Proton conduction within the reaction center of Rhodobacter capsulatus: The electrostatic role of the protein
- Centre National de la Recherche Scientifique, Gif-Yvette (France)
- Argonne National Lab., IL (United States)
Light-induced charge separation in the photosynthetic reaction center results in delivery of two electrons and two protons to the terminal quinone acceptor Q[sub B]. In this paper, the authors have used flash-induced absorbance spectroscopy to study three strains that share identical amino acids sequences in the Q[sub B] binding site, all of which lack the protonatable amino acids Glu-L212 and Asp-L213. These strains are the photosynthetically incompetent site-specific mutant Glu-L212/Asp-L213[yields]Ala-L212/Ala-L213 and two different photocompetent derivatives that carry both alanine substitutions and an intergenic suppressor mutation located far from Q[sub B]. At pH 8 in the double mutant, they observe a concomitant decrease of nearly 4 orders of magnitude in the rate constants of second electron and proton transfer to Q[sub B] compared to the wild type. Surprisingly, these rates are increased to about the same extent in both types of suppressor strains but remain >2 orders of magnitude smaller than those of the wild type. In the double mutant, at pH 8, the loss of Asp-L213 and Glu-L212 leads to a substantial stabilization ([>=]60 meV) of the semiquinone energy level. Both types of compensatory mutations partially restore, to nearly the same level, the original free energy difference for electron transfer from primary quinone Q[sub A] to Q[sub B]. The pH dependence of the electron and proton transfer processes in the double-mutant and the suppressor strains suggest that when reaction centers of the double mutant are shifted to lower pH (1.5-2 units), they function like those of the suppressor strains at physiological pH. The data suggest that the main effect of the compensatory mutations is to partially restore the negative electrostatic environment of Q[sub B] and to increase an apparent [open quotes]functional[close quotes] pK of the system for efficient proton transfer to the active site.
- OSTI ID:
- 7028356
- Journal Information:
- Proceedings of the National Academy of Sciences of the United States of America; (United States), Journal Name: Proceedings of the National Academy of Sciences of the United States of America; (United States) Vol. 91:12; ISSN PNASA6; ISSN 0027-8424
- Country of Publication:
- United States
- Language:
- English
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