Kinetics and Energetics of Electron Transfer Reactions in a Photosynthetic Bacterial Reaction Center Assembled with Zinc Bacteriochlorophylls

Abstract

Electron transfer processes were studied in the reaction center (RC) of a Rhodobacter sphaeroides magnesium chelatase (bchD) mutant that assembles with six chemically identical chlorin molecules. A previous study [Jaschke & Beatty, Biochemistry, 2007] and this work show the complete absence of bacteriochlorophyll (containing Mg as the metal) and bacteriopheophytin from the bchD mutant RC. Instead, bacteriochlorophylls containing a Zn atom as the metal (Zn-BChl) occupy the binding sites of the special pair (P), accessory bacteriochlorophyll (B), and primary electron acceptor (H). In spite of significant differences in cofactor composition, electron transfer from excited P through B to H proceeds with high efficiency and with rates nearly identical to the wild type RC. The rate of electron transfer from H to QA is also the same as that observed in the wild type RC. Thus, the protein-cofactor interactions, mainly through electron sharing between the metal of the BChl and the protein, play an important role in adjusting the energies of the cofactors to form an efficient electron transfer system. The study also suggests that the overall electron transfer from P to H is more sensitive to the energy change between P and B than B and H, and can tolerate a large variation in the redox energy of H.