Three dimensional crystal structures have been solved for DsrAB enzymes from Archaeoglobus fulgidus (Figure 2A-B) (20, 36),Desulfovibrio vulgaris (32) (Figure 2C-D), Desulfovibrio gigas (35), and Desulfomicrobium norvegicum (46) at 2, 2.1, 1.76 and 2.5 Å resolution, respectively. The A and B subunits consist of three domains, two of which are structurally similar. The third is a ferredoxin-like domain, thought to have been inserted between two beta strands of domain two after the gene duplication event (20). Each A/B heterodimer in DsrAB harbors two sirohemes (or one siroheme and a sirohydrochlorin moiety, representing a siroheme without metal cofactors) and four [4Fe-4S] clusters that are presumably involved in electron transfer to HSO3- (present in the D. vulgaris structure Figure 2B, D). A third, cysteine disulfide-containing labile
subunit, DsrC, was purified and crystalized covalently bound to the heterotetramer in the D. vulgaris structure (32) (grey and purple in Figure 2B, D) via one of the reduced cysteine residues. In the most recently proposed model of the HSO3-reduction reaction cycle, DsrC in reduced form binds to a SII intermediate at the active site in DsrAB, forming a SI containing hetero-disulfide. Hydrogen sulfide is then produced via an S0-containing protein tri-sulfide intermediate, implicating both cysteine residues in the DsrC C-terminus (7). The final four electrons required for this latter reaction are proposed to emanate from the menaquinone pool, likely implicating the membrane DsrMK(JOP) complex (47), thus coupling HSO3- reduction to proton translocation and energy conservation (7).
Within the DsrAB heterodimer, the A and B subunits are intricately intertwined in the manner of clasped hands. In addition, the C-terminus of the A subunit in one heterodimer crosses over the heterodimer interface to interact extensively with both the A and B subunits of the other (Figure 2A). While crystallographic domain swapping has been demonstrated to be artefactual in many cases, the fact that all four available DsrAB structures (20, 32, 35, 46) exhibit this feature supports the notion that it is a fundamental feature of the DsrAB structure. Besides this crossover interaction, the central interface between heterodimers is quite open, with only limited contacts between two helices of the B subunits from each heterodimer (Figure 2 D, E). Interestingly, at this central interface between heterodimers, the position of the two helices from the B1/B2 subunits responsible for the contacts has been swapped between the A. fuglidus and D. vulgaris structures. All three of the other available structures of DsrAB (32, 35, 46) show the central interfacial helix positioning of theD. vulgaris structure shown in Figure 2D. Native gel electrophoresis followed by mass spectrometry measurements made on preparations of DsrABC from D. vulgaris and D. norvegicumrevealed that the major oligomeric species were A2B2C2 and A2B2C heterohexamer and heteropentamer, the C subunit being somewhat labile (46). These observations reinforce the crystallographic results indicating that DsrAB is a heterotetramer.