Structural insights into the enzyme catalysis from comparison of three forms of dissimilatory sulphite reductase from Desulfovibrio gigas

The crystal structures of two active forms of dissimilatory sulphite reductase (Dsr) from Desulfovibrio gigas, Dsr-I and Dsr-II, are compared at 1.76 and 2.05 A resolution respectively. The dimeric α_2β_2γ_2 structure of Dsr-I contains eight [4Fe–4S] clusters, two saddle-shaped sirohaems and two flat sirohydrochlorins. In Dsr-II, the [4Fe–4S] cluster associated with the sirohaem in Dsr-I is replaced by a [3Fe–4S] cluster. 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 sirohaem has been demetallated in addition to its associated [4Fe–4S] cluster replaced by a [3Fe–4S] centre. In Dsr-I and Dsr-II, the sirohydrochlorin is located in a putative substrate channel connected to the sirohaem. 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 sirohaem in Dsr-I. In Dsr-II, the thioether bond is broken, and the Cysγ104 side-chain moves closer to the bound sulphite at the sirohaem pocket. These different forms of Dsr offer structural insights into a mechanism of sulphite reduction that can lead to S_3O_6^(2−), S_2O_3^(2−) and S^(2−).