Structure of ACC synthase inactivated by the mechanism-based inhibitor l-vinylglycine

l-Vinylglycine (l-VG) is both a substrate for and a mechanism-based inhibitor of 1-aminocyclopropane-1-carboxylate (ACC) synthase. The ratio of the rate constants for catalytic conversion to α-ketobutyrate and ammonia to inactivation is 500/1. The crystal structure of the covalent adduct of the inactivated enzyme was determined at 2.25 Å resolution. The active site contains an external aldimine of the adduct of l-VG with the pyridoxal 5′-phosphate cofactor. The side chain γ-carbon of l-VG is covalently bound to the ε-amino group of Lys273. This species corresponds to one of the two alternatives proposed by Feng and Kirsch [Feng, L. and Kirsch, J.F. (2000) l-Vinylglycine is an alternative substrate as well as a mechanism-based inhibitor of 1-aminocyclopropane-1-carboxylate synthase. Biochemistry 39, 2436–2444] and presumably results from Michael addition to a vinylglycine ketimine intermediate.