The enzyme contains the cofactor 3,5-dihydro-5-methylidene-4H-imidazol-4-one (MIO). This unique cofactor is formed autocatalytically by cyclization and dehydration of the three amino-acid residues alanine, serine and glycine. cf. EC 5.4.3.11, phenylalanine aminomutase (D-beta-phenylalanine forming).