PROTON CONDUCTION WITHIN THE REACTION CENTERS OF RHODOBACTER-CAPSULATUS - THE ELECTROSTATIC ROLE OF THE PROTEIN

MAROTI P HANSON DK BACIOU L SCHIFFER M SEBBAN P

P. Maroti et al., PROTON CONDUCTION WITHIN THE REACTION CENTERS OF RHODOBACTER-CAPSULATUS - THE ELECTROSTATIC ROLE OF THE PROTEIN, Proceedings of the National Academy of Sciences of the United Statesof America, 91(12), 1994, pp. 5617-5621

19

INGLESE

Article

Multidisciplinary Sciences

Proceedings of the National Academy of Sciences of the United Statesof America → ACNP

0027-8424

91

12

1994

5617 - 5621

ISI

0027-8424(1994)91:12<5617:PCWTRC>2.0.ZU;2-E

Light-induced charge separation in the photosynthetic reaction center results in delivery of two electrons and two protons to the terminal q uinone acceptor Q(B). In this paper, we have used flash-induced absorb ance spectroscopy to study three strains that share identical amino ac id sequences in the Q(B) binding site, all of which lack the protonata ble amino acids Glu-L212 and Asp-L213. These strains are the photosynt hetically incompetent site-specific mutant Glu-L212/Asp-L213 --> Ala-L 212/Ala-L213 and two different photocompetent derivatives that carry b oth alanine substitutions and an intergenic suppressor mutation locate d far from Q(B) (class 3 strain, Ala-Ala + Arg-M231 --> Leu; class 4 s train, Ala-Ala + Asn-M43 --> Asp). At pH 8 in the double mutant, we ob serve a concomitant decrease of nearly 4 orders of magnitude in the ra te constants of second electron and proton transfer to Q(B) compared t o the wild type. Surprisingly, these rates are increased to about the same extent in both types of suppressor strains but remain >2 orders o f 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 sta bilization (greater than or equal to 60 meV) of the semiquinone energy level. Both types of compensatory mutations partially restore, to nea rly the same level, the original free-energy difference for electron t ransfer from primary quinone Q(A) to Q(B) The pH dependence of the ele ctron and proton transfer processes in the double-mutant sind the supp ressor strains suggests that when reaction centers of the double mutan t are shifted to lower pH (1.5-2 units), they function like those of t he suppressor strains at physiological pH. Our data suggest that the m ain effect of the compensatory mutations is to partially restore the n egative electrostatic environment of Q(B) and to increase an apparent ''functional'' pK of the system for efficient proton transfer to the a ctive site. This emphasizes the role of the protein in tuning the elec trostatic environment of its cofactors and highlights the possible lon g-range electrostatic effects.