ACC synthase (1-aminocyclopropane-1-carboxylate synthase, EC 4.4.114)
is a key enzyme to control biosynthesis of plant hormone ethylene that
profoundly influences the growth and development of higher plants, such
as germination of seeds, ripening of fruits, abscission of leaves, and
senescence of flowers. ACC synthase (ACS) converts S-adenosylmethionine
(SAM) to ACC in the presence of cofactor pyridoxal-5'-phosphate (PLP or
vitamin B6). The ACS structure has a core fragment in fold type I of PLP-dependent
enzymes and is also superimposable over fold types II and III, thus suggesting
a divergent evolution of PLP-dependent enzymes. The crystal structures
of tomato ACS in complex with PLP and competitive inhibitor aminoethoxyvinylglycine
suggest a concerted accomplishment of the catalysis by cofactor PLP and
the protein residue Tyr152. This mechanism may represent a general scheme
for the diversity of PLP-dependent catalyses.
Ribbon diagram of monomeric ACS.
Superposition of the ACS fragment (golden, fold type I of PLP-dependent
enzymes) over mODC (light-green, fold type III) and TSB (cyan, fold type
II). The PLP binding site is marked with the ball-stick models.