Environmental Engineering Reference
In-Depth Information
4.2.5.1 Oxidation of Thiosulfate to Tetrathionate
Alc. vinosum tetrathionate-forming thiosulfate dehydrogenase (TsdA) (thio-
sulfate:acceptor oxidoreductase EC 1.8.2.2) is a periplasmic, monomeric diheme
30 kDa cytochrome c 554 with an isoelectric point of 4.2. UV-visible and EPR
spectroscopies suggest methionine and cysteine as distal axial ligands of the two
heme irons in TsdA.
4.2.5.2 Oxidation of Thiosulfate to Sulfate by the Sox Multienzyme System
Many GSB and PSB can oxidize thiosulfate completely to sulfate. The Sox complex,
a periplasmic thiosulfate-oxidizing multienzyme complex was first found and char-
acterized in Paracoccus ( Pc .) versutus and Pc. pantotrophus . The Sox gene cluster of
Pc. pantotrophus comprise 15 genes ( soxRSVWXYZABCDEFGH ). The so-called
SoxAX cytochromes are heme-thiolate proteins playing a key role in bacterial
thiosulfate oxidation [ 162 ]. They initiate the reaction cycle of a multienzyme com-
plex in both photo- and chemotrophic SOB by catalyzing the attachment of sulfur
substrates such as thiosulfate to a conserved cysteine present in a carrier protein.
Alc. vinosum possesses five sox genes in two independent loci ( soxBXA and
soxYZ ) encoding proteins related to components of the Pc. pantotrophus Sox
complex. Three sox-encoded proteins were purified and characterized from Alc.
vinosum : the heterodimeric c -type cytochrome SoxXA, the monomeric SoxB
containing a dimanganese center, and the heterodimeric thiosulfate-binding protein
Sox YZ [ 160 , 163 ].
In summary, Sox and Dsr proteins are absolutely essential in Alc. vinosum for the
oxidation of thiosulfate and stored sulfur, respectively. Clusters of dsr and sox
genes are also present in the only distantly related GSB as well as in the other
sulfur-storing chemotrophic and phototrophic SOB. The mechanisms of thiosulfate
oxidation via sulfur deposition and of the oxidation of deposited sulfur seem to
be evolutionary highly conserved. Studies on Alc. vinosum can help to elucidate the
sulfur oxidation pathway in other sulfur-storing bacteria. GSB oxidize sulfide and
thiosulfate to sulfate, with extracellular sulfur globules as an intermediate and
sulfur globule oxidation is strictly dependent on the dSiR system. In GSB,
depending on the strain, sulfite is probably oxidized to sulfate by one or two
mechanisms with different evolutionary origins, using either APS reductase or
the polysulfide reductase-like complex 3 [ 164 ].
5 Conclusions
Anaerobic and aerobic microorganisms play a paramount role in biological cycling
of inorganic sulfur compounds because they have developed enzyme systems to use
inorganic sulfur compounds as electron donors or electron acceptors to facilitate
growth.
Search WWH ::




Custom Search