Electron transfer reactions in Zn-substituted cytochrome P450cam

We have investigated photoinduced electron transfer (ET) reactions between zinc-substituted cytochrome P450cam (ZnP450) and several inorganic reagents by using the laser flash photolysis method, to reveal roles of the electro static interactions in the regulation of the ET reactions. The laser pulse irradiation to ZnP450 yielded a strong reductant, the triplet excited state of ZnP450, (3)ZnP450*, which was able to transfer one electron to anionic redox partners, OsCl62- and Fe(CN)(6)(3-), with formation of the porphyrin pi-cation radical, ZnP450(+). In contrast, the ET reactions from (3)ZnP450* to cationic redox partners, such as Ru(NH3)(6)(3+) and Co(phen)(3)(3+), we re not observed even in the presence of 100-fold excess of the oxidant. One of the possible interpretations for the preferential ET to the anionic red ox partner is that the cationic patch on the P450cam surface, a putative in teraction site for the anionic reagents, is located near the heme (less tha n 10 Angstrom from the heme edge), while the anionic surface is far from th e heme moiety (more than 16 Angstrom from the heme edge), which would yield 8000-fold faster ET rates through the cationic patch. The ET rate through the anionic patch to the cationic partner would be substantially slower tha n that of the phosphorescence process in (3)ZnP450*, resulting in no ET rea ctions to the cationic reagents. These results demonstrate that the asymmet rical charge distribution on the protein surface is critical for the ET rea ction in P450cam.