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.