Respiratory Chain Complex II and Succinate:Quinone Oxidoreductases

Abstract

The superfamily of succinate:quinone oxidoreductases is of particular interest, because these membrane protein complexes can be involved in either aerobic respiration, as succinate:quinone reductases (respiratory complex II), coupling the oxidation of succinate to fumarate to the reduction of quinone to quinol, or anaerobic respiration, as quinol:fumarate reductases, catalyzing the reverse reaction, coupling quinol oxidation to fumarate reduction. Members consist of two hydrophilic subunits, conserved throughout the superfamily, and one large or two small hydrophobic subunits, varying in their composition of bound heme molecules. Based on these functional and structural differences, this contribution describes different functional and structural classifications of the superfamily, provides an overall description of the available high-resolution crystal structures and the different relative orientations of soluble and membrane-embedded subunits. In addition to the compositions and structures of the individual subunits, a focus is on the electron and proton transfer events associated with catalysis. In this context, the di-heme containing members are of special interest, because they support electron transfer across the biological membranes in which they are embedded. The di-heme containing quinol:fumarate reductases, found to be essential in selected human pathogens for host colonization, support an essential transmembrane proton transfer process coupled to transmembrane electron transfer.