5.B.12. The One Electron Transmembrane Transfer Complex (DsrMKJOP) Family
Sulfate-reducing organisms use sulfate as an electron acceptor in an anaerobic respiratory process. Sulfur metabolism is one of the oldest known redox geochemical cycles in our atmosphere. These redox processes utilize different sulfur anions, and the reactions are performed by the gene products of the dsr operon from phylogenetically diverse microorganisms. The operon is involved in the maintenance of environmental sulfur balance. Interestingly, the dsr operon is found to be present in both sulfur anion oxidizing and reducing microorganisms, and in both types of organisms, the sulfite reductase (DsrABC) protein complex plays a vital role (Ghosh and Bagchi 2015).
Genome analysis of sulfate-reducing organisms allowed the identification of two strictly conserved membrane complexes. Pires et al. 2006 reported the purification and characterization of one of these complexes, DsrMKJOP, from Desulfovibrio desulfuricans ATCC 27774. The complex has hemes of the c and b types and several iron-sulfur centers. The corresponding genes in the genome of Desulfovibrio vulgaris have similarly been analyzed. dsrM encodes an integral membrane 339 aa (6 TMSs) cytochrome b, similar to one in the complex of TC# 5.A.3.1.1; dsrK encodes a 547 aa protein homologous to the HdrD subunit of heterodisulfide reductase and similar to subunits in the complexes in 5.B.11.1.1 and 3.D.7.1.1 ; dsrJ encodes a 126 aas (1 N-terminal TMS) triheme periplasmic cytochrome c similar to subunits in 5.B.5.2.1 and 5.A.3.4.1; dsrO encodes a periplasmic 260 aa (1 N-terminal TMS) FeS protein similar to a subunit in 5.A.3.11.1; and dsrP encodes another integral membrane protein (387 aas and 10 TMSs), similar to a subunit in 5.A.3.3.3.
Sequence analysis and EPR studies indicated that DsrJ belongs to a family of multiheme cytochromes c and that its three hemes have different types of coordination, one bis-His, the second His/Met, and the third an unusual His/Cys coordination. The His/Cys-coordinated heme is only partially reduced by dithionite. About 40% of the hemes are reduced by menadiol, but no reduction is observed upon treatment with H2 and hydrogenase, irrespective of the presence of cytochrome c3. The aerobically isolated Dsr complex displays an EPR signal with similar characteristics to the catalytic [4Fe-4S]3+ species observed in heterodisulfide reductases. Further, five different [4Fe-4S]2+/1+centers are observed during a redox titration followed by EPR.The authors speculate that this system accepts electrons from a periplasmic donor (or the quinione pool in the membrane) and transfers them to the cytoplasmic sulfite reductase, DsrABC (Pires et al. 2006). Thus, DsrO and DsrJ are in the periplasm, DsrM and DsrP are transmembrane, and DsrK is on the cytoplasmic side of the membrane.
The generalized reaction proposed to be catalyzed by the Dsr complex is:
electrons (periplasmic electron donor) → electrons (cytoplasmic sulfite reductase, DsrABC)