Mutations that Affect the Donor Midpoint Potential in Reaction Centers from Rhodobacter Sphaeroides

Abstract

Modulation of the redox midpoint potential of the initial electron donor is key for achieving electron transfer with high yields in photosynthetic systems. The initial electron donor in reaction centers of purple nonsulfur bacteria, P, has a much lower potential than the donor in photosystem II reaction centers, which are capable of oxidizing water (for general reviews of chlorophylls and their oxidation potentials, see ref. 1). Isolated bacteriochlorophyll and chlorophyll have similar oxidation potentials of approximately 700 mV and 800 mV, respectively. However, in vivo the midpoint oxidation potential of the donor in bacterial systems, approximately 490 mV, is much lower than that of monomer bacteriochlorophyll, while that of the donor in photosystem II is higher than 1.0 V. This dramatic alteration of the oxidation potentials in the reaction center must be due to interactions between chlorophylls or between the donor and the surrounding protein. This is supported by the observation of changes of approximately 100 mV in the oxidation potentials for bacteriochlorophylls and chlorophylls due to changes in solvent1. In this paper we address the role of specific protein-donor interactions in modulating the potential.