Catalytic Sites for Reduction and Oxidation of Quinones

Abstract

The determination of the structure of the Rps. viridis reaction center at 3 A resolution¹ has provided a great deal of information of direct relevance to the many other membrane proteins which are yet to be solved crystallographically. Two recent reviews^2,3 have discussed putative structures for the quinol oxidizing complexes, and independently suggested a structure (the Cramer-Widger-Saraste-Wikstrom or CWSW structure) for the main subunit of the complexes (the cytochrome b subunit) in which the major structural elements are a set of nine membrane spanning helices. Two of these helices have a special role in providing pairs of histine residues to ligand the two b-type hemes of the complex. In this brief paper, we wish to review the structure of the cytochrome b subunit in the light of additional information from the reaction center structure, and from inhibitor resistant mutants, We will discuss first the structure of the catalytic site of the reaction center at which quinone is reduced, and our attempts to predict a tertiary structure for the analogous site on the photosystem II reaction center. We will examine the location of lesions giving rise to inhibitor resistance, and the information these provide about mechanism. We will then discuss the structure of the ubiquinol:cyt C₂ oxidoreductases of R. capsulatus and R. sphaeroides, the location of the quinone reactive catalytic sites, and the role of inhibitor resistant mutants in elucidating the structure.