title: The unusual iron sulfur composition of the Acidianus ambivalens succinate dehydrogenase complex.
abstract: The succinate dehydrogenase complex of the thermoacidophilic archaeon Acidianus ambivalens was investigated kinetically and by EPR spectroscopy in its most intact form, i.e., membrane bound. Here it is shown that this respiratory complex has an unusual iron-sulfur cluster composition in respect to that of the canonical succinate dehydrogenases known. The spectroscopic studies show that center S3, the succinate responsive [3Fe-4S]1+/0 cluster of succinate dehydrogenases, is not present in membranes prepared from aerobically 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 findings were made in other archaea, namely Acidianus infernus and Sulfolobus solfataricus. Kinetic investigations showed that the A. ambivalens enzyme is reversible, capable of operating as a fumarate reductase - a required activity if this obligate autotroph performs CO2 fixation via a reductive citric acid cycle. Sequencing of the sdh operon confirmed the spectroscopic data. Center S3 ([3Fe-4S]) is indeed replaced by a second [4Fe-4S] center, by incorporation of an additional cysteine, at the cysteine cluster binding motif (CxxYxxCxxxC-->CxxCxxCxxxC). Genomic analysis shows that genes encoding for succinate dehydrogenases similar to the ones here outlined are also present in bacteria, which may indicate a novel family of succinate/fumarate oxidoreductases, spread among the Archaea and Bacteria domains.
authors.affiliation: Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa, Apt 127, 2780, Oeiras, Portugal.
authors.forename: C M
authors.initials: CM
authors.lastname: Gomes
authors.forename: R S
authors.initials: RS
authors.lastname: Lemos
authors.forename: M
authors.initials: M
authors.lastname: Teixeira
authors.forename: A
authors.initials: A
authors.lastname: Kletzin
authors.forename: H
authors.initials: H
authors.lastname: Huber
authors.forename: K O
authors.initials: KO
authors.lastname: Stetter
authors.forename: G
authors.initials: G
authors.lastname: Schäfer
authors.forename: S
authors.initials: S
authors.lastname: Anemüller
mesh_terms: D000595:Amino Acid Sequence; D004578:Electron Spin Resonance Spectroscopy; D007506:Iron-Sulfur Proteins / Q000737:chemistry*; D007700:Kinetics; D008969:Molecular Sequence Data; D016415:Sequence Alignment; D013385:Succinate Dehydrogenase / Q000737:chemistry*; D016943:Sulfolobaceae / Q000201:enzymology*
chemical_list: D007506:Iron-Sulfur Proteins; D013385:Succinate Dehydrogenase