ELECTROSTATIC EFFECTS ON ELECTRON-TRANSFER KINETICS IN THE CYTOCHROME-F PLASTOCYANIN COMPLEX

In a complex of two electron-transfer proteins, their redox potentials can be shifted due to changes in the dielectric surroundings and the electrostatic potentials at each center caused by the charged residues of the partner. These effects are dependent on the geometry of the co mplex. Three different docking configurations (DCs) for intracomptex e lectron transfer between cytochrome f and plastocyanin were studied, d efined by 1) close contact of the positively charged region of cytochr ome f and the negatively charged regions of plastocyanin (DC1) and by (2, 3) close contact of the surface regions adjacent to the Fe and Cu redox centers (DC2 and DC3). The equilibrium energetics for electron t ransfer in DC1-DC3 are the same within approximately +/-0.1 kT. The lo wer reorganization energy for DC2 results in a slightly lower activati on energy for this complex compared with DC1 and DC3. The long heme-co pper distance (similar to 24 Angstrom) in the DC1 complex drastically decreases electronic coupling and makes this complex much less favorab le for electron transfer than DC2 or DC3. DC1-like complexes can only serve as docking intermediates in the pathway toward formation of an e lectron-transfer-competent complex. Elimination of the four positive c harges arising from the lysine residues in the positive patch of cytoc hrome f, as accomplished by mutagenesis, exerts a negligible effect (s imilar to 3 mV) on the redox potential difference between cyt f and PC .