DOCKING AND ELECTRON-TRANSFER BETWEEN CYTOCHROME C(2) AND THE PHOTOSYNTHETIC REACTION-CENTER

Electron transfer between the proteins cytochrome c(2) (cytc(2)) and the photosynthetic reaction center (RC) was studied using the pathways model of Beratan and Onuchic. To investigate the protein structure dependence of this electron transfer reaction, we separated the electronic coupling decay between electron donor and acceptor into three parts: (i) the coupling from the cytochrome heme to the surface of the cytochrome, (ii) the coupling from the RC surface to the bacteriochlorophyll dimer, and (iii) the coupling from the surface of the cytochrome to the surface of the reaction center. Calculating the coupling between the surface amino acids and the redox center allows the simple estimate of inter-protein electronic coupling, and, for a given docked structure, provides a functional criterion for evaluating that structure. The strongest heme to special pair electron transfer pathway in Rps. viridis (an RC with fixed heme-special pair structure) included surface residues on the cytochrome and RC (when each was treated as a separate protein), although it did not invoke the most strongly coupled surface atoms of the RC. To examine the effect of docking orientation on electron transfer in Rb. sphaeroides, in which the cytochrome is not bound to the RC, we generated surface coupling maps of the electronic coupling between the redox sites and solvent exposed atoms in each protein. These maps were compared to individual pathways calculated for two docked cytc(2)/RC structures, one based on electrostatic complementarity, the other based on maximizing electronic coupling. The electronic coupling between donor and acceptor for these two docked structures was remarkably similar, suggesting that efficient electron transfer can be obtained from very different docked orientations of the cytochrome. Other factors influencing intermolecular electron transfer are discussed.