Evidence for the reversibility of the reaction catalyzed by adenosine 5'-phosphosulfate reductase

Fractionation of sulfur isotopes by Desulfovibrio vulgaris mutants lacking hydrogenases or type I tetraheme cytochrome c 3

The sulfur isotope effect produced by sulfate reducing microbes is commonly used to trace biogeochemical cycles of sulfur and carbon in aquatic and sedimentary environments. To test the contribution of intracellular coupling between carbon and sulfur metabolisms to the overall magnitude of the sulfur isotope effect, this study compared sulfur isotope fractionations by mutants of Desulfovibrio vulgaris Hildenborough. We tested mutant strains lacking one or two periplasmic (Hyd, Hyn-1, Hyn-2, and Hys) or cytoplasmic hydrogenases (Ech and CooL), and a mutant lacking type I tetraheme cytochrome (TpI-c 3). In batch culture, wild-type D. vulgaris and its hydrogenase mutants had comparable growth kinetics and produced the same sulfur isotope effects. This is consistent with the reported redundancy of hydrogenases in D. vulgaris. However, the TpI-c 3 mutant (ΔcycA) exhibited slower growth and sulfate reduction rates in batch culture, and produced more H2 and an approximately 50% larger sulfur isotope effect, compared to the wild type. The magnitude of sulfur isotope fractionation in the CycA deletion strain, thus, increased due to the disrupted coupling of the carbon oxidation and sulfate reduction pathways. In continuous culture, wild-type D. vulgaris and the CycA mutant produced similar sulfur isotope effects, underscoring the influence of environmental conditions on the relative contribution of hydrogen cycling to the electron transport. The large sulfur isotope effects associated with the non-ideal stoichiometry of sulfate reduction in this study imply that simultaneous fermentation and sulfate reduction may be responsible for some of the large naturally-occurring sulfur isotope effects. Overall, mutant strains provide a powerful tool to test the effect of specific redox proteins and pathways on sulfur isotope fractionation.

Anaerobic Production of Thiobacillus denitrificans for the Enzyme Rhodanese

A method for the anaerobic growth of Thiobacillus denitrificans in a 140-liter (total capacity) stainless-steel culture vessel is described. As a result of controlling the pH value of cultures, and of ensuring that certain essential nutrients were in excess, cell yields approaching 700 mg (dry weight) per liter were obtained. These were over threefold higher than the best yields hitherto reported. The average rhodanese content of the cells from four cultures was 176,000 units per gram (dry weight). Adenosine-5'-phosphosulfate reductase (average content, 238 units per gram dry weight) and adenylate kinase (average content, 15,300 units per gram, dry weight) were also present.

STUDIES ON THERMOPHILIC SULFATE-REDUCING BACTERIA III. : Adenosine Triphosphate-sulfurylase of Clostridium nigrificans and Desulfovibrio desulfuricans

Akagi, J. M. (University of Kansas, Lawrence) and L. Leon Campbell. Studies on thermophilic sulfate-reducing bacteria. III. Adenosine triphosphate-sulfurylase of Clostridium nigrificans and Desulfovibrio desulfuricans. J. Bacteriol. 84:1194-1201. 1962.-Adenosine triphosphate (ATP)-sulfurylase, which catalyzes the formation of adenosine-5'-phosphosulfate (APS) from ATP and SO(4) (=), has been purified from crude extracts of Clostridium nigrificans and Desulfovibrio desulfuricans by (NH(4))(2)SO(4) fractionation and triethylaminoethyl column chromatography. The enzyme from both sources operates over a broad pH range from 6.0 to 9.5. Below pH 6.0, activity decreases sharply, with no detectable activity at pH 5.0. Of the nucleotides tested (ATP and the triphosphates of deoxyadenosine, uridine, inosine, and guanosine), only ATP was acted upon by the enzyme from either source. The enzyme requires Mg(++) for activity. Incubation of the enzyme from both organisms with ATP and S(35)O(4) (=) in the presence of helium resulted in the formation of an S(35)-labeled nucleotide whose electrophoretic mobility was identical to that of chemically prepared APS. When incubated with ATP and the group VI anions (CrO(4), MoO(4), WO(4)), the enzyme from both organisms formed an unstable intermediate, resulting in the accumulation of pyrophosphate. Thermal stability studies revealed that the ATP-sulfurylase of C. nigrificans was stable at higher temperatures than the enzyme obtained from D. desulfuricans. Exposure of the enzyme from C. nigrificans to 65 C for 2 hr gave virtually no decrease in activity. In contrast, the enzyme from D. desulfuricans was completely inactivated after 30 min at 55 C, after 3 min at 60 C, or after 1 min at 65 C.

COUPLING OF PHOSPHORYLATION AND CARBON DIOXIDE FIXATION IN EXTRACTS OF THIOBACILLUS THIOPARUS

Johnson, Emmett J. (University of Mississippi Medical Center, Jackson), and Harry D. Peck, Jr. Coupling of phosphorylation and carbon dioxide fixation in extracts of Thiobacillus thioparus. J. Bacteriol. 89:1041-1050. 1965.-A cell-free system from Thiobacillus thioparus which fixes large quantities of C(14)O(2) in the presence of ribose-5-phosphate, adenosine triphosphate (ATP), and Mg(++) has been described. The specific activity (0.041 mumole of ribulose-1,5-diphosphate min(-1) mg(-1) protein) of the CO(2)-fixing system approaches that of green plants, and is further evidence for the importance of the role of carboxydismutase in the thiobacilli. In addition to ATP, adenosine diphosphate (ADP) and other nucleoside triphosphates served with varying degrees of effectiveness for the fixation of C(14)O(2). The ATP requirement for CO(2) fixation was partially replaced under aerobic conditions by a combination of SO(3) (=), PO(4) ( identical with), and adenosine monophosphate (AMP). Phosphorylation and CO(2) fixation were separated in time by first incubating SO(3) (=) and AMP aerobically, and then anaerobically introducing C(14)O(3) (=) and ribose-5-phosphate into the reaction mixture. During the first incubation, P(32)O(4) ( identical with) was esterified into nucleotides, mainly ADP, and in the second incubation C(14)O(2) was fixed, with the concomitant utilization of almost equal amounts of the esterified phosphate. These data provide the first in vitro evidence for the mechanism of the coupling of CO(2) fixation and phosphorylation in T. thioparus. The fixation of C(14)O(2) was shown to be almost completely inhibited by AMP. This inhibition was not due to the conversion of ATP to ADP by adenylic kinase, or to the binding of magnesium by the nucleotide. The inhibition was specific for AMP, since other mononucleotides, adenosine, and adenine did not inhibit. The AMP regulation of CO(2) fixation may represent a basic control mechanism in autotrophic metabolism.

Substrate level versus oxidative phosphorylation in the generation of ATP in Thiobacillus denitrificans

Particulate fractions of Thiobacillus denitrificans catalyse that the phosphorylation of ADP to ATP during the oxidation of various inorganic sulphur compounds or NADH via an electron transport chain. On the other hand, a "soluble" cell-free fraction synthesized ATP from APS and inorganic phosphate. The production of ATP was verified either by the firefly luciferin-luciferase enzyme system or by the incorporation of 32Pi into ATP. During the oxidation of sulphide, sulphite and NADH the production of ATP from ADP by particulate fractions is inhibited by compounds that inhibit electron transfer and by uncouplers of oxidative phosphorylation. However, these compounds had little effect on the production of ATP from AMP during the oxidation of sulphite by the soluble fraction. NADH was the most effective electron donor for oxidative phosphorylation. The soluble fraction contained high activities of ATP sulphurylase, inorganic pyrophosphatase and adenylate kinase but ADP sulphurylase activity was relatively low. The effects of inhibitors on ATP production from APS and Pi are compared with those on adenylate kinase and ATP sulphurylase.

Dissimilatory reduction of bisulfite by Desulfovibrio vulgaris

The reduction of bisulfite by Desulfovibrio vulgaris was investigated. Crude extracts reduced bisulfite to sulfide without the formation (detection) of any intermediates such as trithionate or thiosulfate. When the particulate fractions was removed from crude extracts by high-speed centrifugation, the soluble supernatant fraction reduced bisulfite sequentially to trithionate, thiosulfate, and sulfide. Addition of particles or purified membranes to the soluble fraction restored the original activity demonstrated by crude extracts, i.e., reduction of bisulfite to sulfide without the formation of trithionate and/or thiosulfate. By using antiserum directed against bisulfite reductase, the reduction of bisulfite by crude extracts was inhibited. This finding, in addition to several recycling studies of thiosulfate reduction, provided evidence that bisulfite reduction by D. vulgaris operated through the pathway involving trithionate and thiosulfate as intermediates. The role of membranes in this process is discussed.

Reduction of adenylylsulfate and 3'-phosphoadenylylsulfate in phototrophic bacteria

Extracts of 14 species of phototrophic bacteria, partly grown with different sulfur compounds, were tested for their ability to form volatile sulfur compounds from adenylylsulfate (APS) and 3'-phosphoadenylylsulfate (PAPS). The Rhodospirillum species showed marked activities with both APS and PAPS while the Rhodopseudomonas species seem to prefer PAPS. The Chromatiaceae exhibited the strongest activities with APS, whereas Chlorobium limicola had equally high activity with PAPS.

Role of thiosulfate in bisulfite reduction as catalyzed by Desulfovibrio vulgaris

Studies with (35)S-labeled substrates were conducted to investigate the pathway involved in the reduction of sulfite to sulfide by cell-free extracts of the sulfate-reducing organism Desulfovibrio vulgaris. The results showed that accumulation of thiosulfate occurred when crude extracts were incubated under appropriate conditions with sulfite as substrate. With labeled sulfite as substrate, thiosulfate with equal distribution of radioactivity in both sulfur atoms was formed. When the rates of formation of (35)S(2-) from inner- and outer-labeled thiosulfate were compared, the rate of formation from outer-labeled thiosulfate was greater. Time studies with S-(35)SO(3) (2-) showed an increase of (35)S(2-) with time and an increasing ratio of doubly labeled to inner labeled thiosulfate remaining in the reaction mixture. From these studies it is concluded that thiosulfate is a stable intermediate formed from sulfite during the reduction of sulfate by D. vulgaris. Both sulfur atoms are derived from sulfite; during the utilization of thiosulfate, the outer sulfur is reduced to sulfide and the inner sulfur recycles through a sulfite pool.

Formation of thiosulfate from sulfite by Desulfovibrio vulgaris

Crude extracts of Desulfovibrio vulgaris reduced sulfite to sulfide. Ammonium sulfate fractionation of crude extracts separated a thiosulfate-forming system from sulfite- and thiosulfate-reductase activities. Further purification by sucrose density centrifugation separated the thiosulfate-forming system into two components, both of which were required for the reaction. In addition to these two components, cytochrome c(3), ferredoxin, and hydrogenase were required to form thiosulfate from sulfite. By absorption spectra and from the effect of pH and substrate concentration, the ionic species acting as the substrate for thiosulfate-formation was concluded to be bisulfite.

Thiosulfate reductase isolated from Desulfotomaculum nigrificans

A thiosulfate reductase from Desulfotomaculum nigrificans has been partially purified by ammonium sulfate fractionation, diethylaminoethylcellulose chromatography, and sucrose density gradient centrifugation. With inner-and outer-labeled (35)S-thiosulfate, the enzyme reduced only the outer sulfur atom to hydrogen sulfide. The enzyme was inhibited by sulfite and also by several sulfhydryl inhibitors. The K(m) value for this enzyme was calculated to be 1.3 x 10(-1)m. Other inorganic sulfur compounds, such as sulfate, sulfite, tetrathionate, and dithionate, were not reduced by this enzyme.

INORGANIC PYROPHOSPHATASE OF DESULFOVIBRIO DESULFURICANS

Akagi , J. M. (University of Illinois, Urbana) and L. Leon Campbell . Inorganic pyrophosphatase of Desulfovibrio desulfuricans . J. Bacteriol. 86: 563–568. 1963.—The inorganic pyrophosphatase of Desulfovibrio desulfuricans was purified 136-fold by (NH 4 ) 2 SO 4 and ethanol fractionation and diethylaminoethyl cellulose chromatography. Mg ++ or Mn ++ was required for optimal activity; Co ++ was only 65% as effective as Mg ++ . The optimal ratio of Mg ++ to pyrophosphate was 1.0 at pH 8.0. The K s for the pyrophosphatase was found to be in the region of 1.9 × 10 −3 m . Sulfhydryl inhibitors and sodium fluoride had no effect on enzyme activity at a concentration of 10 −3 m . The purified enzyme did not hydrolyze adenosine triphosphate, glycerol phosphate, diphenyl phosphate, or p -nitrophenyl phosphate. Thermal stability studies showed that the enzyme is rapidly inactivated at temperatures above 40 C.

Enzymatic basis for assimilatory and dissimilatory sulfate reduction

Peck, H. D., Jr. (Oak Ridge National Laboratory, Oak Ridge, Tenn.). Enzymatic basis for assimilatory and dissimilatory sulfate reduction. J. Bacteriol. 82: 933-939. 1961.-Two pathways for the reduction of sulfate to sulfite in bacteria have been previously described. The substrate for sulfate reduction by extracts of yeast is 3'-phosphoadenosine-5'-phosphosulfate (PAPS) and, in contrast, the substrate for sulfate reduction in extracts of Desulfovibrio desulfuricans is adenosine-5'-phosphosulfate (APS). The enzymes catalyzing these reductions have been termed PAPS-reductase and APS-reductase, respectively. Since yeasts are "assimilatory sulfate reducers", i.e., reduce only enough sulfate to satisfy nutritional requirements for sulfur, and D. desulfuricans is a "dissimilatory sulfate reducer", i.e., utilizes sulfate as its terminal electron acceptor in anaerobic respiration, the pathway of sulfate reduction was determined in 25 microorganisms to ascertain whether there is a correlation between the pathway of sulfate reduction and the physiological role of sulfate in the metabolism of bacteria. Assimilatory sulfate reducers reduced sulfate in the form of PAPS, and, with one exception, APS-reductase was found only in dissimilatory sulfate reducers. APS-reductase was also found in the Thiobacilli in high specific activity and is involved in the oxidation of reduced sulfur compounds to sulfate.