Journal
JOURNAL OF MOLECULAR BIOLOGY
Volume 334, Issue 2, Pages 269-280Publisher
ACADEMIC PRESS LTD- ELSEVIER SCIENCE LTD
DOI: 10.1016/j.jmb.2003.09.061
Keywords
rhodomycin biosynthesis; helix swapping; protein crystallography; enzyme mechanism; protein structure
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Anthracyclines are aromatic polyketide antibiotics, and several of these compounds are widely used as anti-tumor drugs in chemotherapy. Aclacinomycin-10-hydroxylase (RdmB) is one of the tailoring enzymes that modify the polyketide backbone in the biosynthesis of these metabolites. RdmB, a S-adenosyl-L-methionine-dependent methyltransferase homolog, catalyses the hydroxylation of 15-demethoxy-epsilon-rhodomycin to beta-rhodomycin one step in rhodomycin biosynthesis in Streptomyces purpurascens. The crystal structure of RdmB, determined by multiwave-length anomalous diffraction to 2.1 Angstrom resolution, reveals that the enzyme subunit has a fold similar to methyltransferases and binds S-adenosyl-L-methionine. The N-terminal domain, which consists almost exclusively of alpha-helices, is involved in dimerization. The C-terminal domain contains a typical alpha/beta nucleotide-binding fold, which binds S-adenosyl-L-methionine, and several of the residues interacting with the cofactor are conserved in O-methyltransferases. Adjacent to the S-adenosyl-L-methionine molecule there is a large cleft extending to the enzyme surface of sufficient size to bind the substrate. Analysis of the putative substrate-binding pocket suggests that there is no enzymatic group in proximity of the substrate 15-demethoxy-epsilon-rhodomycin, which could assist in proton abstraction and thus facilitate methyl transfer. The lack of a suitably positioned catalytic base might thus be one of the features responsible for the inability of the enzyme to act as a methyltransferase. (C) 2003 Elsevier Ltd. All rights reserved.
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