Carbon monoxide-reacting pigment from Desulfotomaculum nigrificans and its possible relevance to sulfite reduction

Molecular understanding of heteronuclear active sites in heme-copper oxidases, nitric oxide reductases, and sulfite reductases through biomimetic modelling

Heme-copper oxidases (HCO), nitric oxide reductases (NOR), and sulfite reductases (SiR) catalyze the multi-electron and multi-proton reductions of O2, NO, and SO32-, respectively. Each of these reactions is important to drive cellular energy production through respiratory metabolism and HCO, NOR, and SiR evolved to contain heteronuclear active sites containing heme/copper, heme/nonheme iron, and heme-[4Fe-4S] centers, respectively. The complexity of the structures and reactions of these native enzymes, along with their large sizes and/or membrane associations, make it challenging to fully understand the crucial structural features responsible for the catalytic properties of these active sites. In this review, we summarize progress that has been made to better understand these heteronuclear metalloenzymes at the molecular level though study of the native enzymes along with insights gained from biomimetic models comprising either small molecules or proteins. Further understanding the reaction selectivity of these enzymes is discussed through comparisons of their similar heteronuclear active sites, and we offer outlook for further investigations.

Expression, purification and characterization of an active C491G variant of ferredoxin sulfite reductase from Synechococcus elongatus PCC 7942

Recently developed molecular wire technology takes advantage of [4Fe-4S] clusters that are ligated by at least one surface exposed Cys residue. Mutagenesis of this Cys residue to a Gly opens an exchangeable coordination site to a corner iron atom that can be chemically rescued by an external thiolate ligand. This ligand can be subsequently displaced by mass action using a dithiol molecular wire to tether two redox active proteins. We intend to apply this technique to tethering Photosystem I to ferredoxin sulfite reductase (FdSiR), an enzyme that catalyzes the six-electron reduction of sulfite to hydrogen sulfite and nitrite to ammonia. The enzyme contains a [4Fe-4S]^2+/1+ cluster and a siroheme active site. FdSiR_WT and an FdSiR_C491G variant were cloned from Synechococcus elongatus PCC 7942 and expressed along with the cysG gene from Salmonella typhimurium using the pCDFDuet plasmid. UV/Vis absorbance spectra of both FdSiR_WT and the FdSiR_C491G variant displayed characteristic peaks at 278, 392 (Soret), 585 (α) and 714 nm (charge transfer band), and 278, 394 (Soret), 587 (α) and 714 nm (charge transfer band) respectively. Both enzymes in their as-isolated forms displayed an EPR spectrum characteristic of an S = 5/2 high spin heme. When reduced, both enzymes exhibited the signal of a low spin S = 1/2 [4Fe-4S]¹⁺ cluster. The FdSiR_WT and FdSiR_C491G variant both showed activity using reduced methyl viologen and Synechococcus elongatus PCC 7942 ferredoxin 1 (Fd1) as electron donors. Based on these results, the FdSIR_C491G variant should be a suitable candidate for wiring to Photosystem I.

Hemoproteins in dissimilatory sulfate- and sulfur-reducing prokaryotes

Dissimilatory sulfate and sulfur reduction evolved billions of years ago and while the bacteria and archaea that use this unique metabolism employ a variety of electron donors, H(2) is most commonly used as the energy source. These prokaryotes use multiheme c-type proteins to shuttle electrons from electron donors, and electron transport complexes presumed to contain b-type hemoproteins contribute to proton charging of the membrane. Numerous sulfate and sulfur reducers use an alternate pathway for heme synthesis and, frequently, uniquely specific axial ligands are used to secure c-type heme to the protein. This review presents some of the types and functional activities of hemoproteins involved in these two dissimilatory reduction pathways.

Production of methanethiol and volatile sulfur compounds by the archaeon "Ferroplasma acidarmanus"

Acidophiles are typically isolated from sulfate-rich ecological niches yet the role of sulfur metabolism in their growth and survival is poorly defined. Studies of heterotrophically grown "Ferroplasma acidarmanus" showed that its growth requires a minimum of 100 mM of a sulfate-containing salt. Headspace gas analyses by GC/MS determined that the volatile sulfur compound emitted by active "F. acidarmanus" cultures is methanethiol. In "F. acidarmanus" cultures grown either heterotrophically or chemolithotrophically, methanethiol was produced constitutively. Radiotracer studies with (35)S-labeled methionine, cysteine, and sulfate showed that all three were used in methanethiol production. Additionally, (3)H-labeled methionine was incorporated into methanethiol and was probably used as a methyl-group donor. Methanethiol production in whole cell lysates supplied with SO (3) (2-) indicated that NADPH-dependant sulfite reductase and methyltransferase activities were present. Cell lysates also contained enzymatic activity for methionine-gamma-lyase that cleaved the side chain of either methionine to form methanethiol or cysteine to produce H(2)S. Since methanethiol was detected from the degradation of cysteine, it is likely that sulfide was methylated by a thiol methyltransferase. Collectively, these data demonstrate that "F. acidarmanus" produces methanethiol through the metabolism of methionine, cysteine, or sulfate. This is the first report of a methanethiol-producing acidophile, thus identifying a new contributor to the global sulfur cycle.

Phylogeny of dissimilatory sulfite reductases supports an early origin of sulfate respiration

Microorganisms that use sulfate as a terminal electron acceptor for anaerobic respiration play a central role in the global sulfur cycle. Here, we report the results of comparative sequence analysis of dissimilatory sulfite reductase (DSR) genes from closely and distantly related sulfate-reducing organisms to infer the evolutionary history of DSR. A 1.9-kb DNA region encoding most of the alpha and beta subunits of DSR could be recovered only from organisms capable of dissimilatory sulfate reduction with a PCR primer set targeting highly conserved regions in these genes. All DNA sequences obtained were highly similar to one another (49 to 89% identity), and their inferred evolutionary relationships were nearly identical to those inferred on the basis of 16S rRNA. We conclude that the high similarity of bacterial and archaeal DSRs reflects their common origin from a conserved DSR. This ancestral DSR was either present before the split between the domains Bacteria, Archaea, and Eucarya or laterally transferred between Bacteria and Archaea soon after domain divergence. Thus, if the physiological role of the DSR was constant over time, then early ancestors of Bacteria and Archaea already possessed a key enzyme of sulfate and sulfite respiration.

A dissimilatory sirohaem-sulfite-reductase-type protein from the hyperthermophilic archaeon Pyrobaculum islandicum

A sulfite-reductase-type protein was purified from the hyperthermophilic crenarchaeote Pyrobaculum islandicum grown chemoorganoheterotrophically with thiosulfate as terminal electron acceptor. In common with dissimilatory sulfite reductases the protein has an alpha 2 beta 2 structure and contains high-spin sirohaem, non-haem iron and acid-labile sulfide. The oxidized protein exhibits absorption maxima at 280, 392, 578 and 710 nm with shoulders at 430 and 610 nm. The isoelectric point of pH 8.4 sets the protein apart from all dissimilatory sulfite reductases characterized thus far. The genes for the alpha- and beta-subunits (dsrA and dsrB) are contiguous in the order dsrAdsrB and most probably comprise an operon with the directly following dsrG and dsrC genes. dsrG and dsrC encode products which are homologous to eukaryotic glutathione S-transferases and the proposed gamma-subunit of Desulfovibrio vulgaris sulfite reductase, respectively. dsrA and dsrB encode 44.2 kDa and 41.2 kDa peptides which show significant similarity to the two homologous subunits DsrA and DsrB of dissimilatory sulfite reductases. Phylogenetic analyses indicate a common protogenotic origin of the P. islandicum protein and the dissimilatory sulfite reductases from sulfate-reducing and sulfide-oxidizing prokaryotes. However, the protein from P. islandicum and the sulfite reductases from sulfate-reducers and from sulfur-oxidizers most probably evolved into three independent lineages prior to divergence of archaea and bacteria.

The relationship between structure and function for the sulfite reductases

The six-electron reductions of sulfite to sulfide and nitrite to ammonia, fundamental to early and contemporary life, are catalyzed by diverse sulfite and nitrite reductases that share an unusual prosthetic assembly in their active centers, namely siroheme covalently linked to an Fe4S4 cluster. The recently determined crystallographic structure of the sulfite reductase hemoprotein from Escherichia coli complements extensive biochemical and spectroscopic studies in revealing structural features that are key for the catalytic mechanisms and in suggesting a common symmetric structural unit for this diverse family of enzymes.

Molecular properties of the dissimilatory sulfite reductase from Desulfovibrio desulfuricans (Essex) and comparison with the enzyme from Desulfovibrio vulgaris (Hildenborough)

The dissimilatory sulfite reductase desulfoviridin was purified from the membrane (mSiR) and the soluble fraction (sSiR) of the sulfate-reducing bacterium Desulfovibrio desulfuricans (Essex). Molecular and spectroscopic properties were determined and compared with the properties of the soluble desulfoviridin from Desulfovibrio vulgaris (Hildenborough). The enzymes were isolated as alpha 2 beta 2 gamma n (n = 1-3) multimers with a relative molecular mass of 200 +/- 10 (gel filtration). Both mSiR and sSiR from D. desulfuricans contained 24 +/- 3 Fe and 18 +/- 3 labile sulfide/200 kDa, respectively, and showed identical EPR spectra. Quantification of the high-spin Fe(III) heme resonances at g of approximately 6 indicated that close to 80% of the siroheme moiety in the enzyme from D. desulfuricans was demetallated. D. desulfuricans sulfite reductase showed S = 9/2 EPR signals with the highest apparent g value at g = 17 as reported for SiR from D. vulgaris. Antibodies raised against the alpha, beta and gamma subunit of the D. vulgaris enzyme exhibited cross-reactivity with the subunits of mSiR and sSiR from D. desulfuricans. N-terminal sequences of alpha, beta and gamma subunits of both mSiR and sSiR from D. desulfuricans were identical and showed a high degree of similarity with the sequences of the corresponding subunits obtained from the D. vulgaris enzyme. During gel filtration of sSiR from D. desulfuricans, under non-denaturing conditions, a small protein (molecular mass approximately 11 kDa) was separated. This 11-kDa protein exhibited cross-reactivity with the antibody raised against the gamma subunit of D. vulgaris sulfite reductase. In the case of D. desulfuricans sulfite reductase, the 11-kDa gamma subunit seems not to be an integral part of the protein and can be obtained from the soluble fraction and during purification of the soluble enzyme.

Expression of the gamma-subunit gene of desulfoviridin-type dissimilatory sulfite reductase and of the alpha- and beta-subunit genes is not coordinately regulated

It has been shown [Pierik, A. J., Duyvis, M. G., van Helvoort, J. M. L. M., Wolbert, R. B. G. & Hagen, W. R. (1992) Eur. J. Biochem. 205, 111-115] that desulfoviridin, the dissimilatory sulfite reductase of sulfate-reducing bacteria of the genus Desulfovibrio, contains a third, gamma, subunit (11 kDa), in addition to the well-established alpha (50 kDa) and beta (40 kDa) subunits, and an alpha 2 beta 2 gamma 2 subunit structure has been proposed. Cloning and sequencing of the dsvC gene indicated it to encode a protein of 105 amino acids (11.9 kDa; gamma subunit). The finding that the dsvC gene, located on a 3.5-kb SacII fragment, is transcribed in both Escherichia coli and Desulfovibrio vulgaris as an mRNA of only 400-600 nucleotides, and that both the dsvA and dsvB genes are present on a 7.2-kb SacII fragment, indicates that dsvC forms a separate transcriptional unit. The steady-state level of alpha and beta subunits expressed in D. vulgaris Hildenborough cells is rather constant, while that of the gamma subunit increased strongly in the stationary growth phase. Biochemical analysis of the purified protein, expressed in E. coli, and library comparison of its sequence, have so far failed to establish the function of gamma.

Spore-Forming Thermophilic Sulfate-Reducing Bacteria Isolated from North Sea Oil Field Waters

Thermophilic sulfate-reducing bacteria were isolated from oil field waters from oil production platforms in the Norwegian sector of the North Sea. Spore-forming rods dominated in the enrichments when lactate, propionate, butyrate, or a mixture of aliphatic fatty acids (C 4 through C 6 ) was added as a carbon source and electron donor. Representative strains were isolated and characterized. The isolates grew autotrophically on H 2 -CO 2 and heterotrophically on fatty acids such as formate, propionate, butyrate, caproate, valerate, pyruvate, and lactate and on alcohols such as methanol, ethanol, and propanol. Sulfate, sulfite, and thiosulfate but not nitrate could be used as an electron acceptor. The temperature range for growth was 43 to 78°C; the spores were extremely heat resistant and survived 131°C for 20 min. The optimum pH was 7.0. The isolates grew well in salt concentrations ranging from 0 to 800 mmol of NaCl per liter. Sulfite reductase P582 was present, but cytochrome c and desulfoviridin were not found. Electron micrographs revealed a gram-positive cell organization. The isolates were classified as a Desulfotomaculum sp. on the basis of spore formation, general physiological characteristics, and submicroscopic organization. To detect thermophilic spore-forming sulfate-reducing bacteria in oil field water, polyvalent antisera raised against antigens from two isolates were used. These bacteria were shown to be widespread in oil field water from different platforms. The origin of thermophilic sulfate-reducing bacteria in the pore water of oil reservoirs is discussed.

Purification and characterization of bisulfite reductase (desulfofuscidin) from Desulfovibrio thermophilus and its complexes with exogenous ligands

A dissimilatory bisulfite reductase has been purified from a thermophilic sulfate-reducing bacterium Desulfovibrio thermophilus (DSM 1276) and studied by EPR and optical spectroscopic techniques. The visible spectrum of the purified bisulfite reductase exhibits absorption maxima at 578.5, 392.5 and 281 nm with a weak band around 700 nm. Photoreduction of the native enzyme causes a decrease in absorption at 578.5 nm and a concomitant increase in absorption at 607 nm. When reduced, the enzyme reacts with cyanide, sulfite, sulfide and carbon monoxide to give stable complexes. The EPR spectrum of the native D. thermophilus bisulfite reductase shows the presence of a high-spin ferric signal with g values at 7.26, 4.78 and 1.92. Upon photoreduction the high-spin ferric heme signal disappeared and a typical 'g = 1.94' signal of [4Fe-4S] type cluster appeared. Chemical analyses show that the enzyme contains four sirohemes and eight [4Fe-4S] centers per mol of protein. The molecular mass determined by gel filtration was found to be 175 kDa. On SDS-gel electrophoresis the enzyme presents a main band of 44 to 48 kDa. These results suggest that the bisulfite reductase contains probably one siroheme and two [4Fe-4S] centers per monomer. The dissimilatory bisulfite reductase from D. thermophilus presents some homologous properties with desulfofuscidin, the bisulfite reductase isolated from Thermodesulfobacterium commune (Hatchikian, E.C. and Zeikus, J.G. (1983) J. Bacteriol. 153, 1211-1220).

Anaerobic degradation of aniline and dihydroxybenzenes by newly isolated sulfate-reducing bacteria and description of Desulfobacterium anilini

A new, rod-shaped, Gram-negative, non-sporing sulfate reducer (strain Ani1) was enriched and isolated from marine sediment with aniline as sole electron donor and carbon source. The strain degraded aniline completely to CO2 and NH3 with stoichiometric reduction of sulfate to sulfide. Strain Ani1 also degraded aminobenzoates and further aromatic and aliphatic compounds. The strain grew in sulfide-reduced mineral medium supplemented only with vitamin B12 and thiamine. Cells contained cytochromes, carbon monoxide dehydrogenase, and sulfite reductase P582, but no desulfoviridin. Strain Ani1 is described as a new species of the genus Desulfobacterium D. anilini. Marine enrichments with the three dihydroxybenzene isomers led to three different strains of sulfate-reducing bacteria; each of them could grow only with the isomer used for enrichment. Two strains isolated with catechol (strain Cat2) or resorcinol (strain Re10) were studied in detail. Both strains oxidized their substrates completely to CO2, and contained cytochromes, carbon monoxide dehydrogenase, and sulfite reductase P 582. Desulfoviridin was not present. Whereas the rod-shaped catechol oxidizer (strain Cat2) was able to grow on 18 aromatic compounds and several aliphatic substrates, the coccoid resorcinol-degrading bacterium (strain Re10) utilized only resorcinol, 2,4-dihydroxybenzoate and 1,3-cyclohexanedion. These strains could not be affiliated with existing species of sulfate-reducing bacteria. A further coccoid sulfate-reducing bacterium (strain Hy5) was isolated with hydroquinone and identified as a subspecies of Desulfococcus multivorans. Most-probable-number enumerations with catechol, phenol, and resorcinol showed relatively large numbers (10(4)-10(6) per ml) of aryl compound-degrading sulfate reducers in marine sediment samples.

Desulfotomaculum geothermicum sp. nov., a thermophilic, fatty acid-degrading, sulfate-reducing bacterium isolated with H2 from geothermal ground water

A strictly anaerobic, thermophilic, fatty acids-degrading, sporulating sulfate-reducing bacterium was isolated from geothermal ground water. The organism stained Gram-negative and formed gas vacuoles during sporulation. Lactate, ethanol, fructose and saturated fatty acids up to C18 served as electron donors and carbon sources with sulfate as external electron acceptor. Benzoate was not used. Stoichiometric measurements revealed a complete oxidation of part of butyrate although growth with acetate as only electron donor was not observed. The rest of butyrate was oxidized to acetate. The strain grew chemolithoautotrophically with hydrogen plus sulfate as energy source and carbon dioxide as carbon source without requirement of additional organic carbon like acetate. The strain contained a c-type cytochrome and presumably a sulfite reductase P582. Optimum temperature, pH and NaCl concentration for growth were 54 degrees C, pH 7.3-7.5 and 25 to 35 g NaCl/l. The G + C content of DNA was 50.4 mol %. Strain BSD is proposed as a new species of the spore-forming sulfate-reducing genus Desulfotomaculum, D. geothermicum.

Characterization of a trithionate reductase system from Desulfovibrio vulgaris

A trithionate reductase system was isolated and purified from extracts of Desulfovibrio vulgaris. This system reduced trithionate to thiosulfate and consisted of two proteins. One was bisulfite reductase, an enzyme that reduces bisulfite to trithionate, and the second component was designated TR-1. Both enzymes were required to reduce trithionate to thiosulfate. Flavodoxin and cytochrome c3 from D. vulgaris were tested for their ability to function as electron carriers during trithionate reduction. When molecular hydrogen was the source of electrons for the reduction, both flavodoxin and cytochrome c3 were required. In contrast, when the pyruvate phosphoroclastic system was the reductant, flavodoxin alone participated as the electron carrier. The results indicate that flavodoxin, but not cytochrome c3, interacted with the trithionate reductase system. The cytochrome in the hydrogenase-linked assay functioned as an electron carrier between hydrogenase and flavodoxin.

Sporulation and further nutritional characteristics of Desulfotomaculum acetoxidans

Acetate-oxidizing sulfate-reducing bacteria of the Desulfotomaculum acetoxidans type have been enriched from animal manure, rumen content and dung contaminated freshwater habitats, indicating that they are primarily intestinal bacteria. Sporulation was observed only when acetate was the organic substrate; with butyrate, which allowed faster growth than acetate, spore formation never occurred. The cone-shaped highly refractile areas adjacent to the spores in spore-forming mother cells were shown to be gas vacuoles. Biotin was the only growth factor required by Desulfotomaculum acetoxidans strain 5575 in minimal media with sulfate and acetate or other organic substrates.

Studies on dissimilatory sulfate-reducing bacteria that decompose fatty acids. I. Isolation of new sulfate-reducing bacteria enriched with acetate from saline environments. Description of Desulfobacter postgatei gen. nov., sp. nov

Three strains (2ac9, 3ac10 and 4ac11) of oval to rod-shaped, Gram negative, nonsporing sulfate-reducing bacteria were isolated from brackish water and marine mud samples with acetate as sole electron donor. All three strains grew in simple defined media supplemented with biotin and 4-aminobenzoic acid as growth factors. Acetate was the only electron donor utilized by strain 2ac9, while the other two strains used in addition ethanol and/or lactate. Sulfate served as electron acceptor and was reduced to H2S. Complete oxidation of acetate to CO2 was shown by stoichiometric measurements with strain 2ac9 in batch cultures using sulfate, sulfite or thiosulfate as electron acceptors. With sulfate an average growth yield of 4.8 g cell dry weight was obtained per mol of acetate oxidized; with sulfite or thiosulfate the growth yield on acetate was about twice as high. None of the strains contained desulfoviridin. In strain 2ac9 cytochromes of the b- and c-type were detected. Strain 2ac9 is described as type strain of the new species and genus, Desulfobacter postgatei.

Comparative bioenergetics of sulfate reduction in Desulfovibrio and Desulfotomaculum spp

Extracts of Desulfotomaculum nigrificans, Desulfotomaculum orientis, and Desulfotomaculum ruminis exhibit low levels of inorganic pyrophosphatase but were found to have high levels of pyrophosphate:acetate phosphotransferase. Conversely, extracts of Desulfovibrio gigas, Desulfovibrio vulgaris, and Desulfovibrio desulfuricans Norway 4 were shown to have high levels of inorganic pyrophosphatase but negligible amounts of pyrophosphate:acetate phosphotransferase. Both enzymes are reductant activated and appear to have an analogous function in removing pyrophosphate formed during the activation of sulfate. Conservation of the bond energy of pyrophosphate in Desulfotomaculum eliminates the necessity for invoking electron-transfer-coupled phosphorylation to account for the growth of these bacteria on lactate plus sulfate. Relative growth yields of Desulfovibrio vulgaris and Desulfotomaculum orientis on lactate plus sulfate indicate that the latter does not carry out significant electron-transfer-coupled phosphorylation in this mode of growth.

A new anaerobic, sporing, acetate-oxidizing, sulfate-reducing bacterium, Desulfotomaculum (emend.) acetoxidans

A new strictly anaerobic, polarly flagellated, sporing, acetate-oxidizing, sulfate-reducing bacterium was isolated from anaerobic fresh or sea water mud samples. The oxidation of acetate to CO2 is stoichiometrically linked to the formation of H2S from sulfate. Ethanol, butanol and butyrate are also used. Hydrogen, lactate or pyruvate are not used as electron donors; organic substances are not fermented. A cytochrome of the b-type and a supposed sulfite reductase, P582, were detected spectrophotometrically. An emended description of the genus Desulfotomaculum is proposed which includes the new bacterium as the species Desulfotomaculum acetoxidans.

Metal accumulation by bacteria with particular reference to dissimilatory sulphate-reducing bacteria

Dissimilatory sulphate-reducing bacteria, genera Desulfovibrio and Desulfotomaculum, exhibit a superior ability, over assimilatory organisms, to extract three amounts of metals from culture media. This property does not appear to be solely a function of the presence of H2S. In media containing elevated amounts of Fe, electron dense particles, provisionally identified as FeS, are deposited within the cells of dissimilatory bacteria.

Effect of enzymic assay conditions on sulfite reduction catalysed by desulfoviridin from Desulfovibrio gigas

The type and the amount of end products resulting from sulfite reduction catalysed by a single partially purified desulfoviridin preparation from Desulfovibrio gigas were shown to depend upon the enzymic assay conditions employed. Both manometric and spectrophotometric assays were used, with reduced methyl viologen serving as the electron donor in each system. Trithionate, thiosulfate, tetrathionate and sulfide were identified as possible end products. In the manometric assays, sulfide production was favoured by high reduced methyl viologen concentrations, low sulfite concentrations and a pH value of 7.0 as opposed to 6.0. In the spectrophotometric assays, results approaching the stoichiometric conversion of sulfite to sulfide were obtained only at high initial reduced methyl viologen concentrations.

Observations on the bisulfite reductase (P582) isolated from Desulfotomaculum nigrificans

The bisulfite reductase (P582) from Desulfotomaculum nigrificans was purified to homogeneity as judged by polyacrylamide gel electrophoresis. By colorimetric methods of analysis, the products of bisulfite reduction by this enzyme were determined to be trithionate, thiosulfate, and sulfide. Of these, trithionate was consistently found to be the major product, whereas the latter two were formed in lesser quantities. When [(35)S]bisulfite was incorporated as substrate, no labeled sulfide was detected. Furthermore, when trithionate and thiosulfate were isolated from reaction mixtures and chemically degraded, (35)S was found in all three sulfur atoms of trithionate; however, only the inner sulfur atom of thiosulfate was radioactive. From these data we conclude that the bisulfite reductase of D. nigrificans reduces bisulfite to trithionate and that thiosulfate and sulfide are endogenous side products of the reaction.

Siroheme: a new prosthetic group participating in six-electron reduction reactions catalyzed by both sulfite and nitrite reductases

Ferredoxin-nitrite reductase (EC 1.7.7.1) of spinach, an enzyme that catalyzes the six-electron reduction of nitrite to ammonia, contains siroheme, the new type of prosthetic group recently found in several sulfite reductases (both assimilatory and dissimilatory) that can catalyze the reduction of sulfite to sulfide, also a six-electron reduction. The prosthetic group of sulfite reductase had previously been shown to be an iron-tetrahydroporphyrin of the isobacteriochlorin type (adjacent pyrrole rings reduced) with eight carboxylate side chains. This finding suggests that both types of "multi-electron" reduction processes may share common mechanistic features.

Isolation of a bisulfite reductase activity from Desulfotomaculum nigrificans and its identification as the carbon monoxide-binding pigment P582

Crude preparations of Desulfotomaculum nigrificans were found to reduce bisulfite to trithionate, thiosulfate, and sulfide. The bisulfite reductase of this organism was partially purified and observed to reduce bisulfite to trithionate as the major product and with thiosulfate and sulfide as minor products. The enzyme exhibited spectral properties identical to the carbon monoxide-reacting pigment (P582) isolated from this organism. It is concluded that the bisulfite reductase of D. nigrificans is P582 and that this organism utilizes a pathway which involves trithionate during the reduction of bisulfite to sulfide.