The Pathway, Kinetics and Thermodynamics of Electron Transfer in Wild Type and Mutant Reaction Centers of Purple Nonsulfur Bacteria

Abstract

SummaryThe light driven electron transfer reactions of photosynthesis are best characterized in the reaction centers of purple nonsulfur bacteria such as Rhodobacter sphaeroides. These reactions are extraordinarily fast: the first of them taking place within a few picoseconds. Because photosynthetic electron transfer is initiated by light, and the redox active cofactors have distinct spectral signatures, both the kinetics and thermodynamics of the reaction can be studied on the timescale of the actual reactions. This, in combination with the availability of detailed structural information, has made the electron transfer reactions of purple nonsulfur bacteria some of the most completely characterized electron transfer reactions in biological systems. In addition to the spectroscopic tools which have been used to characterize the electron transfer reactions, the reaction center has also been the subject of mutagenesis studies. The ease with which genetic engineering can be performed in these bacteria, coupled with the structurally robust nature of the reaction center, has resulted in many mutations which affect the early electron transfer reactions. Critical protein-cofactor interactions have now been identified and analyzed which affect the metal content, redox potentials, electronic structure and intermolecular coupling of the bacteriochlorophylls and bacteriopheophytins of the reaction center. In combination, these studies have suggested various possible mechanisms for early electron transfer which involve both the electronic states of the cofactors as well as nuclear conformational changes in the surrounding protein.