Crystal structure of RebC, a flavoprotein involved in rebeccamycin biosynthesis

Rebeccamycin, an antitumor compound, is synthesized by L. aerocolonigenes from the precursor L‐tryptophan [ Biosci. Biotechnol. Biochem. (2003) 67 : –38]. One step in the biosynthetic pathway involves the net eight‐electron oxidation of chlorinated chromopyrrolic acid to generate the rebeccamycin aglycone, which is carried out by the enzymatic pair, RebP and RebC [ PNAS (2005) 102 : –6]. Substitution of RebC with StaC – a homologue with 65% identity to RebC – results in the production of the staurosporine aglycone in lieu of the rebeccamycin aglycone [ JACS (2006) 128 : –98]. While both RebC and StaC are annotated as flavin monooxygenases, only RebC is able to bind flavin adenine dinucleotide [ JACS (2006) 128 : –98]. The structure of RebC bound to flavin adenine dinucleotide (FAD) was recently solved with 1.8Å resolution using phases from a samarium derivative. The structure of RebC bears much resemblance to phenol hydroxylase, a flavin‐dependent monooxygenase. This suggests that RebC is also a flavin‐dependent monooxygenase, which might react with a yet‐unidentified product produced by RebP. Residues in the structure near FAD are potentially responsible for the differences in FAD‐binding and activity found between RebC and StaC. We are using the crystal structure as a guide to investigate the properties of various RebC mutants, in hope of determining the structural basis for enzyme specificity. This research is supported by the NIH (CLD) and by a Howard Hughes Predoctoral Fellowship (KSR).