COUPLING OF CYTOCHROME AND QUINONE TURNOVERS IN THE PHOTOCYCLE OF REACTION CENTERS FROM THE PHOTOSYNTHETIC BACTERIUM RHODOBACTER-SPHAEROIDES

A minimal kinetic model of the photocycle, including both quinone (Q-6 ) reduction at the secondary quinone-binding site and (mammalian) cyto chrome 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 co ntinuous illumination. The typical rate of photochemical excitation by a laser diode at 810 nm was 2.200s(-1), and the rates of stationary t urnover of the reaction center (one-half of that of cytochrome photoox idation) were 600 +/- 70s(-1) at pH6 and 400 +/- 50s(-1) at pH 8. The rate of turnover showed strong pH dependence, indicating the contribut ion of different rate-limiting processes. The kinetic limitation of th e photocycle was attributed to the turnover of the cytochrome c bindin g site (pH < 6), light intensity and quinone/quinol exchange (6 < pH < 8), and proton-coupled second electron transfer in the quinone accept or complex (pH > 8). The analysis of the double-reciprocal plot of the rate of turnover versus light intensity has proved useful in determin ing the light-independent (maximum) turnover rate of the reaction cent er (445 +/- 50s(-1) at pH7.8).