Electrogenic steps in the redox reactions catalyzed by photosynthetic reaction‐centre complex fromRhodopseudomonas viridis

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Abstract
Electrogenic and redox events in the reaction-centre complexes from Rhodopseudomonas viridis have been studied. In contrast to the previous points of view it is shown that all the four hemes of the tightly bound cytochrome c have different Em values (-60, +20, +310 and +380 mV). The first three hemes reveal .alpha. absorption maxima at 554 nm, 552 nm and 556 nm respectively. The 380-mV heme displays a split a band with a maximum at 559 nm and a shoulder at 592 nm. Such a splitting is due to non-degenerated Qx and Qy transitions in the ion-porphyrin ring as demonstrated by magnetic circular dichroism specra. Fast kinetic measurements show that, at redox potentials when only high-potential hemes c-559 and c-556 are reduced, heme c-559 appears to be the electron donor to P960+ (.tau. = 0.32 .mu.s) whereas heme c-556 serves to rereduce c-559 (.tau. = 2.5 .mu.s). Upon reduction of the third heme (c-552), the P-960+ reduction rate increases twofold (.tau. = 0.17 .mu.s) and all photoinduced redox events within the cytochrome appear to be complete in less than 1 .mu.s after flash. The following sequence of the redox centers is tentatively suggested: c-554, c-556, c-552, c-559, P-960. To study electrogenesis, the reaction-centre complexes from Rps. viridis were incorported into asolectin liposomes, and fast kinetics of laser flash-induced electric potential potential difference has been measured in proteoliposomes adsorbed on a phospholipid-impregnated film. The electrical difference induced by a single 15-ns flash was found to be as high as 100 mV. The photoelectric response has been found to involve four electrogenic stages associated with (I) QA reduction by P-960; (II) reduction of P-960+ by heme c-559; (III) reduction of c-559 by c-556 and (IV) protonation of QB2-. The relative contributions of stages I, II, III and IV are found to be equal to 70%, 15%, 5% and 10%, respectively, of the overall electrogenic process. At the same time, the first three respective distances along the axis normal to the membrane plane covered by electrons, calculated from X-ray data of Deisenhofer et al. [J. Mol. Biol. 180, 385-398 (1984)], are 22%, 18.5 and 26%. This indicates that the efficiency of electrogenic phases depends first of all upon the value of the dielectric constant of the respective membrane regions rather than upon the distance between the redox groups involved. The efficiency is higher, the deeper these groups are immersed in the membrane.

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