Coupling of cytochrome and quinone turnovers in the photocycle of reaction centers from the photosynthetic bacterium Rhodobacter sphaeroides.

AUTOR(ES)

Osváth, S

RESUMO

A minimal kinetic model of the photocycle, including both quinone (Q-6) reduction at the secondary quinone-binding site and (mammalian) cytochrome c oxidation at the cytochrome docking site of isolated reaction centers from photosynthetic purple bacteria Rhodobacter sphaeroides, was elaborated and tested by cytochrome photooxidation under strong continuous illumination. The typical rate of photochemical excitation by a laser diode at 810 nm was 2.200 s-1, and the rates of stationary turnover of the reaction center (one-half of that of cytochrome photooxidation) were 600 +/- 70 s-1 at pH 6 and 400 +/- 50 s-1 at pH 8. The rate of turnover showed strong pH dependence, indicating the contribution of different rate-limiting processes. The kinetic limitation of the photocycle was attributed to the turnover of the cytochrome c binding site (pH < 6), light intensity and quinone/quinol exchange (6 < pH < 8), and proton-coupled second electron transfer in the quinone acceptor complex (pH > 8). The analysis of the double-reciprocal plot of the rate of turnover versus light intensity has proved useful in determining the light-independent (maximum) turnover rate of the reaction center (445 +/- 50 s-1 at pH 7.8).

Documentos Relacionados

• Femtosecond coherent transient infrared spectroscopy of reaction centers from Rhodobacter sphaeroides.
• A quorum-sensing system in the free-living photosynthetic bacterium Rhodobacter sphaeroides.
• Specific alteration of the oxidation potential of the electron donor in reaction centers from Rhodobacter sphaeroides.
• Study of wild type and genetically modified reaction centers from Rhodobacter capsulatus: structural comparison with Rhodopseudomonas viridis and Rhodobacter sphaeroides.
• Initial electron-transfer in the reaction center from Rhodobacter sphaeroides.