Acidianus ambivalens complex II typifies a novel family of succinate dehydrogenases

Complex II from the thermoacidophilic archaeon Acidianus ambivalens, an arc hetype of an emerging class of succinate dehydrogenases (SDH), was extracte d from intact membranes and purified to homogeneity. The complex contains o ne molecule of covalently bound FAD and 10 Fe atoms. EPR studies showed tha t the complex contains the canonical centres S1 ([2Fe-2S](2+/1+)) and S2 ([ 4Fe-4S](+2/+1)) but lacks centre S3 ([3Fe-4S](+1/0)); these observations ag ree with the fact that the iron-sulfur subunit contains an extra cysteine t hat may allow the binding of a new centre, most probably a tetranuclear one . Succinate-driven oxygen consumption is observed in intact membranes indic ating that in vivo, complex II operates as a succinate:quinone oxidoreducta se, despite missing the typical anchor domain subunits. The pure complex wa s found to contain bound caldariella quinone, the enzyme physiological part ner. An alternative membrane anchoring for this new type of SDHs, based on the amphipathic nature of the putative helices found in SdhC, is suggested. (C) 2001 Academic Press.