The enzyme aminocyclopropane-1-carboxylic acid synthase (ACC synthase, ACS) (EC 4.4.1.14) catalyzes the synthesis of 1-Aminocyclopropane-1-carboxylic acid (ACC), a precursor for ethylene, from S-Adenosyl methionine (AdoMet, SAM), an intermediate in the Yang cycle and activated methyl cycle and a useful molecule for methyl transfer: Like other PLP dependent enzymes, it catalyzes the reaction through a quinonoid zwitterion intermediate and uses cofactor pyridoxal phosphate (PLP, the active form of vitamin B6) for stabilization.

The reaction catalyzed by 1-aminocyclopropane-1-carboxylic acid synthase (ACS) is the committed and rate-limiting step in the biosynthesis of ethylene [20], a gaseous plant hormone that is responsible for the initiation of fruit ripening, shoot and root growth and differentiation, leaf and fruit abscission, flower opening, and flower and leaf senescence.

[4][5] Proposed steps of the reaction mechanism: The aldehyde of coenzyme PLP reacts to form an imine (Schiff base) linkage with the catalytic domain lysine (278) residue of ACS.

ACC synthase reaches optimal activity in conditions of pH 8.5 and with Km = 20 um relative to its substrate, SAM.

Stresses such as wounding, noxious chemicals, auxin, flooding, and indole-3-acetic acid (IAA) promote ethylene synthesis, creating a positive feedback cycle with ACC synthase, up-regulating its activity.

ACC Synthase is also competitively inhibited by aminoethoxyvinylglycine (AVG) and aminooxyacetic acid (AOA), inhibitors to many pyridoxal phosphate-mediated enzymic reactions.

The proximity of the electronegative oxygen from Tyr (152) to the C-γ-S bond suggests a crucial role in the formation of ACC.

[7] X-ray crystallography with aminoethoxyvinylglycine (AVG) a competitive inhibitor confirmed Tyrosine's role in the γ elimination.