Roles of the proximal hydrogen bonding network in cytochrome P450_cam-catalyzed oxygenation

Structural and functional roles of the hydrogen bonding network that surrounds the heme-thiolate coordination of P450_cam from Pseudomonas putida were investigated. A hydrogen bond between the side chain amide of Gln360 and the carbonyl oxygen of the axial Cys357 was removed in Q360L. The side chain hydrogen bond and the electrostatic interaction between the polypeptide amide proton of Gln360 and the sulfur atom of Cys357 were simultaneously removed in Q360P. The increased electron donation of the axial thiolate in Q360L and Q360P was evidenced by negative shifts of their reduction potentials by 45 and 70 mV, respectively. Together with the results on L358P in which the amide proton at position 358 was removed (Yoshioka, S., Takahashi, S., Ishimori, K., Morishima, I. J. Inorg. Biochem. 2000, 81, 141-151), we propose that the side chain hydrogen bond and the electrostatic interaction of the amide proton with the thiolate ligand cause ∼45 and ∼35 mV of positive shifts, respectively, of the redox potential of the heme in P450_cam. The resonance Raman spectra of the ferrous-CO form of the Q360 mutants showed a downshifted Fe-CO stretching mode at 482∼483 cm^-1 compared with that of wild-type P450_cam at 484 cm^-1. The Q360 mutants also showed the upshift by 4∼5 cm^-1 of the Fe-NO stretching mode in the ferrous-NO form. These Raman results indicate the increase in the σ-electron donation of the thiolate ligand in the reduced state of the Q360 mutants and were in contrast to the increased π-back-donation of the thiolate in L358P having an upshifted Fe-CO stretching mode at 489 cm^-1. The catalytic activities of the Q360 mutants for the unnatural substrates were similar to those of the wild-type enzyme, indicating that the increased σ-electron donation does not promote the O-O bond heterolysis in the Q360 mutants, although the increased π-electron donation in L358P promoted the heterolysis of the O-O bond. We conclude that the functions of the proximal hydrogen bonding network in P450_cam are to stabilize the heme-thiolate coordination, and to regulate the redox potential of the heme iron. Furthermore, we propose that the π-electron donation, not the σ-electron donation, of the thiolate ligand promotes the heterolysis of the O-O bond of dioxygen.