QM/MM study of the product-enzyme complex in P450_cam catalysis

The enzyme-product complex in P450_cam (CYP101) has been studied by combined quantum mechanical/molecular mechanical (QM/MM) calculations. The central iron(III) porphyrin complex and part of the catalytic product (5-exo-hydroxycamphor) are treated with density functional theory, while the protein/solvent environment is represented by the CHARMM force field. The computations indicate a doublet minimum at an Fe-O distance of ca. 2.2 Å, and a flat, barrierless potential for the dissociation of the Fe-O bond. Comparisons with analogous calculations on the isolated QM system in the gas phase show that inclusion of the protein/solvent environment lowers the activation energy for bond dissociation in the doublet state because of interactions within the binding pocket and accounts for a significant stabilization of the quartet and sextet states. The theoretical results allow for a tentative interpretation of recent ENDOR data (Davydov, R.; Makris, T. M. ; Kofman, V.; Werst, D. E.; Sligar, S. G.; Hoffman, B. M. J. Am. Chem. Soc. 2001, 123, 1403).