ELECTRON-TUNNELING PATHS IN VARIOUS ELECTROSTATIC COMPLEXES BETWEEN CYTOCHROME-C AND PLASTOCYANIN - ANISOTROPY OF THE COPPER LIGAND INTERACTIONS AND DEPENDENCE OF THE IRON COPPER ELECTRONIC COUPLING ON THE METALLOPROTEIN ORIENTATION

The Pathways method of Beratan and Onuchic is applied to analysis and comparison of electron-tunneling paths between the iron(II) and copper(II) sites in six configurations of the complex between ferrocytochrome c and cupriplastocyanin. Standard parametrization is used, and electronic coupling in aromatic rings is treated as both attenuated and unattenuated; the results did not significantly depend on this choice. In one methodological improvement, crystallographically detected water molecules are included inside the proteins, and the vicinity of the protein-protein interface is hydrated in a molecular simulation. in another improvement, anisotropy of metal-ligand electronic coupling in the blue copper site is explicitly recognized. Standard calculations, in which the copper(II)-ligand coupling is treated as isotropic. showed the so-called northern equatorial configuration to have the greatest iron(II)-copper(II) coupling, i.e., the best tunneling paths. Other configurations provide neither few excellent paths nor many good ones. Scaled calculations, in which the anisotropy of the copper(II)-ligand coupling is treated in three different ways, favor the paths ending with Cys 84 and disfavor or abolish those ending with the other three ligands. These scaled calculations consistently showed the northern equatorial and the so-called ''maximum-overlap, rotated'' configurations to have greater coupling than the other electrostatically-stabilized configurations of the diprotein complex. The so-called maximum-overlap configuration, which has optimal electrostatic docking, lags behind the other two configurations in electronic coupling. Calculations of electronic coupling of the iron(II) and copper(II) sites to the surfaces of the respective proteins and matching of the surface patches in the two proteins were consistent with the analysis of iron(II)-copper(II) couplings. There are steric obstacles to contacts between the ''conductive'' surface patches in the maximum-overlap configuration but not in the northern equatorial configuration. This theoretical study corroborates experimental findings in this laboratory that the diprotein complex with optimal electrostatic interactions is not optimal for the intracomplex electron-transfer reactions. In studies of metalloprotein complexes by the Pathways method, internal and external hydration, anisotropy of metal-ligand coupling, and steric effects should all be considered.