Cys123α of E. coli succinyl-CoA synthetase was mutated to alanine, serine, threonine and valine in order to test whether the succinyl moiety is transferred to the thiol of Cys123α as part of the catalytic mechanism. The mutant proteins were still active, but showed differences in catalysis and in structure from wild-type enzyme, demonstrating why this residue is highly conserved.