The effect of cytochrome c, hexammineruthenium and ubiquinone-10 on the kinetics of photoelectric responses of Rhodospirillum rubrum reaction centres

Abstract

Rhodospirillum rubrum reaction centre (RC) complexes with or without antenna bacteriochlorophyll have been incorporated into liposomes; the latter were associated with a phospholipid-impregnated collodion film, and flash-induced electric responses were measured. It is shown that in the antenna-free RC-proteoliposomes a very rapid phase of charge separation (less than 200 ns) due to P-870 → Q A electron transfer has a polarity (negative inside) opposite to that of the intact chromatophores or of the antenna-containing RC-proteoliposomes. In the antenna-free RC-proteoliposomes (but not in the chromatophores), P-870 is readily accessible to membrane-impermeable cationic electron donors like cytochrome c and Ru(NH 3) 2+ 6, which indicates localization of P-870 near the outer surface of the membrane. Oxidation of externally added cytochrome c 2+ by P-870 + in these proteoliposomes initiates an additional slower phase of ΔΨ generation comprising about 25% of the total photoelectric response. The dependence of the slower phase rate on the cytochrome c concentration is characteristic of a saturating reaction with K m of 0.9 μM and k max of approx. 6 · 10 3 s −1 at low ionic strength. A similar electrogenic phase can be observed without cytochrome c in the presence of sufficiently high concentrations of artificial redox dyes, e.g., 2 mM TMPD or more than approx. 50 μM PMS. The new phase of ΔΨ generation is tentatively ascribed to e − transfer within reaction centre molecule from the protein surface to the porphyrin ring of P-870 +.