Dissimilatory sulfite reductase

Sulfite reductase catalyzes the six-electron reduction of sulfite to sulfide, m essential enzymatic reaction in the dissimilatory sulfate reduction process. Several different types of dissimilatory sulfite reductases were already isolated from sulfate reducers, namely desul-foviridin (148-150), desulforubidin (151, 152), P-582 (153, 154), and desulfofuscidin (155). In addition to these four enzymes, an assimila-tory-type sulfite reductase was also isolated from D. vulgaris. Although all these enzymes have significantly different subunit composition and amino acid sequences, it is interesting to note that, as will be discussed later, all of them share a unique type of cofactor. 

Marritt, S. and Hagen, W.R. 1996. Dissimilatory sulfite reductase revisited. The desulfoviridin molecule does contain 20 iron ions, extensively demetallated sirohaem, and an S = 9/2 iron-sulfur cluster. European Journal of Biochemistry 238 724—727. 

Pierik, A.J. and Hagen, W.R. 1991. S = 9/2 EPR signals are evidence against coupling between the siroheme and the Fe/S cluster prosthetic groups in Desulfovibrion vulgaris (Hildenborough) dissimilatory sulfite reductase. European Journal of Biochemistry 195 505-516. 

Minz D., Flax J. L., Green S. J., Muyzer G., Cohen Y., Wagner M., Rittmann B. E., and Stahl D. A. (1999b) Diversity of sulfate-reducing bacteria in oxic and anoxic regions of a microbial mat characterized by comparative analysis of dissimilatory sulfite reductase genes. Appl. Environ. Microbiol. 65, 4666-4671. 

SRB are essentially ubiquitous in aqueous environments that contain organic carbon and sulfate (e.g., subsurface aquifers and lake sediments). Moreover, analysis of a key gene associated with sulfate reduction (dissimilatory sulfite reductase) indicates that microbial sulfate reduction is an ancient trait, suggesting that organisms may have contributed to sulfide mineral formation throughout much of Earth history (Wagner et al. 1998). SRB are tolerant to environmental extremes of heat (some are hyperthermophiles) and salinity (some are halophiles). 

Mulkidjanian AY, Koonin EV, Makarova KS, Mekhedov SL, Sorokin A, Wolf YI, Dufresne A, Partensky F, Burd H, Kaznadzey D, Haselkom R, Galperin MY (2006) The cyanobacterial genome core and the origin of photosynthesis. Proc Natl Acad Sci USA 103 13126-13131 Murphy MJ, Siegel LM (1973) Siroheme and sirohydrochlorin. The basis for a new type of porphyrin-related prosthetic group common to both assimilatory and dissimilatory sulfite reductase. J Biol Chem 248 6911-6919... 

The dissimilatory sulfite reductases from bacteria have been divided into four classes according to their visible absorption spectra desulfoviridin (628 nm), desulforubidin (545 nm), desulfofuscidin (576 nm) and P-582 (582 nm) [99]. They are all heterooligomers of high molecular mass, and when assayed in vitro form a mixture of products containing trithionate, thiosulfate and sulfide. The product distribution varies with the assay conditions, and so it is still a matter of dispute whether the physiological product of these enzymes is sulfide or not [100] (Fig. 4). A similar dissimilatory sulfite reductase has also been purified from the thermophilic archaeon Archaeoglobus fulgidus [101]. 

Klein M., Friedrich M., Roger A. J., Hugenholtz P., Fishbain S., Abicht H., Blackall L. L., Stahl D. A. and Wagner M. (2001) Multiple lateral transfers of dissimilatory sulfite reductase genes between major lineages of sulfate reducing prokaryotes. J. Bacterial. 183, 6028-6035. 

Lui, S.M. and J.A. Cowan (1994). Direct reversible protein electrochemistry at a pyrol54ic graphite electrode. Characterization of the redox thermodynamics of the Fe4S4-siroheme prosthetic center in the hexameric dissimilatory sulfite reductase and the monomeric assimilatory snlflte reductase from desulfovibrio vulgaris (Hilden-hourgh). Systematic pH titration experiments and implications for active site chemistry./.Am. Chem. Soc. 116, 11538-11549. 

Lui, S.M., A. Soriano, and J.A. Cowan (1993). Enzymatic reduction of inorganic anions. Pre-steady-state kinetic analysis of the dissimilatory sulfite reductase (desul-foviridin) from desulfovibrio vulgaris (Hildenborough). Mechanistic implications. J. Am. Chem. Soc. 115, 10483-10486. 

A number of different enzymes can carry out the reduction of nitrite to either ammonium or nitric oxide and/or nitrous oxide. The latter types are involved with the denitrification process (Payne, 1973) and will not be considered here. Among the enzymes that catalyze the six-electron reduction of nitrite to ammonia, several different types are recognized. These are (I) assimilatory NiRs that function in biosynthetic nitrate assimilation of higher plants, algae, and fungi, (2) ammonia-forming dissimilatory NiRs involved in anaerobic nitrate respiration of diverse bacteria, and (3) assimilatory and dissimilatory sulfite reductases... 

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