PHOTOINDUCED ELECTRON-TRANSFER REACTIONS OF PLASTOCYANIN WITH THE TRIPLET-STATE OF ZINC MYOGLOBIN AND WITH THE ZINC MYOGLOBIN CATION-RADICAL - PROTEIN-PROTEIN ORIENTATION IN THE ABSENCE OF STRONG ELECTROSTATIC INTERACTIONS

Various redox properties and electron-transfer reactions of myoglobin have been studied, but very few of these studies have dealt with reactions between myoglobin and other redox proteins. We report kinetics of two such photoinduced bimolecular reactions-reduction of cupriplastocyanin by the triplet state of zinc myoglobin (the forward reaction) and oxidation of cuproplastocyanin by the cation radical of zinc myoglobin (the back reaction). A widely-used version of Debye-Huckel theory reproduces the dependence on ionic strength of the forward reaction but not of the back reaction. Dependence on ionic strength of the forward rate constant is treated by van Leeuwen theory, which recognizes monopole-monopole, monopole-dipole, and dipole-dipole interactions between the protein molecules. Fitting of the kinetic results reveals the overall protein-protein orientation for electron transfer. Myoglobin lacks distinct charged patches on its surface, its net charge is nearly zero, and its heme is buried. In the absence of significant electrostatic attraction to cupriplastocyanin, an area near the heme crevice in zinc myoglobin seems to abut the electroneutral hydrophobic patch in plastocyanin, which is proximate to the copper atom. This finding is compared with our previous findings concerning zinc cytochrome c, which has positively charged patch surrounding the exposed heme edge. In that case, strong electrostatic attraction keeps the exposed heme near the acidic patch, which is remote from the copper atom. Protein-protein orientation for electron transfer depends on an interplay between electrostatic interactions and donor-acceptor coupling, among other factors.