Light and Redox-Linked H⁺ Translocation: Pumps, Cycles, and Stoichiometry

Abstract

Two mechanisms of H⁺ translocation have been discussed thus far, the uptake and release of H⁺ by ubi- and plastoquinone (Chap. 5), and the release of H⁺ in the photosynthetic water splitting reaction (Chap. 6.8). The probable involvement of protein acid-base groups in the pathway of H⁺ uptake to QA and QB in the photosynthetic reaction center was discussed in Chap. 5, section 5.8. Particular proteins are known to function in H⁺ translocation. Many of the ideas on structure and function of proteinaceous H⁺ pumps originated with bacteriorhodopsin (section 7.2). An H⁺ pump is an energy-linked H⁺ translocation system in a transmembrane protein or protein complex. Transmembrane H⁺ pump function has also been documented for the electron transport protein complex cytochrome oxidase of respiratory membranes (section 7.3), and has been proposed for the cytochrome bc ₁ (b ₆ f) complex in respiratory and photosynthetic membranes (section 7.4). The bacteriorhodopsin and cytochrome oxidase pump systems, as well as the chloroplast CF_o-CF₁ ATPase pump, all have a characteristic maximum rate of H⁺ translocation, 200–500 H⁺/pump-s, corresponding to a common millisecond rate-limiting step and implying a common mechanism.