Mitochondrial succinyl-CoA synthetase catalyzes the reversible reaction of succinylCoA, GDP, and orthophosphate to form succinate, CoASH, and GTP. The enzyme is a heterodimer consisting of an alpha chain and a beta chain that determines whether the reaction involves adenine nucleotides or guanine nucleotides (as here). The biochemical properties of the chicken enzyme have been studied in detail (Hamilton and Ottaway 1981) but the genes encoding its component proteins have not been cloned. The molecular properties of these proteins are inferred from those of their pigeon homologues (Johnson et al. 1998).