Crystal structures of 1-aminocyclopropane-1-carboxylate (ACC) synthase in complex with aminoethoxyvinylglycine and pyridoxal-5 '-phosphate provide new insight into catalytic mechanisms

The structures of tomato 1-aminocyclopropane-1-carboxylate synthase (ACS) i n complex with either cofactor pyridoxal-5'-phosphate (PLP) or both PLP and inhibitor aminoethoxyvinylglycine have been determined by x-ray crystallog raphy. The structures showed good conservation of the catalytic residues, s uggesting a similar catalytic mechanism for ACS and other PLP-dependent enz ymes. However, the proximity of Tyr(152) to the C-gamma -S bond of model su bstrate S-adenosylmethionine implies its critical role in the catalysis. Th e concerted accomplishment of catalysis by cofactor PLP and a protein resid ue, as proposed on the basis of the ACS structures in this paper, may repre sent a general scheme for the diversity of PLP-dependent catalyses. PLP-dep endent enzymes have been categorized into four types of folds. A structural comparison revealed that a core fragment of ACS in fold type I is superimp osable over tryptophan synthase beta subunit in fold type II and mouse orni thine decarboxylase in fold type III, thus suggesting a divergent evolution of PLP-dependent enzymes.