Succinyl-CoA synthetases are mitochondrial enzymes that catalyze the reversible conversion of succinyl-CoA to succinate in the only step of the tricarboxylic acid cycle coupled with substrate-level phosphorylation of either ADP or GDP. In eukaryotes, the functional enzyme is a heterodimeric complex composed of a single alpha subunit and one of two isoforms of a beta subunit that determines the substrate specificity, resulting in an ATP- or GTP-generating enzyme. (Adapted from PMID:37445899.)