On the redox properties of three bisulfite reductases from the sulfate-reducing bacteria

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.

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.

S = 9/2 EPR signals are evidence against coupling between the siroheme and the Fe/S cluster prosthetic groups in Desulfovibrio vulgaris (Hildenborough) dissimilatory sulfite reductase

Sulfite reductases contain siroheme and iron-sulfur cluster prosthetic groups. The two groups are believed to be structurally linked via a single, common ligand. This chemical model is based on a magnetic model for the oxidized enzyme in which all participating iron ions are exchange coupled. This description leads to two serious discrepancies. Although the iron-sulfur cluster is assumed to be a diamagnetic cubane, [4Fe-4S]2+, all iron appears to be paramagnetic in Mössbauer spectroscopy. On the other hand, EPR spectroscopy has failed to detect anything but a single high-spin heme. We have re-addressed this problem by searching for new EPR spectroscopic clues in concentrated samples of dissimilatory sulfite reductase from Desulfovibrio vulgaris (Hildenborough). We have found several novel signals with effective g values of 17, 15.1, 11.7, 9.4, 9.0, 4. The signals are interpreted in terms of an S = 9/2 system with spin-Hamiltonian parameters g = 2.00, D = -0.56 cm-1, magnitude of E/D = 0.13 for the major component. In a reductive titration with sodium borohydride the spectrum disappears with Em = -205 mV at pH 7.5. Contrarily, the major high-spin siroheme component has S = 5/2, g = 1.99, D = +9 cm-1, magnitude of E/D = 0.042, and Em = -295 mV. The sum of all siroheme signals integrates to 0.2 spin/half molecule, indicating considerable demetallation of this prosthetic group. Rigorous quantification procedures for S = 9/2 are not available, however, estimation by an approximate method indicates 0.6 S = 9/2 spin/half molecule. The S = 9/2 system is ascribed to an iron-sulfur cluster. It follows that this cluster is probably not a cubane, is not necessarily exchange-coupled to the siroheme, and, therefore, is not necessarily structurally close to the siroheme. It is suggested that this iron-sulfur prosthetic group has a novel structure suitable for functioning in multiple electron transfer.

Low-spin sulfite reductases: a new homologous group of non-heme iron-siroheme proteins in anaerobic bacteria

Two new low molecular weight proteins with sulfite reductase activity, isolated from Methanosarcina barkeri (DSM 800) and Desulfuromonas acetoxidans (strain 5071), were studied by EPR and optical spectroscopic techniques. Both proteins have visible spectra similar to that of the low-spin sulfite reductase of Desulfovibrio vulgaris strain Hildenborough and no band at 715 nm, characteristic of high-spin Fe3+ complexes in isobacteriochlorins is observed. EPR shows that as isolated the siroheme is in a low-spin ferric state (S = 1/2) with g-values at 2.40, 2.30 and 1.88 for the Methanosarcina barkeri enzyme and g-values at 2.44, 2.33 and 1.81 for the Desulfuromonas acetoxidans enzyme. Chemical analysis shows that both proteins contain one siroheme and one [Fe4S4] center per polypeptidic chain. These results suggest that the low molecular weight, low-spin non-heme iron siroheme proteins represent a new homologous class of sulfite reductases common to anaerobic microorganisms.

Characterization of a new type of dissimilatory sulfite reductase present in Thermodesulfobacterium commune

A new type of dissimilatory bisulfite reductase, desulfofuscidin, was isolated from the nonsporeforming thermophilic sulfate-reducing microorganism Thermodesulfobacterium commune. The molecular weight of the enzyme was estimated at 167,000 by sedimentation equilibrium, and the protein was pure by both disc electrophoresis and ultracentrifugation. The bisulfite reductase was a tetramer and had two types of subunits with an alpha(2)beta(2) structure and an individual molecular weight of 47,000. The enzyme exhibited absorption maxima at 576, 389, and 279 nm, with a weak band at 693 nm. Upon the addition of dithionite, the absorption maxima at 576 and 693 nm were weakened, and a new band appeared at 605 nm. The protein reacted with CO in the presence of dithionite to give a complex with absorption peaks at 593, 548, and 395 nm. The extinction coefficients of the purified enzyme at 576, 389, and 279 nm were 89,000, 310,000, and 663,000 M(-1) cm(-1), respectively. Siroheme was detected as the prosthetic group. The protein contains 20 to 21 nonheme iron atoms and 16 to 17 acid-labile sulfur groups per molecule. The data suggest the presence of four sirohemes and probably four (4Fe-4S) centers per molecule by comparison with desulfoviridin, the dissimilatory sulfite reductase from Desulfovibrio species. The protein contains 36 cysteine residues and is high in acidic and aromatic amino acids. The N-terminal amino acids of the alpha and beta subunits were threonine and serine, respectively. With reduced methyl viologen as electron donor, the major product of sulfite reduction was trithionate, and the pH optimum for activity was 6.0. The enzyme was stable to 70 degrees C and denatured rapidly above this temperature. The dependence of T. commune bisulfite reductase activity on temperature was linear between 35 and 65 degrees C, and the Q(10) values observed were above 3. The presence of this new type of dissimilatory bisulfite reductase in T. commune is discussed in terms of taxonomic significance.