PHACS (putative human ACS), also known as ACC or ACS (1-aminocyclopropane-1-carboxylate synthase homolog), is a 501 amino acid protein that belongs to the alpha family of pyridoxal-5-prime-phosphate enzymes. PHACS is expressed in a wide range of tissues and shares structural similarity with AAT (aspartate aminotransferase), TAT (tyrosine aminotransferase) and enzymes that catalyze beta-elimination reactions on amino acids. PHACS consists of overlapping aminotransferase I and beta-eliminating lyase domains and is involved in the deamination of L-vinylglycine. The plant homolog ACS is the key metabolic intermediate in the biosynthesis of phytohormone ethylene, which is essential for the growth and development of plants.