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

doi:10.1073/pnas.0700579104

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

*Department of Biochemistry and Food Chemistry, University of Turku, FIN-20014, Turku, Finland; and
^†Department of Medical Biochemistry and Biophysics, Karolinska Institutet, S-171 77 Stockholm, Sweden

Edited by Christopher T. Walsh, Harvard Medical School, Boston, MA, and approved February 16, 2007 (received for review January 22, 2007)

Abstract

Aclacinomycin (Acl) oxidoreductase (AknOx) catalyzes the last two steps in the biosynthesis of polyketide antibiotics of the Acl 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-Å 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 8α-Nδ1-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.

Footnotes

^‡To whom correspondence may be addressed. E-mail: jarnie{at}utu.fi or gunter.schneider{at}ki.se
Author contributions: I.A. and A.S. contributed equally to this work; P.M., J.N., and G.S., designed research; I.A. and A.S. performed research; I.A., A.S., J.N., and G.S. analyzed data; and I.A., A.S., P.M., J.N., and G.S. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Data deposition: The crystallographic data reported in this paper have been deposited with the Protein Data Bank, www.pdb.org (PDB ID code 2IPI).
This article contains supporting information online at www.pnas.org/cgi/content/full/0700579104/DC1.
Abbreviations:

Acl,

aclacinomycin;

AknOx,

Acl oxidoreductase;

GOOX,

glucooligosaccharide oxidase;

PCMH,

p-cresol methylhydroxylase.