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

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 ci absorption maxima at 554 nm, 552 nm and 556 nm respectively. The 380-mV heme displays a split ci band with a maximum at 559 nm and a shoulder at 552 nm. Such a splitting is due to non-degenerated Qx and Q, transitions in the ironporphyrin ring as demonstrated by magnetic circular dichroism spectra. 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 P-960' (z = 0.32 ps) whereas heme c-556 serves to rereduce c-559 (z = 2.5 ps). Upon reduction of the third heme (c-552), the P-960' reduction rate increases twofold (z = 0.17 ps) and all photoinduced redox events within the cytochrome appear to be complete in less than 1 ps after the 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 incorporated into asolectin liposomes, and fast kinetics of laser flash-induced electric potential difference has been measured in proteoliposomes adsorbed on a phospholipid-impregnated film. The electrical difference induced by a single 15-11s 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; (11) reduction of P-960+ by heme c-559; (111) reduction of c-559 by c-556 and (IV) protonation of Qg-. The relative contributions of stages I, 11,111 and IV are found to be equal to 70%, 15%, 5% and lo%, 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.The concept of transmembrane electron flow in the coupling sites of redox chains was put forward by Mitchell virtually simultaneously with the general chemiosmotic principle of energy coupling via protonmotive force [l]. Whereas the general formulation of the chemiosmotic theory proved to be accepted by the majority of bioenergeticists, there is still considerable debate on the electrogenic transmembrane electron transfer as a mechanism of ApH generation.There are many indications that electron transfer across the coupling membrane does indeed occur in reaction centre complexes of photosynthetic bacteria (reviewed...

Tuning Heme Redox Potentials in the Cytochrome c Subunit of Photosynthetic Reaction Centers

“…Heme Redox Potentials-The experimental determination of the heme redox potentials is performed by varying the solution redox potential and monitoring the heme redox states by UVvisible spectroscopy (6,7). In doing so, the special pair remains continuously in the reduced state because of its high redox potential of 500 mV (3) and the lack of strong illumination at a wavelength suitable for an effective photo-oxidation.…”

“…In the cyt c complex of the R. viridis RC, electrons are transferred through a linear chain of low-high-low-high potential cyt c hemes (heme number 4 -3-2-1) to the special pair chlorophyll dimer where photo-oxidation takes place (5,6). The redox potentials of the four heme groups were determined by various experimental approaches (5)(6)(7)(8).…”

“…The redox potentials of the four heme groups were determined by various experimental approaches (5)(6)(7)(8). Accordingly, the midpoint potentials of hemes 1 to 4 are at 370, 20, 300, and Ϫ60 mV, respectively (7).…”

Shortcut of the photosynthetic electron transfer in a mutant lacking the reaction center‐bound cytochrome subunit by gene disruption in a purple bacterium, Rubrivivax gelatinosus

“…The electron transfer function and characterization of hemes in the cytochrome subunit has been widely studied in various purple bacteria (e.g, R. viridis [2][3][4][5][6][7][8][9][10][11][12], Rhodospirillum molischianum [13], Rubrivivax gelatinosus [14][15][16], Rhodof erax fermentans [17], Chromatium vinosum [18], and Rhodopseudomonas acidophila [19]). These studies have presented that the four hemes in the cytochrome subunit differ in redox midpoint potentials (Em) and peak wavelengths of the a-absorption bands, as shown in a recent review by Nitschke and Dracheva *Corresponding author.…”

The roles of the two proton input channels in cytochrome c oxidase from Rhodobacter sphaeroides probed by the effects of site-directed mutations on time-resolved electrogenic intraprotein proton transfer

“…It is also associated with the transmembrane pumping of at least one, or probably two, protons (3,23). Vectorial movements of these protons within the membrane-incorporated enzyme result in the generation of a transmembrane electric potential difference (⌬⌿) that can be monitored electrometrically with submicrosecond time resolution using the method developed by Drachev (24)(25)(26).…”