X-ray structure of the γ-subunit of a dissimilatory sulfite reductase: Fixed and flexible C-terminal arms

The X-ray structure of the γ-subunit of the dissimilatory sulfite reductase (DsrC) from Archaeoglobus fulgidus was determined at 1.12 and 2.1 Å resolution, in the two crystal forms named DsrC nat and DsrC ox the latter being cocrystallized with the oxidizing agent tert-butyl hydroperoxide. The fold corresponds to that of the homologous protein from Pyrobaculum aerophilum but is significantly more compact. The most interesting, highly conserved C-terminal arm adopts a well-defined conformation in A. fulgidus DsrC in contrast to the completely disordered conformation in P. aerophilum DsrC. The functional relevance of both conformations and of a potentially redox-active disulfide bond between the strictly invariant Cys103 and Cys114 are discussed.