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 188.8.131.52, phenylalanine aminomutase (D-beta-phenylalanine forming).
(E)-cinnamate is both a substrate and an intermediate of the reaction. To account for the distinct (3alpha)-beta-amino acid stereochemistry catalyzed by the enzyme, the cinnamate skeleton must rotate the C1-Calpha and Cipso-Cbeta bonds 180° in the active site prior to exchange and rebinding of theNH2/H pair to the cinnamate
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to 2.38 A resolution, space group C2. A (E)-cinnamate molecule is bound in the active site, lying above the 4-methylidene-1H-imidazol-5(4H)-one cofactor and under a loop region that includes residues 80-97, which define the top of the active site. The (E)-cinnamate molecule lies about 3.4 A above the methylidene carbon of the 4-methylidene-1H-imidazol-5(4H)-one moiety. The carboxylate of the cinnamate makes a salt bridge interaction with a strongly conserved residue R325, which serves to position the product in the active site. The plane of the aromatic ring of the cinnamate is displaced about 20° from the perpendicular relative to the pi-bond plane of the propenoate carboncarbon double bond. The aromatic ring is bound relatively loosely in the active site, making only one direct hydrophobic interaction with residue L104