This enzyme is a member of the aromatic amino acid lyase family, other members of which are EC
4.3.1.23 (tyrosine ammonia-lyase), EC
4.3.1.24 (phenylalanine 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 [4]. This unique cofactor is formed autocatalytically by cyclization and dehydration of the three amino-acid residues alanine, serine and glycine [5]. This enzyme catalyses the first step in the degradation of histidine and the product, urocanic acid, is further metabolized to glutamate [2,3].