Sulfite reductases

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. 

Desulfoviridin was found in D. gigas, D. salexigens, and D. vulgaris Hildenborough. Desulfoviridin is composed of three different subunits organized in a 0 2/3272 configuration with a total molecular mass of approximately 200 kDa. The molecular mass of the a, (3, and 7 subunits was calculated to be 50, 40, and 11 kDa, respectively (156). 

Desulforubidin was found in strains of the Desulfomicrobium genus and has been described as the sulfite reductase of this genus. The subunit composition and molecular mass are similar to what was observed for desulfoviridin. However, in desulforubidin all sirohydrochlorins are metalated as proved by Mossbauer spectroscopy (152). The as-isolated protein contains four [4Fe-4S] clusters two of them are exchange-coupled to two paramagnetic sirohemes. 

Low-spin sulfite reductase were isolated from D. vulgaris (160), De-sulfuromonas acetoxidans (161), and Methanosarcina barker) (DSM 800) (162). The D. vulgaris protein has a molecular mass of 27 kDa and contains a single [4Fe-4S] cluster and one siroheme. The EPR spectrum shows a rhombic signal with g values at 2.44, 2.36, and 1.77, characteristic of a ferriheme low-spin system. This is a unique 

Molecular hydrogen plays a major role on the oxidation-reduction processes involved in bacterial energetics, as well as in the degradation and conversion of biomass related with all major elemental cycles. Hydrogenase has a key role on this process and catalyzes the reversible oxidation of dihydrogen, important in bacterial anaerobic metabolism  

As sulfite reductase is a water-soluble enzyme, previously it has been extracted without the aid of a detergent. The water-extracted enzyme has a molecular mass of 200 kDa (subunits, 55 and 45 kDa) its absorption peaks are at 390, 410, 585, and 628 nm (Kobayashi et al., 1972). Steuber et al. (1994) purified the enzyme with the aid of a detergent from D. desulfuricans. The detergent-extracted enzyme shows the absorption peaks at 391, 410, 583, and 630 nm, and its molecular mass is also 200 kDa but consists of three subunits (50, 45, and 11 kDa). The detergent-extracted enzyme catalyzes the reduction of hydrogensulfite with ferrocytochrome 

it has been established that cytochrome c3 functions as the electron donor for sulfite reductase. 

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In 1973, the first naturally occurring isobacteriochlorin, iron-containing siroheme, was isolated1 from a sulfite reductase of Escherichia coli. Later it was also discovered in sulfite and nitrite reductases of numerous bacteria and plants.2 Iron-free sirohydrochlorins (also called factor II) were discovered in vitamin B12 producing bacteria.3-4 Together with factor III. a sirohydrochlorin methylated in the 20-position, the reduced forms of factor II and factor III were identified as biosynthetic intermediates in the biosynthesis of vitamin B12.5... 

Eschenbrenner M, E Coves, M Fontecave (1995) The flavin reductase activity of the flavoprotein component of sulfite reductase bom Escherichia coli. J Biol Chem 270 20550-20555. 

Murphy Ml, LM Siegel, H Kamin (1973) Reduced nicotinamide adenine dinucleotide phosphate-sulfite reductase of enterobacteria. II. Identification of a new class of heme prosthetic group an iron-tetrahydroporphyrin (isobacteriochlorin type) with eight carboxylic acid groups. J Biol Chem 248 2801-3814. 

The sulfite reductase from the hyperthermophilic methanogen Methanocaldococcus jannashii is able to reduce the otherwise toxic sulfite to sulfide that is required for growth. In contrast to most organisms that use nicotinamides and cytochromes as electron carriers, this organism uses a coenzyme p42o-dependent reductase (Johnson and Mukhopadhyay 2005). 

Johnson EF, B Mukhopadhyay (2005) A new type of sulfite reductase, a novel coenzyme F420 -dependent enzyme, from the methanarchaeon Methanocaldocccus jannaschii. J Biol Client 280 38776-38786. 

There are a number of more complex Fe-S clusters whose structures are known and some of them are illustrated in Figure 2.10 (Beinert, 2000 Holm et al., 1996). In the sulfite reductase of Escherichia coli a 4Fe-4S cluster is linked via a cysteine to the... 

Figure 2.10 Schematic structures of (a) sulfite reductase of Escherichia coli in which a 4Fe-4S cluster is linked via a cysteine to the iron in a sirohaem (b) P cluster of nitrogenase (c) FeMoCo cluster of nitrogenase (d) the binuclear site in Desulforibrio gigas hydrogenase.

There is some evidence that the iron-sulfur protein, FhuF, participates in the mobilization of iron from hydroxamate siderophores in E. coli (Muller et ah, 1998 Hantke, K. unpublished observations). However, a reductase activity of FhuF has not been demonstrated. Many siderophore-iron reductases have been shown to be active in vitro and some have been purified. The characterization of these reductases has revealed them to be flavin reductases which obtain the electrons for flavin reduction from NAD(P)H, and whose main functions are in areas other than reduction of ferric iron (e.g. flavin reductase Fre, sulfite reductase). To date, no specialized siderophore-iron reductases have been identified. It has been suggested that the reduced flavins from flavin oxidoreductases are the electron donors for ferric iron reduction (Fontecave et ah, 1994). Recently it has been shown, after a fruitless search for a reducing enzyme, that reduction of Co3+ in cobalamin is achieved by reduced flavin. Also in this case it was suggested that cobalamins and corrinoids are reduced in vivo by flavins which may be generated by the flavin... 

Arendsen, A.F., Verhagen, M.F.J.M., Wolbert, R.B.G., Pierik, A.J., Stams, A.J.M., Jetten, M.S.M., and Hagen, W.R. 1993. Thedissimilatory sulfite reductase fromDesulfosarcina variabilis is a desulforubidin containing uncoupled metalated siroheme and S = 9/2 iron-sulfur clusters. Biochemistry 32 10323-10330. 

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. 

Figure 2 Structures of the actrive sites of metalloenzymes containing metal-sulfur cluster units, (a) Fe only hydrogenase, H-cluster (Hoxfarm) (b) Sulfite reductase (c) NiFe carbon monoxide dehydrogenase, C-cluster and (d) NiFe carbon monoxide dehydrogenase, A-cluster, which functions as acetyl-CoA synthase...

Cytochromes, catalases, and peroxidases all contain iron-heme centers. Nitrite and sulfite reductases, involved in N-O and S-O reductive cleavage reactions to NH3 and HS-, contain iron-heme centers coupled to [Fe ] iron-sulfur clusters. Photosynthetic reaction center complexes contain porphyrins that are implicated in the photoinitiated electron transfers carried out by the complexes. 

Crane, B. R., Getzoff, E. D., The relationship between structure and function for the sulfite reductases,... 

Although electron transfers in biological systems are generally expected to be non-adiabatic, it is possible for some intramolecular transfers to be close to the adiabatic limit, particularly in proteins where several redox centers are held in a very compact arrangement. This situation is found for example in cytochromes C3 of sulfate-reducing bacteria which contain four hemes in a 13 kDa molecule [10, 11], or in Escherichia coli sulfite reductase where the distance between the siroheme iron and the closest iron of a 4Fe-4S cluster is only 4.4 A [12]. It is interesting to note that a very fast intramolecular transfer rate of about 10 s was inferred from resonance Raman experiments performed in Desulfovibrio vulgaris Miyazaki cytochrome Cj [13]. 

The ability to catalyse the evolution or oxidation of H2 may have been exploited by the earliest life forms as H2 would have been present in the early prebiotic environments. The origins of the proton-dependent chemiosmotic mechanism for ATP synthesis may also reflect the formation of proton gradients created by hydrogenases on either side of the cytoplasmic membrane. In addition, it has been speculated that the coupling of H2 and S metabolisms was also of fundamental importance in the origin of life. These two processes seem intimately coupled in the bifunctional sulfhydrogenase found in Pyrococcus furiosus (a combination of subunits for hydrogenase and sulfite reductase) which can dispose of excess reductant either by the reduction of protons to H2 or S° to H2S (Ma et al. 1993 Pedroni et al. 1995). 

Pedroni, P., Della Volpe, A., Galli, G., Mura, G. M., Pratesi, C. and Grandi, G. (1995) Characterization of the locus encoding the [Ni-Fe] sulfhydrogenase from the archaeon Pyrococcus furiosus Evidence for a relationship to bacterial sulfite reductases. Microbiology, 141, 449-58. 

Belinsky MI. 1996. Hyperfine evidence of strong double exchange in multimetallic [Fe4S4]-Fe) active center of Escherichia coli sulfite reductase. J Biol Inorg Chem 1 186-8. 

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