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Biomedical subjects

V K Shah

Publications and source records attributed to V K Shah.

At least 91 records · Page 5Linked to original sources
PubMed1975

Nitrogenase. VI. Acetylene reduction assay: Dependence of nitrogen fixation estimates on component ratio and acetylene concentration.

Acetylene reduction, an assay for nitrogenase activity (nitrogen:(acceptor) oxidoreductase, EC 1.7.99.2), Is dependent on the ratio of the two protein components of nitrogenase as well as on C2H2 concentration. As the component I : component II ratio (based on activity) is increased, the C2H2 reduction : N2 fixation ratio decreases to a minimum of 3.4 and then increases. The minimum is found at a ratio near 1 : 1. At a component I : component II ratio of 20 : 1, the C2H2 reduction : N2 fixation ratio is 5.3. Acetylene exhibits substrate inhibition in assays for nitrogenase activity. Both the apparent Km and Ki for acetylene vary as a function of the relative concentrations of components I and II present in the assay. When the more labile component II is limiting in the assay and "saturating" levels of C2H2 (above 0.1 atm) are used, N2-fixation capacity may be greatly under-estimated.

Acetylene↗
PubMed1975

Biochemistry and genetics of Klebsiella pneumoniae mutant strains unable to fix N2.

Selected mutant strains of Klebsiella pneumoniae that are unable to fix nitrogen have been characterized according to nitrogenase component activity as well as antigenic cross-reacting material. The lesions in these strains have been mapped by transduction, and the results indicate that there are at least five genes specifically responsible for nitrogen fixation in vivo. Besides genes that specify the structure of the two nitrogenase components, there is a gene for a factor that is required for component I activity and a gene that codes for a factor possibly involved in electron transport to component II. A mutation in another site does not allow the organism to produce either of the nitrogenase components. All of these genes are co-transducible with the gene that specifics the structure of histidinol dehydrogenase.

Alcohol Oxidoreductases↗
PubMed1974

Mutant of Azotobacter vinelandii that hyperproduces nitrogenase component II.

A mutant strain of Azotobacter vinelandii that is unable to fix N(2) produces high levels of nitrogenase component II. Activities of revertants from this mutant strain indicate that a single genetic lesion is responsible for both hyperproduction of component II and the inability to produce component I.

Azotobacter↗
PubMed1974

Effect of molybdenum starvation and tungsten on the synthesis of nitrogenase components in Klebsiella pneumonia.

Klebsiella pneumoniae M5a1 grows well in the presence or absence of molybdenum in media containing excess NH(4) (+). However, growth on N(2) is completely dependent on the presence of molybdenum in the medium. Tungstate competes with the molybdate requirement during growth on N(2). In molybdenum-depleted medium, neither protein component of nitrogenase is active and neither component can be detected antigenically. These data provide evidence that molybdenum is an inducer of nitrogenase synthesis.

Bacterial Proteins↗
PubMed1974

Regulation of nitrogenase synthesis by oxygen in Klebsiella pneumoniae.

Klebsiella pneumoniae does not fix N(2) under aerobic conditions. The two protein components required for nitrogenase activity were studied during aeration of cells in nitrogen-free media. Component II of nitrogenase was inactivated more slowly in vivo than component I during aeration. The rate of loss of component II was less than the rate of component II synthesis during derepression. No inactive components were detected in cells that had been growing on NH(4) (+) and then aerated in nitrogen-free medium. This supports the hypothesis that O(2) somehow represses the formation of nitrogenase.

Aerobiosis↗
PubMed1974

Activation of inactive nitrogenase by acid-treated component I.

When Azotobacter vinelandii was derepressed for nitrogenase synthesis in a N-free medium containing tungstate instead of molybdate, an inactive component I was synthesized. Although this inactive component I could be activated in vivo upon addition of molybdate to the medium, it could not be activated in vitro when molybdate was added to the extracts. Activation occurred, however, when an acid-treated component I was added to extracts of cells derepressed in medium containing tungstate. Acid treatment completely abolished component I activity. Mutant strains UW45 and UW10 were unable to fix N(2). Both strains synthesized normal levels of component II but produced inactive component I. Acid-treated component I activated inactive component I in extracts of mutant strain UW45 but not mutant strain UW10. This activating factor could be obtained from N(2)-fixing Klebsiella pneumoniae, Clostridium pasteurianum, and Rhodospirillum rubrum.

Azotobacter↗
PubMed1973

Internal membrane control in Azotobacter vinelandii.

Azotobacter vinelandii was grown on N(2), NH(4) (+), or NO(3) (-), and an internal membrane network was observed by electron microscopy of thin sections of cells. Cells obtained in early exponential growth contained less internal membrane than did cells from cultures in late exponential growth. It seems likely that O(2) has a role in regulating the amount of internal membrane structure.

Ammonium Chloride↗
PubMed1973

Protection of nitrogenase in Azotobacter vinelandii.

The site or sites that protect nitrogenase from O(2) inactivation in vivo are sensitive to sodium azide or 2,4-dinitrophenol. Both components of nitrogenase can be synthesized when oxidative phosphorylation is disrupted.

Acetates↗
PubMed1972

Electron paramagnetic resonance of nitrogenase and nitrogenase components from Clostridium pasteurianum W5 and Azotobacter vinelandii OP.

The electron paramagnetic resonance of nitrogenase components, separately and together with the other reactants in the nitrogenase system (namely, reductant and Mg.ATP), have been examined at low temperatures (<20 degrees K). The MoFe protein, component I or molybdoferredoxin, in the oxidized (but not oxygen-inactivated) state yields signals with g-values of 4.3, 3.7, and 2.01, and when reduced has no observable electron paramagnetic resonance. The Fe protein, component II, or azoferredoxin, yields a signal with g-values of 2.05, 1.94, and 1.89 in the reduced state that is converted by Mg.ATP into an axial signal with g-values near 2.05 and 1.94, and a second split signal near g = 4.3. The Fe protein has no definite electron paramagnetic resonance in the oxidized (not oxygen-denatured) state under these conditions. The Mg.ATP complex of reduced Fe protein reduces the MoFe protein, whereas dithionite alone does not reduce the MoFe protein. Reoxidation of the system by substrate leads to disappearance of the Fe protein signal and the reappearance of the MoFe protein signal. Thus Mg.ATP, which is hydrolyzed during substrate reduction, converts the Fe protein to a reductant capable of transferring electrons to MoFe protein, after which substrate reduction occurs.

Adenosine Triphosphate↗