Light and Redox-Linked H+ Translocation: Pumps, Cycles, and Stoichiometry
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 bc1 (b6f) complex in respiratory and photosynthetic membranes (section 7.4). The bacteriorhodopsin and cytochrome oxidase pump systems, as well as the chloroplast CFo-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.
KeywordsPolypeptide Photosynthesis Carotenoid Succinate Quinone
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