Conservation of energy based on the reduction of sulfate is of fundamental importance for the biogeochemical sulfur cycle. A key enzyme of this ancient anaerobic process is the dissimilatory sulfite reductase (dSir), which catalyzes the six-electron reduction of sulfite to hydrogen sulfide under participation of a unique magnetically coupled siroheme-[4Fe-4S] center. We determined the crystal structure of the enzyme from the sulfate-reducing archaeon Archaeoglobus fulgidus at 2-A resolution and compared it with that of the phylogenetically related assimilatory Sir (aSir). dSir is organized as a heterotetrameric (alphabeta)(2) complex composed of two catalytically independent alphabeta heterodimers. In contrast, aSir is a monomeric protein built of two fused modules that are structurally related to subunits alpha and beta except for a ferredoxin domain inserted only into the subunits of dSir. The [4Fe-4S] cluster of this ferredoxin domain is considered as the terminal redox site of the electron transfer pathway to the siroheme-[4Fe-4S] center in dSir. While aSir binds one siroheme-[4Fe-4S] center, dSir harbors two of them within each alphabeta heterodimer. Surprisingly, only one siroheme-[4Fe-4S] center in each alphabeta heterodimer is catalytically active, whereas access to the second one is blocked by a tryptophan residue. The spatial proximity of the functional and structural siroheme-[4Fe-4S] centers suggests that the catalytic activity at one active site was optimized during evolution at the expense of the enzymatic competence of the other. The sulfite binding mode and presumably the mechanism of sulfite reduction appear to be largely conserved between dSir and aSir. In addition, a scenario for the evolution of Sirs is proposed.

The iron-sulfur flavoenzyme adenylylsulfate (adenosine 5 -phosphosulfate, APS) reductase catalyzes reversibly the reduction of APS to sulfite and AMP. The structures of APS reductase from the hyperthermophilic Archaeoglobus fulgidus in the two-electron reduced state and with sulfite bound to FAD are reported at 1.6-and 2.5-Å resolution, respectively. The FAD-sulfite adduct was detected after soaking the crystals with APS. This finding and the architecture of the active site strongly suggest that catalysis involves a nucleophilic attack of the N5 atom of reduced FAD on the sulfur atom of APS. In view of the high degree of similarity between APS reductase and fumarate reductase especially with regard to the FAD-binding ␣-subunit, it is proposed that both subunits originate from a common ancestor resembling archaeal APS reductase. The two electrons required for APS reduction are transferred via two [4Fe-4S] clusters from the surface of the protein to FAD. The exceptionally large difference in reduction potential of these clusters (؊60 and ؊500 mV) can be explained by interactions of the clusters with the protein matrix.S ulfur compounds are widely used for energy conservation by several bacterial lineages and one hyperthermophilic order of Archaea, the Archaeoglobales. In the dissimilatory pathway the terminal electron acceptor sulfate becomes reduced to hydrogen sulfide, a process that is used for energy conservation (1). This pathway has to be distinguished from the assimilatory sulfate reduction in plants and bacteria, where the hydrogen sulfide is used for the biosynthesis of cysteine and S-containing cofactors. In the sulfur-oxidizing pathway sulfide, elemental sulfur, and thiosulfate serve as electron donors, and sulfate is the final product.The pathways of dissimilatory sulfate reduction and sulfur oxidation involve three key enzymes localized in the cytoplasm or at the cytoplasmic aspect of the inner membrane. Sulfate has to be activated to adenylylsulfate (adenosine 5Ј-phosphosulfate, APS) by ATP-sulfurylase (EC 2.7.7.4) at the expense of ATP, APS reductase (EC 1.8.99.2) converts APS to sulfite and AMP, and sulfite is reduced to sulfide by sulfite reductase (EC 1.8.7.1): AdeO 3 POSO 3 2Ϫ (APS) ϩ 2e Ϫ 3 AdeOPO 3 2Ϫ (AMP) ϩ SO 3 2Ϫ . In the sulfur-oxidizing pathway these three enzymes operate in the reverse direction, liberating electrons and ATP.The molecular parameters including mass, subunit composition, and cofactor stoichiometry of APS reductase, which had been characterized from diverse sulfate-reducing and sulfuroxidizing organisms, have been a matter of debate. Most recently, highly active APS reductase from sulfate-reducing and sulfur-oxidizing bacteria and archaea was isolated as a 1:1 ␣␤-heterodimer with a molecular mass of Ϸ95 kDa (2). The ␣-subunit (Ϸ75 kDa) contains a noncovalently bound flavinadenine dinucleotide (FAD), and the ␤-subunit (Ϸ20 kDa) contains two [4Fe-4S] clusters of the ferredoxin type (2, 3). The spectroscopic properties of these APS reductases, especially UV͞visible and EPR sp...