Structural characterization of CalO2: A putative orsellinic acid P450 oxidase in the calicheamicin biosynthetic pathway

Although bacterial iterative Type I polyketide synthases are now known to participate in the biosynthesis of a small set of diverse natural products, the subsequent downstream modification of the resulting polyketide products remains poorly understood. Toward this goal, we report the X‐ray structure determination at 2.5 Å resolution and preliminary characterization of the putative orsellenic acid P450 oxidase (CalO2) involved in calicheamicin biosynthesis. These studies represent the first crystal structure for a P450 involved in modifying a bacterial iterative Type I polyketide product and suggest the CalO2‐catalyzed step may occur after CalO3‐catalyzed iodination and may also require a coenzyme A‐ (CoA) or acyl carrier protein‐ (ACP) bound substrate. Docking studies also reveal a putative docking site within CalO2 for the CLM orsellinic acid synthase (CalO5) ACP domain which involves a well‐ordered helix along the CalO2 active site cavity that is unique compared with other P450 structures. Proteins 2009. © 2008 Wiley‐Liss, Inc.