Proton-Coupled Electron Transfer Reactions of QB in Reaction Centers from Photosynthetic Bacteria

Abstract

The reaction center from purple bacteria mediates the initial steps of a light driven proton pump, coupling light induced electron transfer to proton uptake. The key steps in this reaction involve the two electron reduction of the secondary quinone QB with the concomitant uptake of 2 protons: 2H++2e−+QB→QBH2 Recent advances in studies of reaction centers from purple bacteria have provided insight into the pathway of proton transfer into the RC and the mechanism of coupling proton and electron transfer reactions. Studies using site directed mutagenesis have identified 3 residues near QB that are important for proton transfer in RCs from Rb. sphaeroides. The mutation of Ser L223 and Asp L213 to Ala and Asn, respectively, block proton transfer and the second electron transfer involved in QB reduction. Mutation of Glu L212 to Gln blocks proton transfer associated with QB reduction but 2 electron reduction of QB still occurs. The model explaining these results is that Ser L223 and Asp L213 are involved in the proton transfer pathway for uptake of the first proton which is bound prior to the second electron transfer, whereas Glu L212 is involved in the pathway for the second proton which is bound after the second electron transfer. These results clearly show that specific residues play important roles in proton transfer. However other functionally active mutant RCs have been obtained in which either Asp L213, Ser L223 or Glu L212 were absent indicating that the pathways for proton transfer are not unique. The structure of the RC near the QB site suggests the involvement of water molecules in addition to protein residues in the proton transfer chain to QB. In addition, negatively charged residues near QB increase the proton coupled electron transfer rate by stabilizing a proton in the interior of the RC.