Specificity of respiratory pathways involved in the reduction of sulfur compounds by Salmonella enterica.
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The tetrathionate (Ttr) and thiosulfate (Phs) reductases of Salmonella enterica LT2, together with the polysulfide reductase (Psr) of Wolinella succinogenes, are unusual examples of enzymes containing a molybdopterin active-site cofactor since all formally catalyse sulfur-sulfur bond cleavage. This is in contrast to the oxygen or hydrogen transfer reactions exhibited by other molybdopterin enzymes. Here the catalytic specificity of Ttr and Phs has been compared using both physiological and synthetic electron-donor systems. Ttr is shown to catalyse reduction of trithionate but not sulfur or thiosulfate. In contrast, Phs cannot reduce tetrathionate or trithionate but allows whole cells to utilize elemental sulfur as an electron acceptor. Mechanisms are proposed by which the bacterium is able to utilize an insoluble sulfur substrate by means of reactions at the cytoplasmic rather than the outer membrane. (+info)
Product analysis of bisulfite reductase activity isolated from Desulfovibrio vulgaris.
(2/5)
Bisulfite reductase was purified from extracts of Desulfovibrio vulgaris. By colorimetric analyses trithionate was found to be the major product, being formed in quantities 5 to 10 times more than two other detectable products, thiosulfate and sulfide. When [35S]bisulfite was used as the substrate, all three products were radioactively labeled. Degradation of [35S]trithionate showed that all of its sulfur atoms were equally labeled. In contrast, [35S]thiosulfate contained virtually all of the radioactivity in the sulfonate atom while the sulfane atom was unlabeled. These results, in conjunction with the funding that the sulfide was radioactive, led to the conclusion that bisulfite reductase reduced bisulfite to trithionate as the major product and sulfide as the minor product; the reason for the unusual labeling pattern found in the thiosulfate molecule was not apparent at this time. When bisulfite reductase was incubated with [35S]bisulfite in the presence of another protein fraction, FII, the thiosulfate formed from this reaction contained both sulfur atoms having equal radioactivity. This discovery, plus the fact that trithionate was not reduced to thiosulfate under identical conditions, led to the speculation that bisulfite could be reduced to thiosulfate by another pathway not involving trithionate. (+info)
Characterization of a novel thiosulfate-forming enzyme isolated from Desulfovibrio vulgaris.
(3/5)
An enzyme that formed thiosulfate from bisulfite and trithionate was purified from extracts of Desulfovibrio vulgaris. This enzyme, designated as "thiosulfate-forming" enzyme, required the presence of both bisulfite and trithionate. Various 35S-labeling studies showed that thiosulfate was formed from bisulfite and the inner sulfur atom of trithionate. This involved a nucleophilic attack by the bisulfite ion, resulting in the displacement of the two outer sulfonate groups of trithionate that recycled to participate as free bisulfite in subsequent reactions. This reaction required a reduction, presumably by a concerted mechanism with thiosulfate formation. The natural electron carrier cytochrome c3 participated in this reductive formation of thiosulfate. This reaction was coupled to the bisulfite reductase-catalyzed reaction, which resulted in the reconstruction of a thiosulfate-forming pathway from bisulfite. (+info)
Mechanism of adsorption of hard and soft metal ions to Saccharomyces cerevisiae and influence of hard and soft anions.
(4/5)
The applicability of the hard-and-soft principle of acids and bases in predicting metal adsorption characteristics in a biological context was investigated for metabolism-independent uptake of the metal ions Sr2+, Mn2+, Zn2+, Cu2+, Cd2+, and Tl+ by Saccharomyces cerevisiae. Metal adsorption increased with external metal concentration (5 to 50 microM), although some saturation of uptake of the harder ions examined, Sr2+, Mn2+, and Zn2+, was evident at the higher metal concentrations. Cation displacement experiments indicated that, with the exception of Tl+, relative covalent bonding (H+ displacement) of the metals was greater at low metal concentrations, while weaker electrostatic interactions (Mg2+ plus Ca2+ displacement) became increasingly important at higher concentrations. These results were correlated with curved Scatchard and reciprocal Langmuir plots of metal uptake data. Saturation of covalent binding sites was most marked for the hard metals, and consequently, although no relationship between metal hardness and ionic/covalent bonding ratios was evident at 10 microM metal, at 50 microM the ratio was generally higher for harder metals. Increasing inhibition of metal uptake at increasing external anion concentrations was partially attributed to the formation of metal-anion complexes. Inhibitory effects of the hard anion SO42(-) were most marked for uptake of the hard metals Sr2+ and Mn2+, whereas greater relative effects on adsorption of the softer cations Cu2+ and Cd2+ were correlated with complexation by the soft anion S2O32(-). Inhibition of uptake of the borderline metal Zn2+ by SO42(-) and that by S2O32(-) were approximately equal.(ABSTRACT TRUNCATED AT 250 WORDS) (+info)
Effect of pH on sulfite oxidation by Thiobacillus thiooxidans cells with sulfurous acid or sulfur dioxide as a possible substrate.
(5/5)
The oxidation of sulfite by Thiobacillus thiooxidans was studied at various pH values with changing concentrations of potassium sulfite. The optimal pH for sulfite oxidation by cells was a function of sulfite concentrations, rising with increasing substrate concentrations, while that by the cell extracts was unaffected. The sulfite oxidation by cells was inhibited at high sulfite concentrations, particularly at low pH values. The results from kinetic studies show that the fully protonated form of sulfite, sulfurous acid or sulfur dioxide, is the form which penetrates the cells for the oxidation. (+info)