Quantitative transfer of the molybdenum cofactor from xanthine oxidase and from sulphite oxidase to the deficient enzyme of the nit-1 mutant of Neurospora crassa to yield active nitrate reductase

Method for assessing the content of molybdenum enzymes in the internal organs of fish

Molybdenum enzymes (Mo-enzymes) contain a molybdenum cofactor (MoCo) in the active site. These enzymes are potentially interesting for studying the survival mechanism of fish under hypoxic water conditions. This is because Mo-enzymes can synthesize nitric oxide from nitrates and nitrites, which are present in high concentrations under hypoxic water conditions. However, there is currently no method for assessing the Mo-enzymes content in the fish internal organs. Methods capable of determining Mo-enzymes content in the fish are of major importance. For this purpose, a method for quantitative determination of MoCo from plant tissues was modified. We demonstrated the Mo-enzyme content assessment by isolated MoCo from the fish's internal organs and the Neurospora crassa nit-1 extract containing inactive NADPH nitrate reductase. The Mo enzyme content was calculated using a calibration curve in nM of nitrites as a product of restored NADPH reductase activity after complementation with MoCo. Here we present a robust laboratory method which can be used to assess the content of Mo-enzymes in the internal organs of fish.•Mo-enzymes play a crucial role in detoxifying toxic compounds. Therefore, it is important to develop a method to accurately determine the amount of Mo-enzymes present. Notably, the method demonstrated the efficiency and accuracy as detected high content of Mo-enzymes in the liver and intestines (P < 0.0001). The obtained data on the distribution of Mo-enzymes in the internal organs of this species correspond to that of other vertebrates. Here, we present a rapid, sensitive, accurate and accessible method.•The developed method is simple and easy to use. Importantly, the protocol does not require complex manipulations, and the equipment used is available in most laboratories. The article provides step-by-step instructions for reproducing the method.

The History of the Molybdenum Cofactor—A Personal View

The transition element molybdenum (Mo) is an essential micronutrient for plants, animals, and microorganisms, where it forms part of the active center of Mo enzymes. To gain biological activity in the cell, Mo has to be complexed by a pterin scaffold to form the molybdenum cofactor (Moco). Mo enzymes and Moco are found in all kingdoms of life, where they perform vital transformations in the metabolism of nitrogen, sulfur, and carbon compounds. In this review, I recall the history of Moco in a personal view, starting with the genetics of Moco in the 1960s and 1970s, followed by Moco biochemistry and the description of its chemical structure in the 1980s. When I review the elucidation of Moco biosynthesis in the 1990s and the early 2000s, I do it mainly for eukaryotes, as I worked with plants, human cells, and filamentous fungi. Finally, I briefly touch upon human Moco deficiency and whether there is life without Moco.

Study of Different Variants of Mo Enzyme crARC and the Interaction with Its Partners crCytb5-R and crCytb5-1

The mARC (mitochondrial Amidoxime Reducing Component) proteins are recently discovered molybdenum (Mo) Cofactor containing enzymes. They are involved in the reduction of several N-hydroxylated compounds (NHC) and nitrite. Some NHC are prodrugs containing an amidoxime structure or mutagens such as 6-hydroxylaminopurine (HAP). We have studied this protein in the green alga Chlamydomonas reinhardtii (crARC). Interestingly, all the ARC proteins need the reducing power supplied by other proteins. It is known that crARC requires a cytochrome b₅ (crCytb5-1) and a cytochrome b₅ reductase (crCytb5-R) that form an electron transport chain from NADH to the substrates. Here, we have investigated NHC reduction by crARC, the interaction with its partners and the function of important conserved amino acids. Interactions among crARC, crCytb5-1 and crCytb5-R have been studied by size-exclusion chromatography. A protein complex between crARC, crCytb5-1 and crCytb5-R was identified. Twelve conserved crARC amino acids have been substituted by alanine by in vitro mutagenesis. We have determined that the amino acids D182, F210 and R276 are essential for NHC reduction activity, R276 is important and F210 is critical for the Mo Cofactor chelation. Finally, the crARC C-termini were shown to be involved in protein aggregation or oligomerization.

Molybdenum metabolism in the alga Chlamydomonas stands at the crossroad of those in Arabidopsis and humans

Molybdenum (Mo) is a very scarce element whose function is fundamental in living beings within the active site of Mo-oxidoreductases, playing key roles in the metabolism of N, S, purines, hormone biosynthesis, transformation of drugs and xenobiotics, etc. In eukaryotes, each step from Mo acquisition until its incorporation into a biologically active molybdenum cofactor (Moco) together with the assembly of this Moco in Mo-enzymes is almost understood. The deficiency in function of a particular molybdoenzyme can be critical for the survival of the organism dependent on the pathway involved. However, incapacity in forming a functional Moco has a pleiotropic effect in the different processes involving this cofactor. A detailed overview of Mo metabolism: (a) specific transporters for molybdate, (b) the universal biosynthesis pathway for Moco from GTP, (c) Moco-carrier and Moco-binding proteins for Moco transfer and (d) Mo-enzymes, is analyzed in light of recent findings and three systems are compared, the unicellular microalga Chlamydomonas, the plant Arabidopsis and humans.

Occurrence of Nitrate Reductase and Molybdopterin in Xanthomonas maltophilia

Fifteen of 23 ATCC strains and 2 of 9 clinical isolates of Xanthomonas maltophilia, all of which grew aerobically on ammonia, but not nitrate, as a sole nitrogen source, reduced nitrate to nitrite. X. maltophilia failed to grow anaerobically on complex medium with or without nitrate, so it is considered an obligate aerobe. Nitrate-reducing strains contained reduced methyl viologen nitrate reductase (MVH-NR) with specific activities ranging from 49.2 to 192 U mg of protein. Strain ATCC 17666 doubled its cell mass after 3 h of growth on nitrate broth under low aeration, possessed maximal MVH-NR activity, and converted the added nitrate to nitrite, which accumulated. Dissolved oxygen above 15% saturation greatly suppressed nitrite formation. All strains, except ATCC 14535, possessed between 0.25 and 5.05 pmol of molybdopterin mg of protein as measured by the Neurospora crassa nit-1 assay. The molybdopterin activity in the soluble fraction sedimented as a single symmetrical peak with an s(20,w) of 5.1. Studies identified MVH-NR in selected strains as a membrane-bound protein. The deoxycholate-solubilized MVH-NR sedimented as a single peak in sucrose density gradients with an s(20,w) of 8.8. The MVH-NR of X. maltophilia has the physical characteristics of a respiratory nitrate reductase and may enable cells to use nitrate as an electron sink under semiaerobic conditions.

Purification of Two Nitrate Reductases from Xanthomonas maltophilia Grown in Aerobic Cultures

Xanthomonas maltophilia ATCC 17666 is an obligate aerobe that accumulates nitrite when grown on nitrate. Spectra of membranes from nitrate-grown cells exhibited b-type cytochrome peaks and A(615-630) indicative of d-type cytochrome but no absorption peaks corresponding to c-type cytochromes. The nitrate reductase (NR) activity was located in the membrane fraction. Triton X-100-extracted reduced methyl viologen-NRs were purified on DE-52, hydroxylapatite, and Sephacryl S-300 columns to specific activities of 52 to 67 mumol of nitrite formed per min per mg of protein. The cytochrome-containing NR(I) separated on sodium dodecyl sulfate-polyacrylamide gel electrophoresis into a 135-kDa alpha-subunit, a 64-kDa beta-subunit, and a 23-kDa gamma-subunit with relative band intensities indicative of a 1:1:1 alpha/beta/gamma subunit ratio and a M(r) of 222,000. The electronic spectrum of dithionite-reduced purified NR displayed peaks at 425, 528, and 558 nm, indicative of the presence of a cytochrome b, an interpretation consistent with the pyridine hemochrome spectrum formed. The cytochrome b of the NR was reduced under anaerobic conditions by menadiol and oxidized by nitrate with the production of nitrite. This NR contained 0.96 Mo, 12.5 nonheme iron, and 1 heme per 222 kDa: molybdopterin was detected with the Neurospora crassa nit-1 assay. A smaller reduced methyl viologen-NR (169 kDa), present in various concentrations in the Triton X-100 preparations, lacked a cytochrome spectrum and did not oxidize menadiol. The characteristics of the NRs and the absence of c-type cytochromes provide insights into why X. maltophilia accumulates nitrite.

Function and structure of the molybdenum cofactor carrier protein from Chlamydomonas reinhardtii

The molybdenum cofactor (Moco) forms the catalytic site in all eukaryotic molybdenum enzymes and is synthesized by a multistep biosynthetic pathway. The mechanism of transfer, storage, and insertion of Moco into the appropriate apo-enzyme is poorly understood. In Chlamydomonas reinhardtii, a Moco carrier protein (MCP) has been identified and characterized recently. Here we show biochemical evidence that MCP binds Moco as well as the tungstate-substituted form of the cofactor (Wco) with high affinity, whereas molybdopterin, the ultimate cofactor precursor, is not bound. This binding selectivity points to a specific metal-mediated interaction with MCP, which protects Moco and Wco from oxidation with t((1/2)) of 24 and 96 h, respectively. UV-visible spectroscopy showed defined absorption bands at 393, 470, and 570 nm pointing to ene-diothiolate and protein side-chain charge transfer bonds with molybdenum. We have determined the crystal structure of MCP at 1.6 Angstrom resolution using seleno-methionated and native protein. The monomer constitutes a Rossmann fold with two homodimers forming a symmetrical tetramer in solution. Based on conserved surface residues, charge distribution, shape, in silico docking studies, structural comparisons, and identification of an anionbinding site, a prominent surface depression was proposed as a Moco-binding site, which was confirmed by structure-guided mutagenesis coupled to substrate binding studies.

Isolation, Purification, and Characterization of Nitrate Reductase from a Salt-Tolerant Rhodotorula glutinis Yeast Strain Grown in the Presence of Tungsten

The salt-tolerant Rhodotorula glutinis yeast strain grew in medium containing nitrate, 1 mM tungsten, and trace amounts of molybdenum (as impurities from the reagents used). Isolation of electrophoretically homogenous preparation of nitrate reductase from the Rh. glutinis cells grown under these growth conditions is described. The isolated nitrate reductase is a molybdenum-containing homodimer with molecular mass of 130 kD, containing 0.177 mol of Mo per mol of the enzyme. The activity of the enzyme is maximal at pH 7.0 and 35-45 degrees C and is inhibited by low concentrations of azide and cyanide. The enzyme is almost insensitive to 1 mM tungsten.

Characterization of Molybdenum-Free Nitrate Reductase from Haloalkalophilic Bacterium Halomonas sp. Strain AGJ 1-3

Nitrate reductase from the haloalkalophilic denitrifying bacterium Halomonas sp. strain AGJ 1-3 was isolated and purified to homogeneity. The isolated enzyme belongs to a novel family of molybdenum-free nitrate reductases. It presents as a 130-140 kD monomeric protein with specific activity of 250 micromol/min per mg protein. The enzyme reduces not only nitrate, but also other anions, thus showing polyoxoanion reductase activity. Enzyme activity was maximal at pH 7.0 and 70-80 degrees C.

The plant Mo-hydroxylases aldehyde oxidase and xanthine dehydrogenase have distinct reactive oxygen species signatures and are induced by drought and abscisic acid

The plant molybdenum-cofactor (Moco) and flavin-containing enzymes, xanthine dehydrogenase (XDH; EC 1.2.1.37) and aldehyde oxidase (AO; EC 1.2.3.1) are thought to play important metabolic roles in purine metabolism and hormone biosynthesis, respectively. Their animal counterparts contribute to reactive oxygen species (ROS) production in numerous pathologies and here we examined these enzymes as potential sources of ROS in plants. Novel in-gel assay techniques and Moco sulfurase mutants, lacking a sulfur ligand in their Moco active center, were employed to demonstrate that the native tomato and Arabidopsis XDHs are capable of producing O, but not H2O2, while the animal counterpart was shown to produce both, O and H2O2. Superoxide production was dependent on Moco sulfuration when using hypoxanthine/xanthine but not NADH as substrates. The activity was inhibited by diphenylene iodonium (DPI), a suicide inhibitor of FAD containing enzymes. Analysis of XDH in an Arabidopsis Atxdh1 T-DNA insertion mutant and RNA interference lines revealed loss of O activity, providing direct molecular evidence that plant XDH generates superoxides. Contrary to XDH, AO activity produced only H2O2 dissimilar to native animal AO, that can produce O as well. Surprisingly, H2O2 accumulation was not sensitive to DPI. Plant ROS production and transcript levels of AO and XDH were rapidly upregulated by application of abscisic acid and in water-stressed leaves and roots. These results, supported by in vivo measurement of ROS accumulation, indicate that plant AO and XDH are possible novel sources for ROS increase during water stress.

Molybdenum-free nitrate reductases from vanadate-reducing bacteria

Two catalytically distinct molybdenum-free dissimilatory nitrate reductases, a soluble periplasmic and a membrane-bound one, were isolated from the vanadate-reducing facultatively anaerobic bacterium Pseudomonas isachenkovii and purified to electrophoretic homogeneity. The enzymes did not contain molybdenum, the periplasmic enzyme contained vanadium, whereas the membrane-bound enzyme was vanadium-free. Both nitrate reductases lacked molybdenum cofactor. This fact was proved by reconstitution of the apoprotein of the nitrate reductase of Neurospora crassa nit-1 mutant. This is the first demonstration of molybdenum-free and molybdenum cofactor-free nitrate reductases.

The Chlamydomonas reinhardtii MoCo carrier protein is multimeric and stabilizes molybdopterin cofactor in a molybdate charged form

In Chlamydomonas reinhardtii, molybdopterin cofactor (MoCo) able to reconstitute active nitrate reductase (NR) with apoenzyme from the Neurospora crassa mutant nit-1 was found mostly bound to a carrier protein (CP). This protein is scarce in the algal free extracts and has been purified 520-fold. MoCoCP is a protein of 64 kDa with subunits of 16.5 kDa and an isoelectric point of 4.5. In contrast to free MoCo, MoCo bound to CP was remarkably protected against inactivation under both aerobic conditions and basic pH. MocoCP transferred active MoCo to apoNR in vitro without addition of molybdate, though reconstituted activity was 20% higher in the presence of molybdate. Incubation with tungstate specifically inhibited MoCoCP activity but had no effect on the activity of free MoCo released from milk xanthine oxidase. MoCoCP did not charge molybdate unless in the presence of N. crassa extracts. Our data support that MoCoCP stabilizes MoCo in an active form charged with molybdate to provide MoCo to apomolybdoenzymes.

Characterization of moeB--part of the molybdenum cofactor biosynthesis gene cluster in Staphylococcus carnosus

Transposon mutagenesis of Staphylococcus carnosus led to the identification of a gene cluster comprising nine genes that are important for molybdenum cofactor biosynthesis. Two nitrate-reductase-negative Tn917-insertion mutants were defective in MoeB. In cell-free extracts of an moeB mutant, the molybdenum-cofactor-deficient nitrate reductase could be reconstituted with a low-molecular-mass component (most likely free molybdenum cofactor) from an S. carnosus mutant that is defective in the nitrate reductase structural genes. The expression of moeB was studied in response to oxygen and nitrate. Primer-extension studies indicated that anaerobiosis and nitrate each enhance transcription of moeB.

Enzymatic properties and effect of ionic strength on periplasmic nitrate reductase (NAP) from Desulfovibrio desulfuricans ATCC 27774

Some sulfate reducing bacteria can induce nitrate reductase when grown on nitrate containing media being involved in dissimilatory reduction of nitrate, an important step of the nitrogen cycle. Previously, it was reported the purification of the first soluble nitrate reductase from a sulfate-reducing bacteria Desulfovibrio desulfuricans ATCC 27774 (S.A. Bursakov, M.-Y. Liu, W.J. Payne, J. LeGall, I. Moura, and J.J.G. Moura (1995) Anaerobe 1, 55-60). The present work provides further information about this monomeric periplasmic nitrate reductase (Dd NAP). It has a molecular mass of 74 kDa, 18.6 U specific activity, KM (nitrate) = 32 microM and a pHopt in the range 8-9.5. Dd NAP has peculiar properties relatively to ionic strength and cation/anion activity responses. It is shown that monovalent cations (potassium and sodium) stimulate NAP activity and divalent (magnesium and calcium) inhibited it. Sulfate anion also acts as an activator in KPB buffer. NAP native form is protected by phosphate anion from cyanide inactivation. In the presence of phosphate, cyanide even stimulates NAP activity (up to 15 mM). This effect was used in the purification procedure to differentiate between nitrate and nitrite reductase activities, since the later is effectively blocked by cyanide. Ferricyanide has an inhibitory effect at concentrations higher than 1 mM. The N-terminal amino acid sequence has a cysteine motive C-X2-C-X3-C that is most probably involved in the coordination of the [4Fe-4S] center detected by EPR spectroscopy. The active site of the enzyme consists in a molybdopterin, which is capable for the activation of apo-nit-1 nitrate reductase of Neurospora crassa. The oxidized product of the pterin cofactor obtained by acidic hidrolysis of native NAP with sulfuric acid was identified by HPLC chromatography and characterized as a molybdopterin guanine dinucleotide (MGD).

Purification and characterization of the membrane-bound nitrate reductase isoenzymes of Bradyrhizobium japonicum

Two respiratory membrane-bound nitrate reductase (NR) isoenzymes, NRI and NRII, have been purified for the first time from one single microorganism. Triton X-100-solubilized NRs were purified by a three-step procedure of differential centrifugation, Q-Sepharose chromatography, and gel filtration on Sephacryl S-300. Both isoenzymes were purified to homogeneity by the criteria of NR activity staining in polyacrylamide gels run under non-denaturating conditions and coincident staining of the protein band by silver nitrate. NRI is composed of three subunits of 116 kDa, 68 kDa, and 56 kDa, whereas NRII is composed of four subunits of 116 kDa, 68 kDa, 59 kDa, and 56 kDa. The 116-kDa subunit of NRI and the 59-kDa subunit of NRII exhibited immunological cross-reactivity with the respiratory NR of Pseudomonas stutzeri strain ZoBell.

Molybdenum cofactor biosynthesis in Neurospora crassa: biochemical characterization of pleiotropic molybdoenzyme mutants nit-7, nit-8, nit-9A, B and C

Available mutants of molybdenum cofactor (MoCo) biosynthesis of Neurospora crassa were studied for converting factor activity and for in vitro molybdate repair of nitrate reductase (NR) activity. Mutant nit-7 was found to contain an activity that fits the functional definition of converting factor activity in Escherichia coli. Its high molecular weight fraction converts a low molecular weight compound from nit-1 and nit-8 into biologically active molybdopterin (MPT). Like nit-1, mutant nit-8 is devoid of this activity. Mutants nit-9 A, B and C contain a protein-bound precursor form of MoCo, which is presumed to be MPT bound to apo-NR. It is converted into active MoCo as part of NR in the presence of reduced glutathione and high exogenous molybdate concentrations. The NR apoenzyme of nit-1 is needed to detect the total amount of MoCo after molybdate repair, because mutants nit-9 A, B and C build no detectable content of functional NR apoenzyme. Evidence is presented for the transfer of MPT from demolybdo-NR to free NR apoenzyme.

Purification and characterization of the assimilatory nitrate reductase of Azotobacter vinelandii

1. A soluble reduced Methyl Viologen-dependent assimilatory nitrate reductase from Azotobacter vinelandii strain UW136 grown aerobically on nitrate was purified to homogeneity by the criteria of nitrate reductase activity staining, and coincidence of a Coomassie Blue-staining protein band on polyacrylamide gels run under non-denaturing conditions. The specific activity was 3 mumol of NO2- formed/min per mg of protein. 2. Gel filtration on Superose-12 and SDS/PAGE showed that the enzyme had an M(r) of 105,000 and was monomeric. The enzyme contained 1 Mo atom, 4 Fe atoms and 4 acid-labile sulphide atoms per molecule; no evidence for the presence of cytochrome or FAD was found. 3. Mo was present in a molybdenum cofactor, which on extraction was capable of activating apo-(nit-1) nitrate reductase present in crude extracts of nit-1 mutants of Neurospora crassa. 4. As isolated, the enzyme had e.p.r. signals assigned to Mo(V) with g-values g1 = 2.023; g2 = 1.998; g3 = 1.993 and with gav. = 2.004 indicating an unusual environment of Mo in this enzyme. 5. Reduction with S2O4(2-) bleached the e.p.r. signals which, on reoxidation after the addition of NO3(2-) to initiate enzyme turnover, exhibited at short times Mo(V) signals similar to those of dissimilatory nitrate reductases, with g1 = 1.998; g2 = 1.989; g3 = 1.981 and gav. = 1.989. Prolonged incubation subsequently gave a mixture of both e.p.r. species. 6. Neither NADH nor NADPH was effective as an electron donor, but reduced Methyl Viologen (apparent Km 998 microM) and reduced Bromophenol Blue (apparent Km 158 microM) were effective. With these donors the apparent Km values for nitrate were 70 microM and 217 microM respectively.

Direct transfer of molybdopterin cofactor to aponitrate reductase from a carrier protein in Chlamydomonas reinhardtii

A Chlamydomonas reinhardtii molybdenum cofactor (MoCo)-carrier protein (CP), capable of reconstituting nitrate reductase activity with apoprotein from the Neurospora crassa mutant nit-1, was subjected to experiments of diffusion through a dialysis membrane and gel filtration. CP bonded firmly MoCo and did not release it efficiently unless aponitrate reductase was present in the incubation mixture. Stability of MoCo bound to CP against air and heat was very similar to that of free-MoCo released from milk xanthine oxidase. Our data strongly suggest that MoCo is directly transferred from CP to aponitrate reductase to form an active enzyme.

Involvement of the narJ or narW gene product in the formation of active nitrate reductase in Escherichia coli

Two membrane-bound nitrate reductases, NRA and NRZ, exist in Escherichia coli. Both isoenzymes are composed of three structural subunits, alpha, beta, and gamma encoded by narG/narZ, narH/narY and narI/narV, respectively. The genes are in transcription units which also contain a fourth gene encoding a polypeptide, delta, which is not part of the final enzyme. A strain which is devoid of, or does not express, the nar genes, was used to investigate the role of the delta and gamma polypeptides in the formation and/or processing of the nitrate reductase. When only the alpha and beta polypeptides are produced, an (alpha beta) complex exists which is inactive and soluble. When the alpha, beta and delta polypeptides are produced, the (alpha beta) complex is active with artificial donors such as benzyl viologen but is soluble. When the alpha, beta and gamma polypeptides are produced, the (alpha beta) complex is inactive but partially binds the membrane. It was concluded that the gamma polypeptide is involved in the binding of the (alpha beta) complex to the membrane while the delta polypeptide is indispensable for the (alpha beta) nitrate reductase activity. The activation by the delta polypeptide does not seem to involve the insertion of the redox centres of the enzyme since the purified inactive (alpha beta) complex was shown to contain the four iron-sulphur centres and the molybdenum cofactor, which are normally present in the native purified enzyme. The extreme sensitivity of this inactive complex to thermal denaturation or tryptic treatment favours the idea that the delta polypeptide promotes the correct assembly of the alpha and beta subunits. Although this corresponds to the definition of a chaperone protein this possibility has been rejected. In this study we have also demonstrated that the delta or gamma polypeptide encoded by one nar operon can be substituted successfully for by its respective counterpart from the other nar operon to give an active membrane bound heterologous nitrate reductase enzyme.

Regulation of molybdenum cofactor species in the green alga Chlamydomonas reinhardtii

Molybdenum cofactor (MoCo) of molybdoenzymes is constitutively produced in cells of the green alga Chlamydomonas reinhardtii grown in ammonium media, under which conditions certain molybdoenzymes are not synthesized. In soluble form, MoCo was found to be present in several forms: (i) as a low Mr free species; (ii) bound to a MoCo-carrier protein of about 50 kDa that could release MoCo to directly reconstitute in vitro nitrate reductase activity in the nit-1 mutant of Neurospora crassa, but not to Thiol-Sepharose which, in contrast, bonded free MoCo; and (iii) bound to other proteins, putatively constitutive molybdoenzymes, which only released MoCo after a denaturing treatment. The amount of total MoCo (free, carrier-bound and heat releasable forms) was dependent on the growth phase of cell cultures. Constitutive levels of total MoCo in ammonium-grown cells markedly increased when cells were transferred to media lacking ammonium (nitrate, urea or nitrogen-free media). This increase did not require de novo protein synthesis and was stimulated by light. Levels of both total MoCo and free plus carrier-bound MoCo seemed to be unrelated to either nitrate reductase synthesis or functioning of nit-1 and nit-2 genes responsible for nitrate reductase structure and regulation, respectively. Results suggest that MoCo is continuously synthesized in C. reinhardtii and that its levels are regulated by ammonium in a way independent of nitrate reductase synthesis.

Regulation of the chlA locus of Escherichia coli K12: involvement of molybdenum cofactor

The chlA locus encodes functions required for the biosynthesis of the molybdopterin part of the molybdenum cofactor. Mutants, carrying gene fusions at the chlA locus, which place beta-galactosidase expression under the control of the chlA promoter, have been isolated employing lambda placMu1 as the mutagen. The mutants exhibited beta-galactosidase expression which was greatly enhanced when grown anaerobically. Secondary mutations at the chlB, D, E or G loci did not affect the high level of expression. The fnr gene product was not required for the anaerobic expression. Bacteriophage lambda transducing phages were isolated which carried the phi(chlA-lac) mutations and were used to construct chlA+/phi(clA-lac) merodiploids. The merodiploids exhibited a much lower level of expression but showed the same characteristics as strains carrying lac fusions to the single chromosomal chlA locus. Genetic evidence is presented which strongly suggests that the molybdenum cofactor is a repressor of chlA expression. The anaerobic enhancement of chlA expression is mediated via a mechanism that is distinct from the molybdenum cofactor effect.

Menaquinol-nitrate oxidoreductase of Bacillus halodenitrificans

When grown anaerobically on nitrate-containing medium, Bacillus halodenitrificans exhibited a membrane-bound nitrate reductase (NR) that was solubilized by 2% Triton X-100 but not by 1% cholate or deoxycholate. Purification on columns of DE-52, hydroxylapatite, and Sephacryl S-300 yielded reduced methyl viologen NR (MVH-NR) with specific activities of 20 to 35 U/mg of protein that was stable when stored in 40% sucrose at -20 degrees C for 6 weeks. 3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxypropone-1-sulfonat e (CHAPSO) and dodecyl-beta-D-maltoside stimulated enzyme activity three- to fourfold. Membrane extractions yielded purified NR that separated after electrophoresis into a 145-kDa alpha subunit, a 58-kDa beta subunit, and a 23-kDa gamma subunit. The electronic spectrum of dithionite-reduced, purified NR displayed peaks at 424.6, 527, and 557 nm, indicative of the presence of a cytochrome b, an interpretation consistent with the pyridine hemochrome spectrum formed. Analyses revealed a molybdenum-heme-non-heme iron ratio of 1:1:8 for the NR and the presence of molybdopterin. Electron paramagnetic resonance (EPR) signals characteristic of iron-sulfur centers were detected at low temperature. EPR also revealed a minor signal centered in the g = 2 region of the spectra. Upon reduction with dithionite, the enzyme displayed signals at g = 2.064, 2.026, 1.906, and 1.888, indicative of the presence of low-potential iron-sulfur centers, which resolve most probably as two [4Fe-4S]+1 clusters. With menadiol as the substrate for nitrate reduction, the Km for nitrate was 50-fold less than that seen when MVH was the electron donor. The cytochrome b557-containing enzyme from B. halodenitrificans is characterized as a menaquinol-nitrate:oxidoreductase.

Molybdenum cofactor requirement for in vitro activation of apo-molybdoenzymes of Escherichia coli

The apo-nitrate reductase precursor in an Escherichia coli chlB mutant preparation obtained following growth in the presence of tungstate is activated by incubation with protein FA and a heat-treated preparation from an E. coli crude extract. We show that the requirement for heat-treated E. coli crude extract can be fulfilled by material obtained from either of two heat-denatured purified E. coli molybdoenzymes, namely nitrate reductase or trimethylamine N-oxide reductase. Apo-trimethylamine N-oxide reductase precursor in the tungstate-grown chlB preparation can be activated in a similar manner with material from either heat-denatured molybdoenzyme. The active component in the denatured molybdoenzyme preparations is shown to be the molybdenum cofactor by Neurospora crassa nit1 molybdenum cofactor assay, size estimation and fluorimetric analysis. The direct demonstration of the requirement for molybdenum cofactor in the E. coli tungstate-grown chlB complementation system is an important step towards the molecular definition of the activation process and an understanding of the mechanism of cofactor acquisition during molybdoenzyme biosynthesis.

Purification and further characterization of the second nitrate reductase of Escherichia coli K12

Two nitrate reductases, nitrate reductase A and nitrate reductase Z, exist in Escherichia coli. The nitrate reductase Z enzyme has been purified from the membrane fraction of a strain which is deleted for the operon encoding the nitrate reductase A enzyme and which harbours a multicopy plasmid carrying the nitrate reductase Z structural genes; it was purified 219 times with a yield of about 11%. It is an Mr-230,000 complex containing 13 atoms iron and 12 atoms labile sulfur/molecule. The presence of a molybdopterin cofactor in the nitrate reductase Z complex was demonstrated by reconstitution experiments of the molybdenum-cofactor-deficient NADPH-dependent nitrate reductase activity from a Neurospora crassa nit-1 mutant and by fluorescence emission and excitation spectra of stable derivatives of molybdoterin extracted from the purified enzyme. Both nitrate reductases share common properties such as relative molecular mass, subunit composition and electron donors and acceptors. Nevertheless, they diverge by two properties: their electrophoretic migrations are very different (RF of 0.38 for nitrate reductase Z versus 0.23 for nitrate reductase A), as are their susceptibilities to trypsin. An immunological study performed with a serum raised against nitrate reductase Z confirmed the existence of common epitopes in both complexes but unambiguously demonstrated the presence of specific determinants in nitrate reductase Z. Furthermore, it revealed a peculiar aspect of the regulation of both nitrate reductases: the nitrate reductase A enzyme is repressed by oxygen, strongly inducible by nitrate and positively controlled by the fnr gene product; on the contrary, the nitrate reductase Z enzyme is produced aerobically, barely induced by nitrate and repressed by the fnr gene product in anaerobiosis.

Isolation, in the intact state, of the pterin molybdenum cofactor from xanthine oxidase

A procedure is described for isolation of the pterin molybdenum cofactor, in the active molybdenum-containing state, starting from purified milk xanthine oxidase. The method depends on the use of anaerobic-glove-cabinet techniques and on working in aqueous solution, in the presence of 1 mM-Na2S2O4. SDS was used to denature the protein, followed by ion-exchange chromatography and gel filtration. The cofactor, obtained at concentrations up to 0.5-1.0 mM, was fully active in the nit-1 assay [Hawkes & Bray (1984) Biochem. J. 214, 481-493], with a specific activity of 22 nmol of NO2-/min per pg-atom of Mo (with 15% molybdate-dependence). The Mr, determined by gel filtration, was about 610, consistent with the structure proposed by Kramer, Johnson, Ribeiro, Millington & Rajagopalan [(1987) J. Biol. Chem. 262, 16357-16363]. At pH 5.9, under anaerobic conditions, the cofactor was stable for at least 300 h at 20-25 degrees C.

Molybdenum cofactor biosynthesis in humans. Identification of two complementation groups of cofactor-deficient patients and preliminary characterization of a diffusible molybdopterin precursor

Molybdenum cofactor deficiency is a devastating disease with affected patients displaying the symptoms of a combined deficiency of sulfite oxidase and xanthine dehydrogenase. Because of the extreme lability of the isolated, functional molybdenum cofactor, direct cofactor replacement therapy is not feasible, and a search for stable biosynthetic intermediates was undertaken. From studies of cocultured fibroblasts from affected individuals, two complementation groups were identified. Coculture of group A and group B cells, without heterokaryon formation, led to the appearance of active sulfite oxidase. Use of conditioned media indicated that a relatively stable, diffusible precursor produced by group B cells could be used to repair sulfite oxidase in group A recipient cells. Although the extremely low levels of precursor produced by group B cells preclude its direct characterization, studies with a heterologous, in vitro reconstitution system suggest that the precursor that accumulates in group B cells is the same as a molybdopterin precursor identified in the Neurospora crassa molybdopterin mutant nit-1, and that a converting enzyme is present in group A cells which catalyzes an activation reaction analogous to that of a converting enzyme identified in the Escherichia coli molybdopterin mutant ChlA1.

The isolation of demolybdo xanthine oxidase from bovine milk

It was deduced many years ago from indirect evidence that demolybdo xanthine oxidase is present in normal bovine milk. This has now been confirmed by isolation of this enzyme form by a method based on the folate-gel affinity-chromatography procedure described Nishino & Tsushima [(1986) J. Biol. Chem. 261, 11242-11246]. Enzymic and spectroscopic properties of demolybdo xanthine oxidase, which retains flavin and iron-sulphur centres, are generally in accordance with expectations. Like the normal enzyme, it yields on denaturation material fluorescing at 460 nm. Molybdenum cofactor activity measured by the Neurospora crassa nit-1 assay in the presence of added molybdate was 33% of that of the normal enzyme. The absorption spectrum in the near-u.v. region differs slightly, but significantly, from that of the active and desulpho forms of the enzyme. It is concluded that the molybdenum cofactor site contains a pterin-like material not identical with that in the normal enzyme. The significance of the occurrence of demolybdo xanthine oxidase in milk is discussed, and evidence in the literature for demolybdo forms of other molybdoenzymes is briefly reviewed. Additional studies on the use of the affinity procedure for large-scale preparation of high-activity xanthine oxidase are described. In agreement with our ability to isolate the demolybdo enzyme, the procedure appears less effective in eliminating the demolybdo than the desulpho enzyme.

The inorganic biochemistry of molybdoenzymes

Molybednum-containing enzymes (Coughlan, 1980; Spiro, 1985) occupy a significant place in the development of the field now termed inorganic biochemistry. The importance of the metal as a biological trace element depends on its involvement in the known, and perhaps other as yet unknown, molybdoenzymes. That it plays a role in biological nitrogen fixation, the process whereby the enzyme nitrogenase in the root nodules of plants converts atmospheric nitrogen into ammonia, was recognized in the 1930s. The metal is also a constituent of a variety of other enzymes, having first been found in a mammalian enzyme, xanthine oxidase, in the 1950s.

Purification and properties of Escherichia coli dimethyl sulfoxide reductase, an iron-sulfur molybdoenzyme with broad substrate specificity

Dimethyl sulfoxide reductase, a terminal electron transfer enzyme, was purified from anaerobically grown Escherichia coli harboring a plasmid which codes for dimethyl sulfoxide reductase. The enzyme was purified to greater than 90% homogeneity from cell envelopes by a three-step purification procedure involving extraction with the detergent Triton X-100, chromatofocusing, and DEAE ion-exchange chromatography. The purified enzyme was composed of three subunits with molecular weights of 82,600, 23,600, and 22,700 as identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The native molecular weight was determined by gel electrophoresis to be 155,000. The purified enzyme contained 7.5 atoms of iron and 0.34 atom of molybdenum per mol of enzyme. The presence of molybdopterin cofactor in dimethyl sulfoxide reductase was identified by reconstitution of cofactor-deficient NADPH nitrate reductase activity from Neurospora crassa nit-I mutant and by UV absorption and fluorescence emission spectra. The enzyme displayed a very broad substrate specificity, reducing various N-oxide and sulfoxide compounds as well as chlorate and hydroxylamine.

Extraction and purification of molybdenum cofactor from milk xanthine oxidase

Molybdenum cofactor (mocofactor) is extracted efficiently, free of impurities and in high concentrations, by acid treatment of xanthine oxidase and subsequent incubation of the precipitate with phosphate buffer containing EDTA, molybdate and oxygen. It is suggested that cofactor is bound to the enzyme via hydrophobic forces as well as via an oxygen-sensitive mechanism. Upon extraction, the capability to complement the apo nitrate reductase of Neurospora crassa nit-1 can be conserved only in the total absence of oxygen. Cysteine and glutathione were shown to protect efficiently free mocofactor from oxidation. Two species of active mocofactor, probably a molybdoform and a demolybdoform, could be separated by means of reversed-phase HPLC with a mobile phase of 5 mM sodium citrate at a pH of 6.5. The mode of interaction between either of these species with thiol reagents is discussed.

Involvement of a protein with molybdenum cofactor in the in vitro activation of nitrate reductase from a chlA mutant of Escherichia coli K12

Chlorate-resistant mutants are pleiotropically defective in molybdoenzyme activities. The inactive derivative of the molybdoenzyme, respiratory nitrate reductase (nitrite: (acceptor) oxidoreductase, EC 1.7.99.4), which is present in cell-free extracts of chlA mutants can be activated by addition of purified protein PA, the presumed active product of the chlA+ locus, but the activity of the purified protein PA is low, since comparatively large amounts of protein PA are required for the activation. Addition of 10 mM tungstate to the growth medium of a chlBchlC double mutant leads to inactivation of both the molybdenum cofactor and protein PA. Protein PA prepared from such cells was unable to potentiate the in vitro activation of nitrate reductase present in the soluble fraction of a chlA mutant. Quantitation of inactive protein PA was determined immunologically using protein PA-specific antiserum. When a heat-treated extract of a wild-type strain was added to purified protein PA or to the supernatant fraction of a chlBchlC double mutant grown with tungstate, a large stimulation in the ability of these preparations to activate chlA nitrate reductase was found. We equate the activator of protein PA with molybdenum cofactor because: (1) both are absent from heated extracts of tungstate-grown chlBchlC double mutant and cofactor defective chlA and chlE mutants; (2) both are present in heated extracts of wild-type strain; and (3) they behave identically on molecular-sieve columns.

The molybdenum iron-sulphur protein from Desulfovibrio gigas as a form of aldehyde oxidase

The molybdenum iron-sulphur protein originally isolated from Desulfovibrio gigas by Moura, Xavier, Bruschi, Le Gall, Hall & Cammack [(1976) Biochem. Biophys. Res. Commun. 72, 782-789] has been further investigated by e.p.r. spectroscopy of molybdenum(V). The signal obtained on extended reduction of the protein with sodium dithionite has been shown, by studies at 9 and 35 HGz in 1H2O and 2H2O and computer simulations, to have parameters corresponding to those of the Slow signal from the inactive desulpho form of various molybdenum-containing hydroxylases. Another signal obtained on brief reduction of the protein with small amounts of dithionite was shown by e.p.r. difference techniques to be a Rapid type 2 signal, like that from the active form of such enzymes. In confirmation that the protein is a molybdenum-containing hydroxylase, activity measurements revealed that it had aldehyde:2,6-dichlorophenol-indophenol oxidoreductase activity. No such activity towards xanthine or purine was observed. Salicylaldehyde was a particularly good substrate, and treatment of the protein with it also gave rise to the Rapid signal. Molybdenum cofactor liberated from the protein was active in the nit-1 Neurospora crassa nitrate reductase assay. It is concluded that the protein is a form of an aldehyde oxidase or dehydrogenase. From the intensity of the e.p.r. signals and from enzyme activity measurements, 10-30% of the protein in the sample examined appeared to be in the functional form. The evolutionary significance of the protein, which may represent a primitive form of the enzyme rather than a degradation product, is discussed briefly.

Involvement of chlA, E, M, and N loci in Escherichia coli molybdopterin biosynthesis

All molybdenum enzymes except nitrogenase contain a common molybdenum cofactor, whose organic moiety is a novel pterin called molybdopterin (MPT). To assist in elucidating the biosynthetic pathway of MPT, two MPT-deficient mutants of Escherichia coli K-12 were isolated. They lacked activities of the molybdenum enzymes nitrate reductase and formate dehydrogenase, did not reconstitute apo nitrate reductase from a Neurospora crassa nit-1 strain, and did not yield form A, a derivative of MPT. By P1 mapping, these two mutations mapped to chlA and chlE, loci previously postulated but never definitely shown to be involved in MPT biosynthesis. The two new mutations are in different genetic complementation groups from previously isolated chlA and chlE mutations and have been designated as chlM and chlN (closely linked to chlA and chlE, respectively). The reported presence of Mo cofactor activity in the chlA1 strain is shown to be due to in vitro synthesis of MPT through complementation between a trypsin-sensitive macromolecule from the chlA1 strain and a low-molecular-weight compound from the nit-l strain.

Purification and characterization of a molybdenum-pterin-binding protein (Mop) in Clostridium pasteurianum W5

A large-scale fractionation scheme purified the major molybdenum(Mo)-binding protein (Mop) from crude extracts of Clostridium pasteurianum, with a 10 and 0.2% yield of Mo and protein, respectively. The apparent molecular weight of the purified molybdoprotein is 5,700, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The protein contains 0.7 mol of Mo per mol of protein with a molecular weight of 5,700. Mop, as isolated, has a peak absorbency at 293 nm. Denaturation and oxidation of the molybdoprotein released multiple pterin like fluorescent compounds. Mop appears to contain a pterin derivative and Mo, but phosphate analysis indicated that the pterin at the very least is not phosphorylated; phosphorylation is required for functional molybdenum cofactor. All treatments used to release the putative Mo-pterin species from Mop failed to yield a molybdopterin that had detectable molybdenum cofactor activity.