The unusual iron sulfur composition of the Acidianus ambivalens succinate dehydrogenase complex

The succinate dehydrogenase complex of the thermoacidophilic archaeon Acidi anus ambivalens was investigated kinetically and by EPR spectroscopy in its most intact form, i.e., membrane bound. Here it is shown that this respira tory complex has an unusual iron-sulfur cluster composition in respect to t hat of the canonical succinate dehydrogenases known. The spectroscopic stud ies show that center S3, the succinate responsive [3Fe-4S](1+/0) cluster of succinate dehydrogenases, is not present in membranes prepared from aerobi cally grown A. ambivalens, nor in partially purified complex fractions. On the other hand, EPR features associated to the remaining centers, clusters S1 ([2Fe-2S](1+/2+)) and S2 ([4Fe-4S](2+/1+)), could be observed. Similar f indings were made in other archaea, namely Acidianus infernus and Sulfolobu s solfataricus. Kinetic investigations showed that the A. ambivalens enzyme is reversible, capable of operating as a fumarate reductase-a required act ivity if this obligate autotroph performs CO2 fixation via a reductive citr ic acid cycle. Sequencing of the sdh operon confirmed the spectroscopic dat a. Center S3 ([3Fe-4S]) is indeed replaced by a second [4Fe-4S] center, by incorporation of an additional cysteine, at the cysteine cluster binding mo tif (CxxYxxCxxxC --> CxxCxxCxxxC). Genomic analysis shows that genes encodi ng for succinate dehydrogenases similar to the ones here outlined are also present in bacteria, which may indicate a novel family of succinate/fumarat e oxidoreductases, spread among the Archaea and Bacteria domains. (C) 1999 Elsevier Science B.V. All rights reserved.