ELECTRON-TRANSFER KINETICS AND ELECTROSTATIC PROPERTIES OF THE RHODOBACTER-SPHAEROIDES REACTION CENTER AND SOLUBLE C-CYTOCHROMES

The kinetics of electron transfer between the Rhodobacter sphaeroides R-26 reaction center and nine soluble c-cytochromes have been analyzed and compared to the patterns of the surface electrostatic potentials for each of the proteins. Characteristic first-order electron-transfer rates for 1:1 complexes formed at low ionic strength between the reac tion center and the different c-cytochromes were identified and found to vary by a factor of almost 100, while second-order rates were found to differ by greater than 10(6). A correlation was found between the location of likely electrostatic interaction domains on each cytochrom e and its characteristic rate of electron transfer. The interaction do mains were identified by mapping electrostatic potentials, calculated from the Poisson-Boltzmann equation, onto simulated ''encounter surfac es'' for each of the cytochromes and the reaction center. For the reac tion center, the c-cytochrome binding domain was found to have almost exclusively net negative potential (<-3 kT) and to be shifted slightly toward the M-subunit side of the reaction center. The location of int eraction domains of complementary, positive potential (>3 kT) differed for each cytochrome. The correspondence between electrostatic, struct ural, and kinetic properties of 1:1 reaction center-cytochrome complex es leads to a proposed mechanism for formation of reaction center-cyto chrome electron-transfer complexes that is primarily driven by the jux taposition of regions of delocalized complementary potential. In this mechanism the clustering of charged residues is of primary importance and not the location of specific residues. A consequence of this mecha nism is that many different sets of charge distributions are predicted to be capable of stabilizing a specific configuration for a reaction center-cytochrome complex. This mechanism for reaction center associat ion with water-soluble c-cytochromes fits molecular recognition mechan isms proposed for c-cytochromes in nonphotosynthetic systems. In gener al, the kinetic scheme for reaction center driven cytochrome oxidation was found to vary between a simple two-state model, involving cytochr ome in free and reaction center bound states, and a three-state model, that includes cytochrome binding in kinetically competent(''proximal' ')and incompetent (''distal'') modes. The kinetically incompetent mode of cytochrome binding is suggested not to be an intrinsic feature of the reaction center-cytochrome association but is likely to be due to variation in the physical state of the reaction center.