Aclacinomycin oxidoreductase (AknOx) from the biosynthetic pathway of the antibiotic aclacinomycin is an unusual flavoenzyme with a dual active site

Aclacinomycin (Acl) oxidoreductase (AknOx) catalyzes the last two steps in the biosynthesis of polyketide antibiotics of the Ad group, the oxidation of the terminal sugar moiety rhodinose to L-aculose. We present the crystal structure of AknOx with bound FAD and the product AclY, refined to 1.65-angstrom resolution. The overall fold of AknOx identifies the enzyme as a member of the p-cresol methylhydroxylase superfamily. The cofactor is bicovalently attached to His-70 and Cys-130 as 8 alpha-N delta 1-histidyl, 6-S-cysteinyl FAD. The polyketide ligand is bound in a deep cleft in the substrate-binding domain, with the tetracyclic ring system close to the enzyme surface and the three-sugar chain extending into the protein interior. The terminal sugar residue packs against the isoalloxazine ring of FAD and positions the carbon atoms that are oxidized close to the N5 atom of FAD. The structure and site-directed mutagenesis data presented here are consistent with a mechanism where the two different reactions of AknOx are catalyzed in the same active site but by different active site residues. Tyr-450 is responsible for proton removal from the C-4 hydroxyl group in the first reaction, the oxidation of rhodinose to cinerulose A. Tyr-378 acts as a catalytic base involved in proton abstraction from C3 of cinerulose A in the second reaction, for formation L-aculose. Replacement of this residue, however, does not impair the conversion of rhodinose to cinerulose A.