Abstract
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-Å resolution respectively. The dimeric α 2 β 2 γ 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 3 O 6 2- , S 2 O 3 2- and S 2- . © 2010 Blackwell Publishing Ltd.