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R R Eady

Publications and source records attributed to R R Eady.

At least 19 recordsLinked to original sources
PubMed1992

Correction for creatine interference with the direct indophenol measurement of NH3 in steady-state nitrogenase assays.

Creatine was identified as a major source of interference with the direct phenol/hypochlorite colorimetric determination of ammonia in nitrogenase reaction mixtures. A method is described for removing other compounds which inhibit color development and for compensating for the interference produced by creatine. This method avoids time-consuming microdiffusion and also routinely makes available the efficiency of ATP hydrolysis coupled to substrate reduction (ATP/2e ratio) with N2 as a reducible substrate. Using this method we determined values for this ratio at 30 degrees C of 4.87 +/- 0.03 during the reduction of protons to H2 and 7.16 +/- 0.14 during the reduction of N2 by the vanadium-containing nitrogenase of Azotobacter chroococcum.

Adenosine Triphosphate↗
PubMed1992

Posttranslational modification of Klebsiella pneumoniae flavodoxin by covalent attachment of coenzyme A, shown by 31P NMR and electrospray mass spectrometry, prevents electron transfer from the nifJ protein to nitrogenase. A possible new regulatory mechanism for biological nitrogen fixation.

A strain of Escherichia coli (71-18) that produces ca. 15% of its soluble cytoplasmic protein as a flavodoxin, the Klebsiella pneumoniae nifF gene product, has been constructed. The flavodoxin was purified using FPLC and resolved into two forms, designated KpFldI and KpFldII, which were shown to have identical N-terminal amino acid sequences (30 residues) in agreement with that predicted by the K. pneumoniae nifF DNA sequence. 31P NMR, electrospray mass spectrometry, UV-visible spectra, and thiol group estimations showed that the single cysteine residue (position 68) of KpFldI is posttranslationally modified in KpFldII by the covalent, mixed disulfide, attachment of coenzyme A. KpFldII was inactive as an electron carrier between the K. pneumoniae nifJ product (a pyruvate-flavodoxin oxidoreductase) and K. pneumoniae nifH product (the Fe-protein of nitrogenase). This novel posttranslational modification of a flavodoxin is discussed in terms of the regulation of nitrogenase activity in vivo in response to the level of dissolved O2 and the carbon status of diazotrophic cultures.

Amino Acid Sequence↗
PubMed1991

Direct electrochemistry of two genetically distinct flavodoxins isolated from Azotobacter chroococcum grown under nitrogen-fixing conditions.

Two genetically distinct flavodoxins, designated AcFldA and AcFldB, were isolated from Azotobacter chroococcum (MCD1155) grown under nitrogen-fixing conditions. AcFldA and AcFldB differ in their midpoint potentials for the semiquinone-hydroquinone couple (Em -305 mV and -520 mV respectively). Only AcFldB was competent to act as an electron donor to the Mo-containing nitrogenase of A. chroococcum. The N-terminal amino acid sequence (20 residues) of AcFldB was identical with that predicted from the nifF DNA sequence of A. vinelandii OP [Bennett, Jacobsen & Dean (1988) J. Biol. Chem. 263, 1364-1369], suggesting that AcFldB is the nifF gene product of A. chroococcum (MCD1155). Direct fast reversible electrochemistry of these flavodoxins has been achieved at a polished edge-plane graphite electrode using the aminoglycoside neomycin as a promoter. The heterogeneous rates of electron transfer between the graphite electrode and AcFldA and AcFldB were determined to be 1.2 x 10(-3) cm.s-1 and 2.0 x 10(-3) cm.s-1 respectively. The natures of two minor species of flavodoxin designated AcFldC and AcFldD, which were resolved by f.p.l.c., are also discussed.

Amino Acid Sequence↗
PubMed1991

Hydrazine is a product of dinitrogen reduction by the vanadium-nitrogenase from Azotobacter chroococcum.

During the enzymic reduction of N2 to NH3 by Mo-nitrogenase, free hydrazine (N2H4) is not detectable, but an enzyme-bound intermediate can be made to yield N2H4 by quenching the enzyme during turnover [Thorneley, Eady & Lowe (1978) Nature (London) 272, 557-558]. In contrast, we show here that the V-nitrogenase of Azotobacter chroococcum produces a small but significant amount of free N2H4 (up to 0.5% of the electron flux resulting in N2 reduction) as a product of the reduction of N2. The amount of N2H4 formed increased 15-fold on increasing the assay temperature from 20 degrees C to 40 degrees C. Activity cross-reactions between nitrogenase components of Mo- and V-nitrogenases showed that the formation of free N2H4 was associated with the VFe protein. These data provide the first direct evidence for an enzyme intermediate at the four-electron-reduced level during the reduction of N2 by V-nitrogenase.

Azotobacter↗
PubMed1990

Iron K-edge X-ray-absorption spectroscopy of the iron-vanadium cofactor of the vanadium nitrogenase from Azotobacter chroococcum.

Iron K-edge e.x.a.f.s. data for the iron-vanadium cofactor (FeVaco) from Azotobacter chroococcum vanadium nitrogenase reported here provide further evidence for the structural similarity between this and the iron-molybdenum nitrogenase cofactor (FeMoco) from Klebsiella pneumoniae molybdenum nitrogenase [Arber, Flood, Garner, Gormal, Hasnain & Smith (1988) Biochem. J. 252, 421-425]. The e.x.a.f.s. data are consistent with the vanadium being present in a V-Fe-S cluster, thus confirming that the N-methylformamide extract of the VFe protein component of A. chroococcum vanadium nitrogenase does indeed contain a polynuclear metal-sulphur cluster. Additionally, a long Fe-Fe distance is observed as 0.369 nm, demonstrating the presence of a long-range order in the cluster.

Azotobacter↗
PubMed1989

Molybdenum nitrogenase of Azotobacter chroococcum. Tight binding of MgADP to the MoFe protein.

The dye-oxidized or dithionite-reduced forms of the MoFe protein of molybdenum nitrogenase of Azotobacter chroococcum were shown to bind 2 mol of MgADP/mol of protein, as determined by column equilibrium techniques. The gel-filtration elution profile of unbound Mg[14C]ADP was not symmetrical, consistent with a low rate of dissociation from the protein. Symmetrical elution profiles were observed for the oxidized Fe protein of nitrogenase, which bound 2 mol of MgADP/mol of protein. The low rate of dissociation of MgADP from MoFe protein was shown by non-equilibrium column techniques, where 1 mol of MgADP/mol of MoFe protein remained tightly bound during chromatography. Very weak binding of MgATP (less than 0.01 mol of MgATP/mol of MoFe protein) to dye-oxidized but not to dithionite-reduced MoFe protein was observed. These results are discussed in terms of their relevance to the catalytic cycle of nitrogenase catalysis.

Adenosine Diphosphate↗
PubMed1989

Vanadium K-edge X-ray-absorption spectroscopy of the functioning and thionine-oxidized forms of the VFe-protein of the vanadium nitrogenase from Azotobacter chroococcum.

Vanadium K-edge X-ray-absorption spectra were collected for samples of thionine-oxidized, super-reduced (during enzyme turnover) and dithionite-reduced VFe-protein of the vanadium nitrogenase of Azotobacter chroococcum (Acl*). Both the e.x.a.f.s and the x.a.n.e.s. (X-ray-absorption near-edge structure) are consistent with the vanadium being present as part of a VFeS cluster; the environment of the vanadium is not changed significantly in different oxidation states of the protein. The vanadium atom is bound to three oxygen (or nitrogen), three sulphur and three iron atoms at 0.215(3), 0.231(3) and 0.275(3) nm respectively.

Azotobacter↗
PubMed1989

Vanadium nitrogenase of Azotobacter chroococcum. MgATP-dependent electron transfer within the protein complex.

The kinetics of MgATP-induced electron transfer from the Fe protein (Ac2V) to the VFe protein (AclV) of the vanadium-containing nitrogenase from Azotobacter chroococcum were studied by stopped-flow spectrophotometry at 23 degrees C at pH 7.2. They are very similar to those of the molybdenum nitrogenase of Klebsiella pneumoniae [Thorneley (1975) Biochem. J. 145, 391-396]. Extrapolation of the dependence of kobs. on [MgATP] to infinite MgATP concentration gave k = 46 s-1 for the first-order electron-transfer reaction that occurs with the Ac2V MgATPAclV complex. MgATP binds with an apparent KD = 230 +/- 10 microM and MgADP acts as a competitive inhibitor with Ki = 30 +/- 5 microM. The Fe protein and VFe protein associate with k greater than or equal to 3 x 10(7) M-1.s-1. A comparison of the dependences of kobs. for electron transfer on protein concentrations for the vanadium nitrogenase from A. chroococcum with those for the molybdenum nitrogenase from K. pneumoniae [Lowe & Thorneley (1984) Biochem. J. 224, 895-901] indicates that the proteins of the vanadium nitrogenase system form a weaker electron-transfer complex.

Adenosine Diphosphate↗
PubMed1989

Structural genes for the vanadium nitrogenase from Azotobacter chroococcum.

Structural genes for the VFe-protein (Ac1V) of the vanadium nitrogenase from Azotobacter chroococcum were cloned and sequenced. The VFe-protein contains three subunit types with Mr of 53,793 (alpha), 52,724 (beta) and 13,274 (delta). alpha and beta subunits show 18 and 15% sequence identity respectively, with alpha and beta subunits of the MoFe-protein of A.chroococcum molybdenum nitrogenase. The genes for the three subunits vnfD (alpha), vnfG (delta) and vnfK (beta) are contiguous and form an operon whose transcription is repressed in response to ammonia. The Fe-protein component of the V-nitrogenase (Ac2V) is the product of nifH* that we have previously cloned and sequenced. This gene was located 2.5 kb upstream of vnfD. A deletion in the vnfD, G and K gene cluster prevents V-dependent nitrogen fixation. A strain defective in both V-nitrogenase and Mo-nitrogenase structural genes showed no residual nitrogen fixing capacity arguing against the presence of a third nitrogen fixation system in this organism.

Amino Acid Sequence↗
PubMed1988

Molybdenum and vanadium nitrogenases of Azotobacter chroococcum. Low temperature favours N2 reduction by vanadium nitrogenase.

A comparison of the effect of temperature on the reduction of N2 by purified molybdenum nitrogenase and vanadium nitrogenase of Azotobacter chroococcum showed differences in behaviour. As the assay temperature was lowered from 30 degrees C to 5 degrees C N2 remained an effective substrate for V nitrogenase, but not Mo nitrogenase, since the specific activity for N2 reduction by Mo nitrogenase decreased 10-fold more than that of V nitrogenase. Activity cross-reactions between nitrogenase components showed the enhanced low-temperature activity to be associated with the Fe protein of V nitrogenase. The lower activity of homologous Mo nitrogenase components, although dependent on the ratio of MoFe protein to Fe protein, did not equal that of V nitrogenase even under conditions of high electron flux obtained at a 12-fold molar excess of Fe protein.

Azotobacter↗
PubMed1988

The vanadium nitrogenase of Azotobacter chroococcum. Purification and properties of the Fe protein.

1. Nitrogenase activity of a strain of Azotobacter chroococcum lacking the structural genes of Monitrogenase (nifHDK) was associated with a V + Fe-containing protein and an Fe-containing protein [Robson, Eady, Richardson, Miller, Hawkins & Postgate (1986) Nature (London) 322, 388-390; Eady, Robson, Richardson, Miller & Hawkins (1987) Biochem. J. 244, 197-207]. 2. The Fe protein was purified to homogeneity by the criterion of Coomassie Blue staining after electrophoresis in 10% or 17% (w/v) polyacrylamide gels in the presence of SDS. One type of subunit, of Mr 32,000 +/- 2000, was found. 3. The native protein had an Mr of 62,500 +/- 2500 and contained approximately 4 Fe atoms and 4 acid-labile sulphide groups per molecule. The amino acid composition was similar to those of other purified Fe proteins, and, characteristically, tryptophan was absent. The specific activities (nmol of protein/min per mg of protein) when assayed under optimum conditions with the VFe protein from this strain were 1211 for H2 evolution under Ar, 337 for NH3 from N2 formation and 349 for C2H2 reduction. Activity of the Fe protein was O2-labile with a t1/2 of 36 s in air. At low temperatures the dithionite-reduced protein exhibited e.p.r. signals consistent with the presence of both S = 1/2 and S = 3/2 spin states. These signals were similar to those given by other nitrogenase Fe proteins, as were the changes in their line shape that occurred in the presence of MgATP or MgADP. The absorbance spectra showed that an increase in absorption occurred in the visible range on reversible oxidation of the dithionite-reduced protein. The oxidized-minus-reduced epsilon 420 was 6000 M-1.cm-1.

Amino Acids↗
PubMed1988

The vanadium- and molybdenum-containing nitrogenases of Azotobacter chroococcum. Comparison of mid-point potentials and kinetics of reduction by sodium dithionite of the iron proteins with bound magnesium adenosine 5'-diphosphate.

The mid-point potentials of the Fe protein components (Ac2 and Ac2* respectively) of the Mo nitrogenase and V nitrogenase from Azotobacter chroococcum were determined in the presence of MgADP to be -450 mV (NHE) [Ac2(MgADP)2-Ac2*ox.(MgADP)2 couple] and -463 mV (NHE) [Ac2* (MgADP)2-Ac2*ox.(ADP)2 couple] at 23 degrees C at pH 7.2. These values are consistent with a flavodoxin characterized by Deistung & Thorneley [(1986) Biochem. J. 239, 69-75] with Em = -522 mV (NHE) being an effective electron donor to both the Mo nitrogenase and the V nitrogenase in vivo. Ac2*ox.(MgADP)2 and Ac2*ox.(MgADP)2 were reduced by SO2.- (formed by the predissociation of dithionite ion, S2O4(2-)) at similar rates, k = 4.7 X 10(6) +/- 0.5 X 10(6) M-1.s-1 and 3.2 X 10(6) +/- 0.2 X 10(6) M-1.s-1 respectively, indicating structural homology at the electron-transfer site associated with the [4Fe-4S] centre in these proteins.

Adenosine Diphosphate↗
PubMed1988

The vanadium-iron protein of vanadium nitrogenase from Azotobacter chroococcum contains an iron-vanadium cofactor.

N-Methylformamide extracts of acid-treated precipitated VFe protein of the V-nitrogenase of Azotobacter chroococcum are yellow-brown in colour and contain vanadium, iron and acid-labile sulphur in the approximate proportions 1:6:5. E.p.r. spectra of the extracts exhibit a weak signal with g values near 4.5, 3.6 and 2.0 characteristic of an S = 3/2 metal-containing centre. The N-methylformamide extracts activated the MoFe protein polypeptides from mutants of nitrogen-fixing bacteria unable to synthesize FeMoco, the active centre of Mo-nitrogenase. The active hybrid protein exhibited the characteristic substrate-reducing phenotype associated with the VFe protein except that it could not reduce N2 to NH3. The above data are interpreted as demonstrating the existence of an iron- and vanadium-containing cofactor, FeVaco, within the VFe protein. It is suggested that nitrogen fixation requires specific interactions between FeVaco or FeMoco and their respective polypeptides. The biosynthesis of these cofactors is discussed.

Azotobacter↗
PubMed1988

Cyanamide: a new substrate for nitrogenase.

(1) Cyanamide (N identical to C-NH2) has been shown to be a substrate for purified Mo-nitrogenases of Klebsiella pneumoniae and Azotobacter chroococcum, with apparent Km values near 0.8 mM. (2) Reduction products were CH4, CH3NH2 and NH3 formed by pathways requiring 6 or 8 electrons: N identical to CNH2 + 6e + 6H+----CH3NH2 + NH3; N identical to CNH2 + 8e + 8H+----CH4 + 2NH3 (3) Acetylene reduction and hydrogen evolution were inhibited more than 75% by cyanamide (10 mM). Cyanamide also inhibited total electron flux at nitrogenase protein component ratios (Fe/MoFe) near 10. (4) Cyanamide was also a substrate for the recently isolated Va-nitrogenase of A. chroococcum, but with an apparent Km of 2.6 mM showed weaker binding and an 8-fold lower Vmax than did either Mo-nitrogenase. (5) The component ratios of nitrogenase proteins favouring CH4 formation was 3.5 Fe/MoFe protein and 1 Fe/VaFe protein.

Acetylene↗
PubMed1988

The vanadium nitrogenase of Azotobacter chroococcum. Reduction of acetylene and ethylene to ethane.

1. The vanadium (V-) nitrogenase of Azobacter chroococcum transfers up to 7.4% of the electrons used in acetylene (C2H2) reduction for the formation of ethane (C2H6). The apparent Km for C2H2 (6 kPa) is the same for either ethylene (C2H4) or ethane (C2H6) formation and much higher than the reported Km values for C2H2 reduction to C2H4 by molybdenum (Mo-) nitrogenases. Reduction of C2H2 in 2H2O yields predominantly [cis-2H2]ethylene. 2. The ratio of electron flux yielding C2H6 to that yielding C2H4 (the C2H6/C2H4 ratio) is increased by raising the ratio of Fe protein to VFe protein and by increasing the assay temperature up to at least 40 degrees C. pH values above 7.5 decrease the C2H6/C2H4 ratio. 3. C2H4 and C2H6 formation from C2H2 by V-nitrogenase are not inhibited by H2. CO inhibits both processes much less strongly than it inhibits C2H4 formation from C2H2 with Mo-nitrogenase. 4. Although V-nitrogenase also catalyses the slow CO-sensitive reduction of C2H4 to C2H6, free C2H4 is not an intermediate in C2H6 formation from C2H2. 5. Propyne (CH3C identical to CH) is not reduced by the V-nitrogenase. 6. Some implications of these results for the mechanism of C2H6 formation by the V-nitrogenase are discussed.

Acetylene↗
PubMed1988

The vanadium-containing nitrogenase of Azotobacter.

Fifty years after a role of vanadium in biological fixation was proposed, it was shown that in addition to their well-characterized molybdendum nitrogenases, Azotobacter chroococcum and Azotobacter vinelandii both have a genetically distinct nitrogenase system in which the conventional molybdoprotein is replaced by a vanadoprotein. Both Mo-nitrogenases and V-nitrogenases have similar requirements for activity: MgATP, a low potential reductant and the absence of oxygen. The genes encoding the V-nitrogenase are expressed only under conditions of Mo-deficiency. V-Nitrogenase of A.chroococcum is made up of a tetrameric VFe protein (Mr 210,000) with an alpha 2 beta 2 structure containing two V atoms, 23 Fe atoms and 20 acid-labile sulphide atoms per tetramer, and a dimeric Fe protein (Mr 64,000) with a gamma 2 structure containing four Fe atoms and four acid-labile sulphide atoms per dimer. Vanadium K-edge X-ray absorption spectroscopy indicates that V in the VFe protein, like Mo in MoFe protein, has S, Fe and possibly O as nearest neighbours. A vanadium- and iron-containing cofactor (FeVaco) can be extracted from the VFe protein and will restore C2H2 reductase, but no nitrogenase activity, to the inactive MoFe protein accumulated by mutants unable to synthesize the molybdenum- and iron-containing co-factor of Mo-nitrogenase. The products of C2H2 reduction by the hybrid protein (C2H6 as well as C2H4) are a characteristic of the VFe protein and provide evidence that FeVaco is, or forms part of the active site of V-nitrogenase.

Azotobacter↗
PubMed1987

Quantitative EPR of an S = 7/2 system in thionine-oxidized MoFe proteins of nitrogenase. A redefinition of the P-cluster concept.

Thionine-oxidized nitrogenase MoFe proteins from Azotobacter vinelandii. Azotobacter chroococcum and Klebsiella pneumoniae exhibit excited-state EPR signals with g = 10.4, 5.8 and 5.5 with a maximal amplitude in the temperature range of 20-50 K. The magnitude of these effective g values, combined with the temperature dependence of the peak area at g = 10.4 from 12 K to 86 K, are consistent with an S = 7/2 system with spin Hamiltonian parameters D = -3.7 +/- 0.7 cm-1, [E] = 0.16 +/- 0.01 cm-1 and g = 2.00. This interpretation predicts nine additional effective g values some of which have been detected as broad features of low intensity at g approximately 10, approximately 2.5 and approximately 1.8. The S = 7/2 EPR is ascribed to the multi-iron exchange-coupled entities known as the P clusters. Quantification relative to the S = 3/2 EPR signal from dithionite-reduced MoFe protein indicates a stoichiometry of one P cluster per FeMo cofactor. Two possible interpretations for these observations, together with data from the literature, are proposed. In the first model there are two P clusters per tetrameric MoFe protein. Each P cluster encompasses approximately 8Fe ions and releases a total of three electrons on oxidation with excess thionine. In the second model the conventional view of four P clusters, each containing approximately 4Fe, is retained. This alternative requires that following one-electron oxidation, the P clusters factorize into two populations, Pa and Pb, only one of which is further oxidized with thionine resulting in the S = 7/2 system. Both models require eight-electron oxidation of tetrameric MoFe protein to reach the S = 7/2 state.

Azotobacter↗