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 electrostatic interactions in the regulation of the ET reactions. The laser pulse irradiation to ZnP450 yielded a strong reductant, the triplet excited state of ZnP450, 3ZnP450*, which was able to transfer one electron to anionic redox partners, OsCl62- and Fe(CN)63-, with formation of the porphyrin π-cation radical, ZnP450+. In contrast, the ET reactions from 3ZnP450* to cationic redox partners, such as Ru(NH3)63+ and Co(phen)33+, were 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 redox partner is that the cationic patch on the P450cam surface, a putative interaction site for the anionic reagents, is located near the heme (less than 10 Å from the heme edge), while the anionic surface is far from the heme moiety (more than 16 Å 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 than that of the phosphorescence process in 3ZnP450*, resulting in no ET reactions to the cationic reagents. These results demonstrate that the asymmetrical charge distribution on the protein surface is critical for the ET reaction in P450cam.
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