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.