Publications

• The octahaem SirA catalyses dissimilatory sulfite reduction in Shewanella oneidensis MR-1.

  Shirodkar S., Reed S., Romine M., Saffarini D.

  Shewanella oneidensis MR-1 is a metal reducer that uses a large number of electron acceptors including thiosulfate, polysulfide and sulfite. The enzyme required for thiosulfate and polysulfide respiration has been recently identified, but the mechanisms of sulfite reduction remained unexplored. An ... >> More

  Shewanella oneidensis MR-1 is a metal reducer that uses a large number of electron acceptors including thiosulfate, polysulfide and sulfite. The enzyme required for thiosulfate and polysulfide respiration has been recently identified, but the mechanisms of sulfite reduction remained unexplored. Analysis of MR-1 cultures grown anaerobically with sulfite suggested that the dissimilatory sulfite reductase catalyses six-electron reduction of sulfite to sulfide. Reduction of sulfite required menaquinones but was independent of the intermediate electron carrier CymA. Furthermore, the terminal sulfite reductase, SirA, was identified as an octahaem c cytochrome with an atypical haem binding site. The sulfite reductase of S. oneidensis MR-1 does not appear to be a sirohaem enzyme, but represents a new class of sulfite reductases. The gene that encodes SirA is located within a 10-gene locus that is predicted to encode a component of a specialized haem lyase, a menaquinone oxidase and copper transport proteins. This locus was identified in the genomes of several Shewanella species and appears to be linked to the ability of these organisms to reduce sulfite under anaerobic conditions. << Less

  Environ. Microbiol. 13:108-115(2011) [PubMed] [EuropePMC]

• The octahaem MccA is a haem c-copper sulfite reductase.

  Hermann B., Kern M., La Pietra L., Simon J., Einsle O.

  The six-electron reduction of sulfite to sulfide is the pivot point of the biogeochemical cycle of the element sulfur. The octahaem cytochrome c MccA (also known as SirA) catalyses this reaction for dissimilatory sulfite utilization by various bacteria. It is distinct from known sulfite reductases ... >> More

  The six-electron reduction of sulfite to sulfide is the pivot point of the biogeochemical cycle of the element sulfur. The octahaem cytochrome c MccA (also known as SirA) catalyses this reaction for dissimilatory sulfite utilization by various bacteria. It is distinct from known sulfite reductases because it has a substantially higher catalytic activity and a relatively low reactivity towards nitrite. The mechanistic reasons for the increased efficiency of MccA remain to be elucidated. Here we show that anoxically purified MccA exhibited a 2-to 5.5-fold higher specific sulfite reductase activity than the enzyme isolated under oxic conditions. We determined the three-dimensional structure of MccA to 2.2 Å resolution by single-wavelength anomalous dispersion. We find a homotrimer with an unprecedented fold and haem arrangement, as well as a haem bound to a CX15CH motif. The heterobimetallic active-site haem 2 has a Cu(I) ion juxtaposed to a haem c at a Fe-Cu distance of 4.4 Å. While the combination of metals is reminiscent of respiratory haem-copper oxidases, the oxidation-labile Cu(I) centre of MccA did not seem to undergo a redox transition during catalysis. Intact MccA tightly bound SO2 at haem 2, a dehydration product of the substrate sulfite that was partially turned over due to photoreduction by X-ray irradiation, yielding the reaction intermediate SO. Our data show the biometal copper in a new context and function and provide a chemical rationale for the comparatively high catalytic activity of MccA. << Less

  Nature 520:706-709(2015) [PubMed] [EuropePMC]

• The Wolinella succinogenes mcc gene cluster encodes an unconventional respiratory sulphite reduction system.

  Kern M., Klotz M.G., Simon J.

  Assimilatory and dissimilatory sulphite reductions are key reactions in the biogeochemical sulphur cycle and several distinct sirohaem-containing sulphite reductases have been characterized. Here, we describe that the Epsilonproteobacterium Wolinella succinogenes is able to grow by sulphite respir ... >> More

  Assimilatory and dissimilatory sulphite reductions are key reactions in the biogeochemical sulphur cycle and several distinct sirohaem-containing sulphite reductases have been characterized. Here, we describe that the Epsilonproteobacterium Wolinella succinogenes is able to grow by sulphite respiration (yielding sulphide) with formate as electron donor. Sulphite is reduced by MccA, a prototypical member of an emerging new class of periplasmic cytochrome c sulphite reductases that, phylogenetically, belongs to a multihaem cytochrome c superfamily whose members play crucial roles in the global sulphur and nitrogen cycles. Within this family, MccA represents an unconventional octahaem cytochrome c containing a special haem c group that is bound via two cysteine residues arranged in a unique CX(15)CH haem c binding motif. The phenotypes of numerous W.succinogenes mutants producing MccA variants underlined the structural importance of this motif. Several open reading frames of the mcc gene cluster were individually inactivated and characterization of the corresponding mutants indicated that the predicted iron-sulphur protein MccC, the putative quinol dehydrogenase MccD (a member of the NrfD/PsrC family) as well as a peptidyl-prolyl cis-trans isomerase, MccB, are essential for sulphite respiration. MccA synthesis in W. succinogenes was found to be induced by sulphite (but not by thiosulphate or sulphide) and repressed in the presence of fumarate or nitrate. Based on the results, a sophisticated model of respiratory sulphite reduction by the Mcc system is presented. << Less

  Mol. Microbiol. 82:1515-1530(2011) [PubMed] [EuropePMC]