ELECTRON TRANSFERS OF REACTION CENTER HYBRID PROTEIN SYSTEMS

The study of the functional details of the interaction of major bioenergetic electron transfer proteins with the redox carriers cytochrome c and quinone can benefit from the extreme conservatism of bioenergetic systems throughout evolution. Because virtually all organisms use quinone and/or c type cytochromes as redox carriers, and because redox carriers from one species can often interact with electron transfer proteins from remotely related species, it is possible to construct from purified proteins a wide variety of photosynthetic reaction center hybrid protein systems in which a flash of light initiates electron transfer reactions through cytochrome c and quinone. Such systems introduce the advantages of single-turnover reactions in which it is possible to uncover the elementary first and second order electron transfer reactions on a microsecond and millisecond time scale, and the ability to carefully control and monitor electron transfer in an exact electron accounting. In addition, as a pulse relaxation technique, it is possible to perform multiple identical experiments on the same sample under the desired conditions of redox poise and inhibition to select the reactions of interest. The particular reactions of cytochrome c and quinone with the major electron transfer proteins photosynthetic reaction center from Rhodopseudomonas sphaeroides, mammalian mitochondrial cytochrome bc(,1) complex and cytochrome oxidase, and Escherichia coli quinol oxidases cytochrome bd complex and cyt o are studied. These reveal the importance of product inhibition and rotational motion in the electron transfer from the bound states of cytochrome c, and the significance of the chemical structure and partitioning of quinone in detergent solubilized and proteoliposome hybrid proteins systems. The effect of artificial quinone pools on the reactions at the Qc and Qz sites and the redox potential sensitivity of the cytochrome b oxidation reaction support a Q-cycle model of bc(,1) complex operation over a b-cycle model. The experiment with the E. coli quinol oxidases support models in which quinol oxidation requires both cytochromes b to be reduced in cytochrome o, but is independent of cytochrome d oxidation state in the cytochrome bd complex