Electron pathways from H2 to nitrate in Paracoccus denitrificans. Effects of inhibitors of the UQ-cytochrome b region

Complex I and its involvement in redox homeostasis and carbon and nitrogen metabolism in Rhodobacter capsulatus

A transposon mutant of Rhodobacter capsulatus, strain Mal7, that was incapable of photoautotrophic and chemoautotrophic growth and could not grow photoheterotrophically in the absence of an exogenous electron acceptor was isolated. The phenotype of strain Mal7 suggested that the mutation was in some gene(s) not previously shown to be involved in CO(2) fixation control. The site of transposition in strain Mal7 was identified and shown to be in the gene nuoF, which encodes one of the 14 subunits for NADH ubiquinone-oxidoreductase, or complex I. To confirm the role of complex I and nuoF for CO(2)-dependent growth, a site-directed nuoF mutant was constructed (strain SBC1) in wild-type strain SB1003. The complex I-deficient strains Mal7 and SBC1 exhibited identical phenotypes, and the pattern of CO(2) fixation control through the Calvin-Benson-Bassham pathway was the same for both strains. It addition, it was shown that electron transport through complex I led to differential control of the two major cbb operons of this organism. Complex I was further shown to be linked to the control of nitrogen metabolism during anaerobic photosynthetic growth of R. capsulatus.

Hydrogen-ubiquinone oxidoreductase activity by the Bradyrhizobium japonicum membrane-bound hydrogenase

The Bradyrhizobium japonicum heterodimeric nickel-iron hydrogenase efficiently catalyzed H2-ubiquinone-1 oxidoreductase activity at rates up to 47% of the maximal rates obtained using the artificial electron acceptor methylene blue. Gel filtration chromatography and SDS-polyacrylamide gel electrophoresis experiments demonstrated that the purified enzyme was a heterodimer containing only the 65 kDa and 33 kDa subunits. Reduced minus oxidized absorption difference spectra demonstrated the absence of detectable cytochromes. The H2-ubiquinone-1 oxidoreductase activity of both the purified heterodimeric hydrogenase and membranes was significantly inhibited by 2-n-heptyl-4-hydroxyquinoline-N-oxide and antimycin A, inhibitors known to act in the quinone region of electron transport chains. Our results are the first report of H2-ubiquinone oxidoreductase activity by a purified hydrogenase.

Nitrate transport and its regulation by O2 in Pseudomonas aeruginosa

Pseudomonas aeruginosa is an obligate respirer which can utilize nitrate as a terminal electron acceptor under anaerobic conditions (denitrification). Immediate, transient regulation of nitrate respiration is mediated by oxygen through the inhibition of nitrate uptake. In order to gain an understanding of the bioenergetics of nitrate transport and its regulation by oxygen, the effects of various metabolic inhibitors on the uptake process and on oxygen regulation were investigated. Nitrate uptake was stimulated by the protonophores carbonyl cyanide m-chlorophenylhydrazone and 2,4-dinitrophenol, indicating that nitrate uptake is not strictly energized by, but may be affected by the proton motive force. Oxygen regulation of nitrate uptake might in part be through redox-sensitive thiol groups since N-ethylmaleimide at high concentrations decreased the rate of nitrate transport. Cells grown with tungstate (deficient in nitrate reductase activity) and azide-treated cells transported nitrate at significantly lower rates than untreated cells, indicating that physiological rates of nitrate transport are dependent on nitrate reduction. Furthermore, tungstate grown cells transported nitrate only in the presence of nitrite, lending support to the nitrate/nitrite antiport model for transport. Oxygen regulation of nitrate transport was relieved (10% that of typical anaerobic rates) by the cytochrome oxygen reductase inhibitors carbon monoxide and cyanide.

Comparison of the membrane-bound hydrogenases from Alcaligenes eutrophus H16 and Alcaligenes eutrophus type strain

Whereas the membrane-bound hydrogenase from Alcaligenes eutrophus H16 is an integral membrane protein and can only be solubilized by detergent treatment, the membrane-bound hydrogenase of Alcaligenes eutrophus type strain was found to be present in a soluble form after cell disruption. For the enzyme of A. eutrophus H16 a new, highly effective purification procedure was developed including phase separation with Triton X-114 and triazine dye chromatography on Procion Blue H-ERD-Sepharose. The purification led to an homogeneous hydrogenase preparation with a specific activity of 269 U/mg protein (methylene blue reduction) and a yield of 45%. During purification and storage the enzyme was optimally stabilized by the presence of 0.2 mM MnCl2. The hydrogenase of A. eutrophus type strain was purified from the soluble extract by a similar procedure, however, with less specific activity and activity yield. Comparison of the two purified enzymes revealed no significant differences: They have the same molecular weight, both consist of two different subunits (Mr = 62,000, 31,000) and both have an isoelectric point near pH 7.0. They have the same electron acceptor specificity reacting with similar high rates and similar Km values. The acceptors reduced include viologen dyes, flavins, quinones, cytochrome c, methylene blue, 2,6-dichlorophenolindophenol, phenazine methosulfate and ferricyanide. Ubiquinones and NAD were not reduced. The two hydrogenases were shown to be immunologically identical and both have identical electrophoretic mobility. For the membrane-bound hydrogenase of A. eutrophus H16 it was demonstrated that this type of hydrogenase in its solubilized, purified state is able to catalyze also the reverse reaction, the H2 evolution from reduced methyl viologen.

The respiratory nitrate reductase from Paracoccus denitrificans. Molecular characterisation and kinetic properties

The respiratory nitrate reductase from Paracoccus denitrificans has been purified in the non-ionic detergent Nonidet P-40. The enzyme comprises three polypeptides, alpha, beta and gamma with estimated relative molecular masses of 127 000, 61 000 and 21 000. Duroquinol or reduced-viologen compounds acted as the reducing substrates. The nitrate reductase contained a b-type cytochrome that was reduced by duroquinol and oxidised by nitrate. A preparation of the enzyme that lacked both detectable b-type cytochrome and the gamma subunit was obtained from a trailing peak of nitrate reductase activity collected from a gel filtration column. Absence of the gamma subunit correlated with failure to use duroquinol as reductant; activity with reduced viologens was retained. It is concluded that in the plasma membrane of P. denitrificans the gamma subunit catalyses electron transfer to the alpha and beta subunits of nitrate reductase from ubiquinol which acts as a branch point in the respiratory chain. A new assay was introduced for both nitrate and quinol-nitrate oxidoreductase activity. Diaphorase was used to couple the oxidation of NADH to the production of duroquinol which acted as electron donor to nitrate reductase. Under anaerobic conditions absorbance changes at 340 nm were sensitive to nitrate concentrations in the low micromolar range. This coupled assay was used to determine that the purified enzyme had Km(NO-3) of 13 microM and a Km of 470 microM for ClO-3, an alternative substrate. With viologen substrates Km(NO-3) of 283 microM and Km(ClO-3) of 470 microM were determined; the enzymes possessed a considerably higher Vmax with either NO-3 or ClO-3 than was found when duroquinol was substrate. Azide was a competitive inhibitor of nitrate reduction in either assay system (Ki = 0.55 microM) but 2-n-heptyl-4-hydroxyquinoline N-oxide was effective only with the complete three-subunit enzyme and duroquinol as substrate, consistent with a site of action for this inhibitor on the b-type cytochrome. The low Km for nitrate observed in the duriquinol assay is comparable with the apparent Km(NO-3) recently reported for intact cells of P. denitrificans [Parsonage, D., Greenfield, A. J. & Ferguson, S. J. (1985) Biochim. Biophys. Acta 807, 81-95]. This similarity is discussed in terms of a possible requirement for a nitrate transport system. The nitrate reductase system from P. denitrificans is compared with that from Escherichia coli.

Investigation of the H2-oxidizing activities of Alcaligenes eutrophus H16 membranes with artificial electron acceptors, respiratory inhibitors and redox-spectroscopic procedures

Membrane particles, prepared from cells of Alcaligenes eutrophus H16 by lysozyme treatment and 100 000 X g centrifugation, catalyzed a H2-dependent reduction of methylene blue, menadione, 2,6-dichlorophenol-indophenol (DCPIP), and O2. While the reaction with methylene blue was not altered by 2-n-heptyl-4-hydroxyquinoline-N-oxide (HQNO), the H2-dependent reductions of menadione, DCPIP and O2 were strongly inhibited, indicating that in these reaction components of the respiratory chain other than the membrane-bound hydrogenase were involved. The effect of pentane extraction of membranes on the H2-dependent reductions of methylene blue and menadione were different from those of DCPIP and O2. This suggested that ubiquinone might not be involved in the pathway of the electrons from H2 to methylene blue or menadione, while it might be involved in the pathway to DCPIP and O2. Because the H2-dependent reduction of menadione is sensitive to HQNO, it follows that HQNO might bind to a site upstream of ubiquinone. Further evidence for this hypothesis came from a new technique to record UV and visible redox-difference spectra of membranes under the conditions of a steady-state electron flow. HQNO did not increase the reduction level of ubiquinone relative to the cytochromes. Neither HQNO nor menadione had any influence on the redox difference patterns of the cytochromes as determined with low temperature and room temperature spectroscopy.