Inevitable Cytochrome P450 Coordination Structure for Enzyme Function: Chemical Model Approach to Elucidation of the Axial Ligand Effect

A distinctive structural feature of P450 is the unusual thiolate coordination to heme. We have succeeded in the preparation of the first synthetic thiolato-iron porphyrin (SR complex) that retains its structure during catalytic oxidation.Experiments using SR complex have revealed that the thiolate ligand greatly accelerates the rate of the O-O bond cleavage and its heterolysis even in highly hydrophobic media.Prostaglandin H2 (PGH2) is catalytically isomerized to prostacyclin or thromboxane A2 by cytochrome P450s. We investigated the isomerization mechanism of PGH2 using SR complex, which is a model of P450 having heme-thiolate structure. Isomerization of endoperoxide proceeded very rapidly with SR, whereas imidazole or chloride-ligated heme had slight or no catalytic activity.The results of kinetic isotope effects in the oxidative demethylation of p-dimethoxybenzene unambiguously showed that the formed active intermediates of heme-thiolates are different from those of hemes coordinated by imidazole or chloride.Novel iron porphyrin-alkanethiolate complexes were prepared in order to examine the influence of the NH-S hydrogen bond on catalytic oxidation. The complexes were characterized by IR, EPR, resonance Raman spectroscopies and crystal structure analysis and their redox potentials were measured.Our work also reports some spectroscopic and electrochemical properties of the first synthetic NO-heme-thiolate complex, prepared by treatment of SR with NO, relevant to NO synthase.

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