The archaeal respiratory supercomplex SoxM from S. acidocaldarius combines features of quinole and cytochrome c oxidases.

The hyperthermoacidophilic archaeon Sulfolobus acidocaldarius has a unique respiratory system with at least two terminal oxidases. Genetic and preliminary biochemical studies suggested the existence of a unique respiratory supercomplex, SoxM. Here we show (i) that all respective genes are translated into polypeptides, and (ii) that the supercomplex can be separated from the alternative oxidase SoxABCD and in that way characterized in a catalytically competent form for the first time. It acts as a quinol oxidase and contains a total of seven metal redox centers. One of it--the blue copper protein sulfocyanin--functionally links two subcomplexes. One is a bb3-type terminal oxidase moiety containing CuA and CuB, whereas the other consists of a Rieske FeS-protein and a homolog to cytochrome b--in this case hosting two hemes As. Based on a 1:1 stoichiometry, 1 mol complex contains 6 mol Fe and 4 mol Cu. Its activity is completely inhibited by cyanide and strongly by aurachin-C and -D derivatives as inhibitors of the quinol binding site. These data suggest that the complex provides two proton pumping sites. Interestingly, subunit-II reveals an unusual pH dependence and is proposed to act as a pH sensor as well as a regulator of catalytic activity via a reversible transition between two states of the CuA ligation. This is a novel hint at how S. acidocaldarius can adapt to and survive in its extreme natural environment.