It is thought that only two of the siroheme moieties of Dsr (one per
heterodimer) support HSO3- reduction
(20). While the A. fulgidus heterotetramer (purified and
crystalized anaerobically) comports four siroheme moieties, the D.
vulgaris structure (purified aerobically) has only two, the
other
two being sirohydrochlorin groups lacking the iron. In this latter
structure, the HSO3- ion is found in
interaction only with the siro-hemes (Figure 2B, dark blue). In theA. fulgidus structure, access to the “bottom” siroheme is
blocked by tryptophan B119 (Figure 3A), whereas in the D.
vulgaris structure, no blocking residue is apparent near the
sirohydrochlorin group (Figure 3B), the corresponding side chain of
threonine B135 facing away from the heme.