1-aminocyclopropane-1-carboxylate synthase (ACS) enzymes catalyze the conversion of S-adenosyl-L-methionine (SAM) into 1-aminocyclopropane-1-carboxylate (ACC), a direct precursor of ethylene. Involved in bacterial flagellin-induced ethylene production. S-adenosyl-L-methionine = 1-aminocyclopropane-1-carboxylate + H(+) + S-methyl-5'-thioadenosine Optimum pH is 7.3. Alkene biosynthesis; ethylene biosynthesis via S-adenosyl-L-methionine; ethylene from S-adenosyl-L-methionine: step 1/2. Homodimer and heterodimer. In vivo, the relevance of heterodimerization with other ACS enzymes is however unsure (By similarity). Interacts with GRF3. Expressed in roots and flowers. By indole-3-acetic acid (IAA) and cycloheximide (CHX). By auxin. By treatment with ozone. Phosphorylated on serine residue by MAP kinase (MPK6). May be processed at its C-terminus. The stability of ACS proteins, and the regulation of such stability, play a central role in ethylene biosynthesis. The phosphorylation of serine residues on the C-terminus increases protein stability. Belongs to the class-I pyridoxal-phosphate-dependent aminotransferase family.