Mutational Studies of the Cytochrome bc1 Complexes

Abstract

SummaryThe ubihydroquinone: cytochrome c oxidoreductase (Cyt bc1 complex) is a multisubunit integral membrane protein present in a wide range of organisms. In the photosynthetic bacteria the Cyt bc1 complex is involved in both light driven cyclic electron transfer as well as respiratory electron transfer. It oxidizes ubihydroquinone generated either by the reaction center or the respiratory dehydrogenases and reduces Cyt c2 (or Cyt cy) which then either re-reduces the reaction center or donates electrons to a terminal cytochrome c oxidase. During electron transfer, protons are translocated from the cytoplasm to the periplasm and contribute to the formation of an electrochemical gradient used for ATP synthesis. A large body of spectroscopic data supports a Q-cycle mechanism of electron transfer through the Cyt bc1 complex. The enzyme is composed of three metal containing subunits, Cyt b, Cyt c1 and the ‘Rieske’ iron sulfur protein. The structural genes encoding these three core subunits (designated pet A, B and C or fbcF, B and C) have now been cloned and sequenced from several photosynthetic bacteria. Two of them, Rhodobacter capsulatus and Rhodobacter sphaeroides, have been genetically modified to allow site-directed mutagenesis, and a number ofsingle site mutants have been obtained in each of the three subunits. In recent years, the analyses of both site-directed mutants and spontaneous inhibitor resistant mutants have led to the identification of the ligands to the metal clusters, the general location of the sites of quinone interaction and more accurate folding models of the individual subunits of the complex. This chapter reviews recent biochemical and genetic studies (inclusive of 1994) that have greatly increased our understanding of how the Cyt bc1 complex functions at the molecular level.