Kinetic limitations in turnover of photosynthetic bacterial reaction center protein

Abstract

The membrane-bound reaction center from purple nonsulfur photosynthetic bacterium Rhodobacter sphaeroides performs light-induced charge separation and exports two molecules of oxidized cytochrome and one molecule of fully reduced quinone (quinol) from two opposite (periplasmic and cytoplasmic) sites of the protein during a single turnover. The rate of the turnover was measured based on cytochrome photooxidation under intense and continuous illumination of a laser diode that mimicks the open field conditions for photosynthetic organisms. At the highest intensity of illumination, the primary photochemistry was driven by a rate constant of 4,800 s-1. Factors limiting the steady-state turnover rate of the reaction center were studied under wide variety of conditions of light intensity, ionic strength, heavy metal ion binding and pH. The reaction center was solubilized in detergent, and reduced mammalian cytochrome c and oxidized ubiquinone were the exogenous electron donor and acceptor to the reaction center, respectively. Depending on the conditions, the kinetic limitation was attributed to light intensity, or to donor and acceptor side reactions. At low light intensities (<1,000 s -1, in terms of initial rate of photochemistry), the turnover rate was limited only by the light intensity. At higher intensities, however, the unbinding of the oxidized cytochrome at the donor side (acidic and neutral pH range) or the proton coupled interquinone electron transfer (alkaline pH range) were the bottlenecks of the turnover of the reaction center. The possible entries and pathways of protons to the secondary quinone are discussed based on multiple turnover of the reaction center.