This enzyme is a member of the aromatic amino acid lyase family, other members of which are EC 4.3.1.3 (histidine ammonia-lyase) and EC 4.3.1.23 (tyrosine ammonia-lyase) and EC 4.3.1.25
(phenylalanine/tyrosine ammonia-lyase). The enzyme contains the cofactor 3,5-dihydro-5-methylidene-4H-imidazol-4-one (MIO), which is common to this family. This unique cofactor is formed
autocatalytically by cyclization and dehydration of the three amino-acid residues alanine, serine and glycine. The enzyme from some species is highly specific for phenylalanine.