Oxidation--reduction potentials of molybdenum and iron--sulphur centres in nitrate reductase from Escherichia coli.
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ABSTRACT:
SUBMITTER: Vincent SP
PROVIDER: S-EPMC1186429 | biostudies-other | 1979 Feb
REPOSITORIES: biostudies-other
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Project description:1. The mid-point reduction potentials of the various groups in xanthine oxidase from bovine milk were determined by potentiometric titration with dithionite in the presence of dye mediators, removing samples for quantification of the reduced species by e.p.r. (electron-paramagnetic-resonance) spectroscopy. The values obtained for the functional enzyme in pyrophosphate buffer, pH8.2, are: Fe/S centre I, -343 +/- 15mV; Fe/S II, -303 +/- 15mV; FAD/FADH-; -351 +/- 20mV; FADH/FADH2, -236 +/-mV; Mo(VI)/Mo(V) (Rapid), -355 +/- 20mV; Mo(V) (Rapid)/Mo(IV), -355 +/- 20mV. 2. Behaviour of the functional enzyme is essentially ideal in Tris but less so in pyrophosphate. In Tris, the potential for Mo(VI)/Mo(V) (Rapid) is lowered relative to that in pyrophosphate, but the potential for Fe/S II is raised. The influence of buffer on the potentials was investigated by partial-reduction experiments with six other buffers. 3. Conversion of the enzyme with cyanide into the non-functional form, which gives the Slow molybdenum signal, or alkylation of FAD, has little effect on the mid-point potentials of the other centres. The potentials associated with the Slow signal are: Mo(VI)/Mo(V) (Slow), -440 +/- 25mV; Mo(V) (Slow)/Mo(IV), -480 +/- 25 mV. This signal exhibits very sluggish equilibration with the mediator system. 4. The deviations from ideal behaviour are discussed in terms of possible binding of buffer ions or anti-co-operative interactions amongst the redox centres.
| S-EPMC1163874 | biostudies-other
Project description:Oxidation-reduction midpoint potentials have been determined for the flavin, cytochrome b557 and Mo-pterin prosthetic groups of spinach (Spinacia oleracea L.) assimilatory nitrate reductase using visible, c.d. and room-temperature e.p.r. potentiometric titrations. At pH 7 and 25 degrees C, the midpoint potential for the FAD/FADH2 couple was determined by c.d. potentiometry to be -280 +/- 10 mV (n = 2). The redox potential for reduction of the haem was determined by visible potentiometry to be -123 +/- 10 mV (n = 1), significantly lower than the previously published value of -60 mV [Fido, Hewitt, Notton, Jones & Nasrulhaq-Boyce (1979) FEBS Lett. 99, 180-182]. Potentials for the Mo(VI)/Mo(V) and Mo(V)/Mo(IV) redox couples, determined by room-temperature e.p.r. potentiometry, were found to be +2 +/- 20 and -6 +/- 20 mV respectively. These values are very similar to the values previously determined for the FAD, haem and Mo-pterin centres in assimilatory nitrate reductase isolated from the unicellular green alga Chlorella vulgaris and indicate a close thermodynamic similarity between the two enzymes.
| S-EPMC1133421 | biostudies-other
Project description:Microcoulometric titrations of NADH:nitrate reductase at 25 degrees C in Mops buffer, pH 7.0, showed that the native enzyme, containing functional FAD, haem and Mo, required addition of five electrons for complete reduction. Reduction of the native enzyme occurred in three waves corresponding to addition of reducing equivalents to the centres in the order: Mo, haem, FAD. Oxidation-reduction midpoint potentials (E'0) for the various redox couples were calculated to be as follows: MoVI/MoV, +16 mV; MoV/MoIV, -27 mV; haemoxidized/haemreduced, -172 mV; FAD/FADH2, -283 mV. The values for the haem and flavin are in excellent agreement with those obtained by visible titrations, namely -164 mV and -288 mV respectively. In contrast, the results for the Mo centre are 28-50 mV more positive than the values previously determined by e.p.r. analysis of frozen enzyme samples poised at defined potentials at 25 degrees C and suggest different pH-dependencies or entropies of reduction for the Mo couples.
| S-EPMC1148944 | biostudies-other
Project description:We test the hypothesis that pyranopterin (PPT) coordination plays a critical role in defining molybdenum active site redox chemistry and reactivity in the mononuclear molybdoenzymes. The molybdenum atom of Escherichia coli nitrate reductase A (NarGHI) is coordinated by two PPT-dithiolene chelates that are defined as proximal and distal based on their proximity to a [4Fe-4S] cluster known as FS0. We examined variants of two sets of residues involved in PPT coordination: (i) those interacting directly or indirectly with the pyran oxygen of the bicyclic distal PPT (NarG-Ser(719), NarG-His(1163), and NarG-His(1184)); and (ii) those involved in bridging the two PPTs and stabilizing the oxidation state of the proximal PPT (NarG-His(1092) and NarG-His(1098)). A S719A variant has essentially no effect on the overall Mo(VI/IV) reduction potential, whereas the H1163A and H1184A variants elicit large effects (ΔEm values of -88 and -36 mV, respectively). Ala variants of His(1092) and His(1098) also elicit large ΔEm values of -143 and -101 mV, respectively. An Arg variant of His(1092) elicits a small ΔEm of +18 mV on the Mo(VI/IV) reduction potential. There is a linear correlation between the molybdenum Em value and both enzyme activity and the ability to support anaerobic respiratory growth on nitrate. These data support a non-innocent role for the PPT moieties in controlling active site metal redox chemistry and catalysis.
| S-EPMC4599019 | biostudies-literature
Project description:The molybdenum and tungsten active site model complexes, derived from the protein X-ray crystal structure of the first W-containing nitrate reductase isolated from Pyrobaculum aerophilum, were computed for nitrate reduction at the COSMO-B3LYP/SDDp//B3LYP/Lanl2DZ(p) energy level of density functional theory. The molybdenum containing active site model complex (Mo-Nar) has the largest activation energy (34.4 kcal/mol) for the oxygen atom transfer from the nitrate to the metal center as compared to the tungsten containing active site model complex (W-Nar) (12.0 kcal/mol). Oxidation of the educt complex is close to thermoneutral (-1.9 kcal/mol) for the Mo active site model complex but strongly exothermic (-34.7 kcal/mol) for the W containing active site model complex, however, the MVI to MIV reduction requires equal amount of reductive power for both metal complexes, Mo-Nar or W-Nar.
| S-EPMC5405038 | biostudies-literature
Project description:XOR (xanthine oxidoreductase) purified from human milk was shown to contain 0.04 atom of Mo and 0.09 molecule of molybdopterin/subunit. On the basis of UV/visible and CD spectra, the human enzyme was approx. 30% deficient in iron-sulphur centres. Mo(V) EPR showed the presence of a weak rapid signal corresponding to the enzyme of low xanthine oxidase activity and a slow signal indicating a significant content of desulpho-form. Resulphuration experiments, together with calculations based on enzymic activity and Mo content, led to an estimate of 50-60% desulpho-form. Fe/S EPR showed, in addition to the well-known Fe/S I and Fe/S II species, the presence of a third Fe/S signal, named Fe/S III, which appears to replace partially Fe/S I. Comparison is made with similarly prepared bovine milk XOR, which has approx. 15-fold higher enzymic activity and Mo content. Taken along with evidence of low Mo content in the milk of other mammals, these findings add further support to the idea that XOR protein plays a physiological role in milk (e.g. in secretion) equal in importance to its catalytic function as an enzyme.
| S-EPMC1138957 | biostudies-literature
Project description:Magnetic interaction between molybdenum and one of the iron-sulphur centres in milk xanthine oxidase [Lowe, Lynden-Bell & Bray (1972) Biochem. J. 130, 239-249] was studied further, with particular reference to the newly discovered Mo(V) e.p.r.(electron-paramagnetic-resonance) signal, Resting II [Lowe, Barber, Pawlik & Bray (1976) Biochem. J. 155, 81-85]. E.p.r. measurements at 35GHz near to 4.2K showed that the interaction has the same sign at all molybdenum orientations and is ferromagnetic. The predicted splitting of the e.p.r. signal from the reduced iron-sulphur centre, Fe/S I, was observed, Providing positive identification of this as the other interacting species. Chemical modification of the molybdenum environment in xanthine oxidase can change the size of the interaction severalfold, but interaction always remains approximately isotropic. The interaction in turkey liver xanthine dehydrogenase is indistinguishable from that in the oxidase. However, a bacterial xanthine dehydrogenase with different iron-sulphur centres shows rather larger interaction. Guanidinium chloride disturbs the iron-sulphur centres of the oxidase, and when this occurs there is a parallel and relatively small change in the interaction. Removal of flavin from the molecule, or raising the pH to 12.0, changes the interaction slightly without affecting the chromophores themselves. It is concluded that the Fe/S I centre and the Mo are at least 1.0nm and probably nearer 2.5nm apart, and that the conformation of the protein between them is relatively stable up to pH 12.
| S-EPMC1183819 | biostudies-other
Project description:E.p.r. spectra of reduced iron-sulphur centres of the aldehyde oxidoreductase (iron-molybdenum protein) of Desulfovibrio gigas were recorded at X-band and Q-band frequencies and simulated. Results are consistent with the view that only two types of [2Fe-2S] clusters are present, as in eukaryotic molybdenum-containing hydroxylases. The data indicate the Fe/SI centre to be very similar, and the Fe/SII centre somewhat similar, to these centres in the eukaryotic enzymes.
| S-EPMC1130529 | biostudies-other
Project description:Denitrification and dissimilatory nitrate reduction to ammonium (DNRA) are processes occurring simultaneously under oxygen-limited or anaerobic conditions, where both compete for nitrate and organic carbon. Despite their ecological importance, there has been little investigation of how denitrification and DNRA potentials and related functional genes vary vertically with sediment depth. Nitrate reduction potentials measured in sediment depth profiles along the Colne estuary were in the upper range of nitrate reduction rates reported from other sediments and showed the existence of strong decreasing trends both with increasing depth and along the estuary. Denitrification potential decreased along the estuary, decreasing more rapidly with depth towards the estuary mouth. In contrast, DNRA potential increased along the estuary. Significant decreases in copy numbers of 16S rRNA and nitrate reducing genes were observed along the estuary and from surface to deeper sediments. Both metabolic potentials and functional genes persisted at sediment depths where porewater nitrate was absent. Transport of nitrate by bioturbation, based on macrofauna distributions, could only account for the upper 10 cm depth of sediment. A several fold higher combined freeze-lysable KCl-extractable nitrate pool compared to porewater nitrate was detected. We hypothesised that his could be attributed to intracellular nitrate pools from nitrate accumulating microorganisms like Thioploca or Beggiatoa. However, pyrosequencing analysis did not detect any such organisms, leaving other bacteria, microbenthic algae, or foraminiferans which have also been shown to accumulate nitrate, as possible candidates. The importance and bioavailability of a KCl-extractable nitrate sediment pool remains to be tested. The significant variation in the vertical pattern and abundance of the various nitrate reducing genes phylotypes reasonably suggests differences in their activity throughout the sediment column. This raises interesting questions as to what the alternative metabolic roles for the various nitrate reductases could be, analogous to the alternative metabolic roles found for nitrite reductases.
| S-EPMC3984109 | biostudies-literature
Project description:Nitrate reductase (NR) is a complex molybdenum cofactor (Moco)-dependent homodimeric metalloenzyme that is vitally important for autotrophic organism as it catalyzes the first and rate-limiting step of nitrate assimilation. Beside Moco, eukaryotic NR also binds FAD and heme as additional redox active cofactors, and these are involved in electron transfer from NAD(P)H to the enzyme molybdenum center where reduction of nitrate to nitrite takes place. We report the first biochemical characterization of a Moco-free eukaryotic NR from the fungus Neurospora crassa, documenting that Moco is necessary and sufficient to induce dimer formation. The molybdenum center of NR reconstituted in vitro from apo-NR and Moco showed an EPR spectrum identical to holo-NR. Analysis of mutants unable to bind heme or FAD revealed that insertion of Moco into NR occurs independent from the insertion of any other NR redox cofactor. Furthermore, we showed that at least in vitro the active site formation of NR is an autonomous process.
| S-EPMC3656317 | biostudies-literature