Catalyzes the formation of 1-aminocyclopropane-1-carboxylate, a direct precursor of ethylene in higher plants (PubMed:7809054, PubMed:11591146). Catalyzes also the conversion of L-vinylglycine (L-VG) to alpha-ketobutyrate and ammonia (PubMed:10704193). Can use S-methylmethionine as substrate (PubMed:14678788). S-adenosyl-L-methionine = 1-aminocyclopropane-1-carboxylate + H(+) + S-methyl-5'-thioadenosine (2S)-2-amino-3-butenoate + H2O = 2-oxobutanoate + NH4(+) Inhibited by L-aminoethoxyvinylglycine (AVG) (PubMed:12228256). Inhibited by L-vinylglycine (L-VG) (PubMed:10704193). Inhibited by S-methylmethionine through a L-VG ketimine intermediate (PubMed:14678788). kcat is 9 sec(-1) per monomer with S-adenosyl-L-methionine as substrate (PubMed:7809054). kcat is 1.8 sec(-1) with L-vinylglycine as substrate (PubMed:10704193). kcat is 2.7 min(-1) with S-methylmethionine as substrate (PubMed:14678788). Alkene biosynthesis; ethylene biosynthesis via S-adenosyl-L-methionine; ethylene from S-adenosyl-L-methionine: step 1/2. Homodimer. Expressed during ripening. Belongs to the class-I pyridoxal-phosphate-dependent aminotransferase family.