Abstract
Sulfite reductase mediates the reduction of sulfite to sulfide in sulfate-reducing bacteria. Here, we compare the crystal structures between two distinct forms of the dissimilatory sulfite reductase (Dsr), desulfoviridin, from Desulfovibrio gigas, Dsr-I and Dsr- II, at 1.76 and 2.1 A resolution, respectively. The dimeric α2β2γ2 structure of Dsr-I contains eight [4Fe-4S] clusters, two saddle-shaped sirohemes and two flat sirohydrochlorins. In Dsr- II, the [4Fe-4S] cluster associated with each of the siroheme in Dsr-I is replaced by a [3Fe-4S] cluster. This structural feature allows Thrβ145 to position itself closer to the [3Fe-4S] in Dsr- II to replace the role of the Cysβ188 that ligates the [4Fe-4S] in Dsr-I. In both Dsr forms, each of the sirohydrochlorins is located in a putative substrate channel connected to the siroheme and capped by a dynamic loop from the ferredoxin domain. The γ-subunit C-terminus is inserted into a positively charged channel formed between the α- and β-subunits, with its conserved terminal Cysγ104 side chain covalently linked to the CHA atom of the siroheme in Dsr-I. In Dsr-II, the thiolate bond is broken, and the Cysγ104 side chain moves closer to the bound sulfite at the siroheme pocket. Moreover, the γ-subunit in the region of the C- terminus reveals another arrangement with an interaction between Cysγ93 and Cysγ104 in both Dsr-I and Dsr-II. Beside the sulfite in the active site, a second sulfite interacting with the conserved Lysγ100 has also been identified, implicating this site as the entry into a putative substrate channel. Electron paramagnetic resonance (EPR) of the active Dsr-I and Dsr-II confirm the co-factor structures, whereas EPR of a third but inactive form, Dsr-III, suggests that the siroheme has been demetallated in addition to its associated [4Fe-4S] cluster replaced by a [3Fe-4S] center. A catalytic mechanism that can lead to S3O62−, S2O32− and S2−, the three distinct products observed in the dissimilatory sulfite reduction, is proposed.