Nonenzymatic oxidative steps accompanying action of the cytochrome p450 enzymes StaP and RebP in the biosynthesis of staurosporine and rebeccamycin

In the biosynthesis of the indolocarbazole natural products staurosporine and rebeccamycin, a complex set of oxidative transformations results in dimerization and oxidative cross-linking of a pair of L-tryptophan monomers. Most intriguing among the oxidative enzymes involved in this pathway is StaP (or its homologue, RebP), which appears to catalyze the four- to eight-electron oxidation of chromopyrrolic acid to give a set of three aglycones, differing in oxidation state in the pyrrole-derived ring. In this work, we sought to clarify the catalytic role of StaP in this process and gain insight into the apparent lack of specificity in its product profile. Through the preparation of a putative intermediate, we show that all steps downstream of aryl-aryl coupling can occur nonenzymatically in solution, pinpointing the function of StaP as a two-electron aryl-aryl coupling catalyst that acts on chromopyrrolic acid. StaP thus joins a small but growing family of aryl-aryl coupling enzymes that use P450-based chemistry to facilitate this oxidative transformation. Furthermore, using ring-deuterated substrate, we show that this aryl-aryl coupling process is not a rate-limiting step in the overall formation of the aglycones. This work expands on our previous studies on the biosynthetic enzymes involved in the staurosporine and rebeccamycin pathways and acts to illuminate the specific roles of StaP and RebP in the context of these biosyntheses.