isoforms of succinyl-CoA synthetase in amniotes, one that specifies synthesis of ATP, and one that synthesises GTP.
*  - ATP-forming - SUCLA2
*  - GTP-forming - SUCLG2
In amniotes, the enzyme is a heterodimer of an α- and a β-subunit. The specificity for either adenosine or guanosine phosphates is defined by the β-subunit, which is encoded by 2 genes. SUCLG2 is GTP-specific and SUCLA2 is ATP-specific, while SUCLG1 encodes the common α-subunit. β variants are produced at different amounts in different tissues, causing GTP or ATP substrate requirements.
Mostly consuming tissues such as heart and brain have more ATP-specific succinyl-CoA synthetase (ATPSCS), while synthetic tissues such as kidney and liver have the more GTP-specific form (GTPSCS). Kinetics analysis of ATPSCS from the breast muscle of pigeons and GTPSCS from pigeon liver showed that their apparent Michaelis constants were similar for CoA, but different for the nucleotides, phosphate, and succinate. The largest difference was for succinate: Kapp of ATPSCS = 5mM versus that of GTPSCS = 0.5mM.