Molybdenum in Biology - An Essential Trace Element
Reviews of molybdoenzymes
A volume in the series Metal Ions in Biological Systems is devoted to molybdenum and tungsten:
A. Sigel and H. Sigel (eds.), Metal Ions in Biological Systems, Vol. 39: Molybdenum and Tungsten: Their Roles in Biological Processes, Marcel Dekker, New York, 2002.
See especially:
Stiefel, E.L., The biogeochemistry of molybdenum and tungsten, Molybdenum and Tungsten: Their Roles in Biological Processes, 2002, 39, 1-29.
Lowe, D.J., Enzymes of the xanthine oxidase family: The role of molybdenum, Molybdenum and Tungsten: Their Roles in Biological Processes, 2002, 39, 455-479.
Turnlund, J.R., Molybdenum metabolism and requirements in humans, Molybdenum and Tungsten: Their Roles in Biological Processes, 2002, 39, 727-739.
Lagarde, F. and Leroy, M., Metabolism and toxicity of tungsten in humans and animals, Molybdenum and Tungsten: Their Roles in Biological Processes , 2002, 39, 741-759.
Schindelin, H., Kisker, C., Rees , D.C. , The molybdenum-cofactor: a crystallographic perspective, Journal Of Biological Inorganic Chemistry , 1997, 2 , 773-781.
Mendel, R.R., The role of the molybdenum cofactor in humans, Biofactors, 2000, 11 , 147-148.
Vorholt, J.A. and Thauer, R. K., Molybdenum and tungsten enzymes in C1 metabolism, Molybdenum and Tungsten: Their Roles in Biological Processes, 2002, 39, 571-619.
L'vov, N.P., Nosikov, A. N., and Antipov, A. N., Tungsten-containing enzymes, Biochemistry-Moscow, 2002, 67, 196-200.
Noodleman, L., Lovell, T., Liu, T. Q., Himo, F., and Torres, R. A., Insights into properties and energetics of iron-sulfur proteins from simple clusters to nitrogenase, Current Opinion in Chemical Biology , 2002, 6, 259-273.
Williams, R.J.P. and da Silva, J. J. R. F., The involvement of molybdenum in life, Biochemical and Biophysical Research Communications, 2002, 292, 293-299.
Hille, R., Molybdenum and tungsten in biology, Trends in Biochemical Sciences , 2002, 27, 360-367.
Molybdenum-containing hydroxylases catalyze the hydroxylation of carbon centers using oxygen derived ultimately from water, rather than O2, as the source of the oxygen atom incorporated into the product, and do not require an external source of reducing equivalents. The mechanism by which this interesting chemistry takes place has been the subject of investigation for some time, and in the last several years the chemical course of the reaction has become increasingly well understood. This review summarizes recent mechanistic and structure/function studies of the molybdenum-containing hydroxylases.
Hille, R., Molybdenum-containing hydroxylases, Archives of Biochemistry and Biophysics, 2005, 433, 107-116.
Review of molybdenum uptake into the cell, via formation of the molybdenum cofactor and its storage, to the final modification of molybdenum cofactor and its insertion into apo-metalloenzymes
Mendel, R.R., Molybdenum: biological activity and metabolism, Dalton Transactions, 2005, 3404-3409.
Schwarz, G., Molybdenum cofactor biosynthesis and deficiency, Cellular and Molecular Life Sciences, 2005, 62, 2792-2810.
Noriega, C., Hassett, D. J., and Rowe, J. J., The mobA gene is required for assimilatory and respiratory nitrate reduction but not xanthine dehydrogenase activity in Pseudomonas aeruginosa, Current Microbiology, 2005, 51, 419-424.
Molybdenum is essential for most biological systems as it is required by enzymes catalyzing reactions in carbon, sulfur and nitrogen metabolism. Molybdenum is biologically inactive unless it is complexed by a cofactor. Mo is bound to a pterin forming the molybdenum cofactor (Moco) which is the active compound at the catalytic site of Mo-enzymes except nitrogenase. In eukaryotes, the most prominent Mo-enzymes are (1) sulfite oxidase, which catalyzes the final step in the degradation of sulfur-containing amino acids and is involved in detoxifying excess sulfite, (2) xanthine dehydrogenase, which is involved in purine catabolism and reactive oxygen production, (3) aldehyde oxidase, which oxidizes a variety of aldehydes and is essential for the biosynthesis of the phytohormone abscisic acid, and in autotrophic organisms and (4) nitrate reductase, which catalyzes the key step in inorganic nitrogen assimilation. Mo-enzymes, except plant sulfite oxidase, need at least one more redox active center, many of them involving iron in electron transfer. The biosynthesis of Moco involves the complex interaction of six proteins and is a process of four steps, which also includes iron as well as copper in an indespensable way. Moco as released after synthesis is likely to be distributed to the apoproteins of Mo-enzymes by putative Moco-carrier proteins. Xanthine dehydrogenase and aldehyde oxidase, but not sulfite oxidase and nitrate reductase, require the postranslational sulfuration of their Mo-site for becoming active. This final maturation step is catalyzed by a Moco-sulfurase enzyme, which mobilizes sulfur from L-cysteine in a pyridoxal phosphate-dependent manner as typical for cysteine desulfurases.
Mendel, R. R. and Bittner, F., Cell biology of molybdenum, Biochimica et Biophysica Acta-Molecular Cell Research, 2006, 1763, 621-635.
Review of molybdenum uptake into the cell, via formation of the molybdenum cofactor and its storage, to the final modification of molybdenum cofactor and its insertion into apo-metalloenzymes
Mendel, R.R., Molybdenum: biological activity and metabolism, Dalton Transactions, 2005, 3404-3409.
Schwarz, G. and Mendel, R. R., Molybdenum cofactor biosynthesis and molybdenum enzymes, Annual Review of Plant Biology, 2006, 57, 623-647
Harrison, R., Milk xanthine oxidase: Properties and physiological roles, International Dairy Journal, 2006, 16, 546-554.
Active sites of molybdoenzymes
Molybdenum enzymes can be grouped on the basis of the structure of the metal centre:
molybdenum hydroxylases |
(pyranopterin)MoOS(OH) |
Xanthine oxidase and xanthine dehydrogenase |
eukaryotic oxotransferases |
(pyranopterin)MoO2(S- Cys) |
sulfite oxidases and plant nitrate reductases. |
bacterial oxotransferases |
(pyranopterin)2MoOX |
pyranopterin, cofactor; X, ligated serine, cysteine or selenocysteine |
The active sites possess a catalytically labile Mo-OH (or possibly Mo-OH 2) group that is transferred to substrate in the course of the hydroxylation reaction. Water rather than molecular oxygen is the ultimate source of the oxygen atom incorporated into product.
Hille, R., Molybdenum enzymes, Essays Biochem., 1999, 34, 125-137.
X-ray absorption spectra of molybdoenzymes
The X-ray absorption spectra at the molybdenum and selenium K- edges and the tungsten L-2,L-3-edges are reported for fourteen Mo(IV) and W(IV,VI) bis(dithiolene) complexes related to the active sites of molybdo- and tungstoenzymes. These and previous XAS results should prove useful in characterizing and refining metric features and structures of enzyme sites.
Musgrave, K.B., Lim, B. S., Sung, K. M., Holm, R. H., Hedman, B., and Hodgson, K. O., X-ray spectroscopy of enzyme active site analogues and related molecules: Bis(dithiolene)molybdenum(IV) and -tungsten(IV,VI) complexes with variant terminal ligands, Inorganic Chemistry, 2000, 39 , 5238-5247.
Aldehyde oxidase
The aldehyde oxidoreductase, isolated from the sulfate reducer Desulfovibrio desulfuricans (ATCC 27774), and the homologous enzyme from Desulfovibrio gigas are members of the xanthine oxidase family of molybdenum-containing enzymes. They have similar substrate specificity. The primary sequences from both enzymes show 68 % identity. Both enzymes are very closely related in their sequences and 3D structures. The comparison allowed confirmation and establishment of features that are essential for their function: conserved residues in the active site, catalytically relevant water molecules and recognition of the physiological electron acceptor docking site.
Rebelo, J., Macieira, S., Dias, J. M., Huber, R., Ascenso, C. S., Rusnak, F., Moura, J. J. G., Moura, I., and Romao, M. J., Gene sequence and crystal structure of the aldehyde oxidoreductase from Desulfovibrio desulfuricans ATCC 27774, Journal of Molecular Biology, 2000, 297 , 135-146.
Arsenate and selenate reductases
In arsenate and selenate respiring bacteria, the oxidation of organic substrates acetate, lactate, pyruvate, glycerol, ethanol or hydrogen can be coupled to the reduction of arsenate and selenate. Arsenate and selenate reductases have been characterized; they all contain molybdenum.
Stolz, J.F., Oremland, R.S., Bacterial respiration of arsenic and selenium, Fems Microbiology Reviews , 1999, 23 , 5, 615-627.
Arsenite Oxidase
Arsenite oxidase from Alcaligenes faecalis NCIB 8687 is a molybdenum/iron protein involved in the detoxification of arsenic. It oxidizes arsenite [(AsO 2-)-O-III], which binds to essential sulfhydryl groups of proteins and dithiols, to the relatively less toxic arsenate [(AsO4 3-)-O-V]. Arsenite oxidase consists of a large subunit of 825 residues and a small subunit of approximately 134 residues. The large subunit contains a Mo site, consisting of a Mo atom bound to two pterin cofactors, and a [3Fe-4S] cluster. The large subunit of arsenite oxidase is similar to other members of the dimethylsulfoxide (DMSO) reductase family of molybdenum enzymes, particularly the dissimilatory periplasmic nitrate reductase from Desulfovibrio desulfuricans , but is unique in having no covalent bond between the polypeptide and the Mo atom. The small subunit has no counterpart among known Mo protein structures but is homologous to the Rieske [2Fe-2S] protein domain of the cytochrome be, and cytochrome b(6)f complexes and to the Rieske domain of naphthalene 1,2-dioxygenase
Ellis, P.J., Conrads, T., Hille, R., and Kuhn, P., Crystal structure of the 100 kDa arsenite oxidase from Alcaligenes faecalis in two crystal forms at 1.64 angstrom and 2.03 angstrom, Structure, 2001, 9, 125-132.
Lebrun, E., Brugna, M., Baymann, F., Muller, D., Lievremont, D., Lett, M. C., and Nitschke, W., Arsenite oxidase, an ancient bioenergetic enzyme, Molecular Biology and Evolution, 2003, 20, 686-693.
Biotin sulfoxide reductase
Vibrational modes associated with a terminal oxo ligand and the two molybdopterin dithiolene ligands have been assigned. The enzyme cycles between mono-oxo- Mo(VI) and des-oxo-Mo(IV) forms with both molybdopterin dithiolene ligands remaining co-ordinated. In both redox states the terminal oxo group at the molybdenum centre is exchangeable with water during redox cycling and originates from the substrate in substrate-oxidized samples. Product-induced changes in the Mo=O stretching frequency provide direct evidence for a product- associated mono-oxo-Mo(VI) catalytic intermediate.
Garton, S.D., Temple, C. A., Dhawan, I. K., Barber, M. J., Rajagopalan, K. V., and Johnson, M. K., Resonance Raman characterization of biotin sulfoxide reductase - Comparing oxomolybdenum enzymes in the Me2SO reductase family, Journal of Biological Chemistry , 2000, 275 , 6798-6805.
Carbon monoxide dehydrogenase
CO dehydrogenase (EC 1.2.99.2) catalyses the oxidation of CO
CO + H2O --> CO2 + 2 e- + 2 H+
It is a selenium-containing molybdo-iron-sulfur- flavoenzyme.It has been crystallized from the aerobic CO utilizing bacteria Oligotropha carboxidovorans and Hydrogenophaga pseudoflava. It has been structurally characterized in its oxidized state. The enzymes are dimers of two heterotrimers. Each heterotrimer is composed of a molybdoprotein, a flavoprotein, and an iron-sulfur protein. The substituents in the first co-ordination sphere of the Mo-ion are the enedithiolate sulfur atoms of the molybdopterin-cytosine dinucleotide, two oxo- and a sulfide-group. Extended X-ray absorption fine structure spectroscopy , along with the crystal structure of CO dehydrogenase at 1.85 Angstrom resolution, have identified a sulfur atom at 2.3 Angstrom from the Mo-ion. The sulfur reacts with cyanide yielding thiocyanate. Both CO dehydrogenase structures show only minor differences. CO oxidation takes place at the molybdoprotein which contains a 1:1 mononuclear complex of molybdopterin-cytosine dinucleotide and a Mo-ion, along with a catalytically essential S- selanylcysteine. The corresponding inactive desulfo-CO dehydrogenase shows a typical desulfo inhibited-type of Mo-electron paramagnetic resonance spectrum. Structural changes at the SeMo-site during catalysis are suggested by the Mo to Se distance of 3.7 A and the Mo-S-Se angle of 113 degrees in the oxidized enzyme which increase to 4.1 Angstrom, and 121", respectively, in the reduced enzyme. The intramolecular electron transport chain in CO dehydrogenase involves the following prosthetic groups and minimal distances: CO --> [Mo of the molybdenum cofactor] - 14.6 Angstrom - [2Fe-2S] I - 12.4 Angstrom - [2Fe-2S] II - 8.7 Angstrom - [FAD]
Meyer, O., Gremer, L., Ferner, R., Ferner, M., Dobbek, H., Gnida, M., Meyer-Klaucke, W., and Huber, R., The role of Se, Mo and Fe in the structure and function of carbon monoxide dehydrogenase, Biological Chemistry , 2000, 381 , 865-876.
See also
Hanzelmann, P., Dobbek, H., Gremer, L., Huber, R., and Meyer, O., The effect of intracellular molybdenum in Hydrogenophaga pseudoflava on the crystallographic structure of the seleno- molybdo-iron-sulfur flavoenzyme carbon monoxide dehydrogenase, Journal of Molecular Biology , 2000, 301 , 1221-1235.
The molybdoenzyme carbon monoxide dehydrogenase (CODH) catalyzes the oxidation Of CO to CO2 in the aerobic bacterium Oligotropha carboxidovorans. The active site in oxidized CODH contains a bimetallic [(CuSMOVI)- S-I(=O)(2)] cluster which was converted into a [(CuSMoIV)-S-I(=O)- OH(2)] cluster upon reduction. The Cu...Mo distance is 3.70 Angstrom in the oxidized form and is increased to 4.23 Angstrom upon reduction.
Gnida, M., Ferner, R., Gremer, L., Meyer, O., and Meyer-Klaucke, W., A novel binuclear [CuSMo] cluster at the active site of carbon monoxide dehydrogenase: Characterization by X-ray absorption spectroscopy, Biochemistry, 2003, 42, 222-230.
Dimethylsulfide dehydrogenase
Hanson, G.R., McDevitt, C. A., and McEwan, A. G., Dimethylsulfide dehydrogenase from rhodovulum sulfidophilum : EPR spectroscopy and biochemical analysis reveal its place in the DMSO reductase family of molybdenum enzymes, Journal of Inorganic Biochemistry, 2001, 86, 248.
Dimethyl sulfoxide (DMSO) reductase
The molybdenum site of the oxidized Mo(VI) dimethyl sulfoxide (DMSO) reductase enzyme possesses one terminal oxygen ligand (Mo=O) at 1.68 Angstrom, four thiolate ligands at 2.44 Angstrom, and one oxygen at 1.92 Angstrom. The dithionite-reduced Mo(IV) enzyme possesses a desoxo species with three or four Mo-S at 2.33 Angstrom and two different Mo-O ligands at 2.16 and 1.92 Angstrom. One exchangeable oxygen ligand, most likely an Mo- OH, in the ESR-active signal-giving species originates from the Mo=O of the oxidized enzyme. The enzyme reduced by dimethylsulfide contains a desoxo active site with four Mo-S at 2.36 and two different Mo-O ligands at 1.94 and 2.14 Angstrom. Recombinant wild-type R. sphaeroides DMSO reductase expressed in Escherichia coli. initially has a dioxo structure (two Mo=O at 1.72 Angstrom and four Mo-S at 2.48 Angstrom) but assumes the wild-type Mo(VI) structure after a cycle of reduction and reoxidation. The site-directed Ser147-->Cys mutant possesses a monooxo active site in the oxidized state (Mo=O at 1.70 Angstrom) with five sulfur ligands (at 2.40 Angstrom), consistent with cysteine 147 coordination to Mo. The dithionite reduced form of the mutant possesses a desoxo site also with five. Mo-S ligands (at 2.37 Angstrom) and one Mo-O at 2.12 Angstrom.
George, G.N., Hilton, J., Temple, C., Prince, R.C., Rajagopalan, K.V., Structure of the molybdenum site of dimethyl sulfoxide reductase, Journal Of The American Chemical Society , 1999, 121 , 1256-1266.
Dimethyl sulfoxide (DMSO) reductase from purple bacteria contains a mononuclear Mo co-ordinated by two molybdopterin guanine dinucleotides as its single cofactor. Crystallographic studies on the enzyme from Rhodobacter sphaeroides and Rhodobacter capsulatus revealed substantial differences in the Mo co-ordination environment in the oxidized Mo(VI) state, despite a close structural similarity in the overall fold of the protein. The crystal structure of DMSO reductase from R. sphaeroides identified a Mo environment with a mono-oxo ligation and an asymmetric co-ordination by the two molybdopterins, with three short and one very long Mo-S bond.
In contrast, two independent crystallographic studies of the enzyme from R. capsulatus revealed two additional Mo co-ordination environments: a pentacoordinated dioxo metal ligation sphere in which one molybdopterin is completely dissociated from the Mo and a heptacoordinated environment with symmetrical metal co-ordination by both molybdopterins and two oxo ligands. In all three structures the side chain of a serine was a ligand to the Mo. EXAFS studies on the R. sphaeroides enzyme suggested a hexacoordinated active site geometry, whereas for the R. capsulatus enzyme EXAFS indicated seven ligands.
Baugh, P.E., Garner, C.D., Charnock, J.M., Collison, D., Davies, E.S., McAlpine, A.S., Bailey, S., Lane, I., Hanson, G.R., McEwan, A.G, X-ray absorption spectroscopy of dimethylsulfoxide reductase from Rhodobacter capsulatus, Journal Of Biological Inorganic Chemistry , 1997, 2 , 634-643.
The 1.3 Angstrom resolution crystal structure of oxidized DMSOR from R. sphaeroides reveals plasticity at the active site. The Mo is discretely disordered and exists in a hexacoordinated and a pentacoordinated ligation sphere. The hexacoordinated model reconciles the existing differences in active site co-ordination of R. sphaeroides DMSO reductase as studied by crystallographic and EXAFS techniques. In addition, the pentacoordinated structure closely resembles one of the reported R. capsulatus crystal structures.
Trieber, C.A., Rothery, R.A., Weiner, J.H., Consequences Of Removal Of A Molybdenum Ligand (Dmsa-Ser-176) Of Escherichia-Coli Dimethyl-Sulfoxide Reductase, Journal Of Biological Chemistry, 1996, 271 , 27339-27345.
The active site geometry in the previously reported 2.2 Angstrom crystal structure of R, sphaeroides DMSO reductase appears to represent an average of the two conformations described here. Thus, structural flexibility at the active site appears to give rise to the observed differences in the Mo co-ordination environment
Li, H.K., Temple , C., Rajagopalan, K. V., and Schindelin, H., The 1.3 angstrom crystal structure of Rhodobacter sphaeroides dimethyl sulfoxide reductase reveals two distinct molybdenum co-ordination environments, Journal of the American Chemical Society , 2000, 122 , 7673-7680.
Ethylbenzene dehydrogenase
The first step in anaerobic ethylbenzene mineralization in denitrifying Azoarcus so. strain EB1 is the oxidation of ethylbenzene to (S)-(-)-1-phenylethanol. Ethylbenzene dehydrogenase, which catalyzes this reaction, is a unique enzyme in that it mediates the stereoselective hydroxylation of an aromatic hydrocarbon in the absence of molecular oxygen. Purified ethylbenzene dehydrogenase contains approximately 0.5 mol of molybdenum, 16 mol of iron, and 15 mol of acid-labile sulfur per mol of holoenzyme, and a molydopterin cofactor. According to sequence analysis and biochemical data ethylbenzene dehydrogenase is a novel member of the dimethyl sulfoxide reductase family of molybdopterin-containing enzymes
Johnson, H.A., Pelletier, D. A., and Spormann, A. M., Isolation and characterization of anaerobic ethylbenzene dehydrogenase, a novel Mo-Fe-S enzyme, Journal of Bacteriology, 2001, 183, 4536-4542.
Molybdopterin cofactor
The molybdopterin cofactor is required for the activity of a variety of oxidoreductases. The xanthine oxidase class of molybdoenzymes requires the molybdopterin cofactor to have a terminal, cyanolysable sulfur ligand. In the sulfite oxidase/nitrate reductase class, an oxygen is present in the same position.
Mutations in both the ma-l gene of Drosophila melanogaster and the hxB gene of Aspergillus nidulans cause loss of activities of those molybdoenzymes that require a cyanolysable sulfur in the active centre. The ma-l and hxB genes encode highly similar proteins containing domains common to pyridoxal phosphate-dependent cysteine transulphurases, including the cofactor binding site and a conserved cysteine, which is the putative sulfur donor.
Key similarities were found with NifS, the enzyme involved in the generation of the iron-sulphur centres in nitrogenase. These similarities suggest an analogous mechanism for the generation of the terminal molybdenum-bound sulfur ligand. Putative homologues have been identified of these genes in organisms, including humans.
Amrani, L., Primus, J., Glatigny, A., Arcangeli, L., Scazzocchio, C., and Finnerty, V., Comparison of the sequences of the aspergillus nidulans hxB and drosophila melanogaster ma-l genes with nifS from azotobacter vinelandii suggests a mechanism for the insertion of the terminal sulfur atom in the molybdopterin cofactor, Mol.Microbiol. , 2000, 38 , 114-125.
All mononuclear molybdoenzymes bind molybdenum in a complex with an organic cofactor termed molybdopterin. In many bacteria, including Escherichia coli, molybdopterin can be further modified by attachment of a guanine monophosphate group to the terminal phosphate of molybdopterin to form molybdopterin guanine dinucleotide. This modification reaction is required for the functioning of many bacterial molybdoenzymes, including the nitrate reductases, dimethylsulfoxide and trimethylamine-N-oxide reductases, and formate dehydrogenases. The guanine monophosphate attachment step is catalyzed by the cellular enzyme MobA. The crystal structure of the 21.6 kDa E. coli MobA has been determined.
Stevenson, C.E.M., Sargent, F., Buchanan, G., Palmer, T., and Lawson, D. M., Crystal structure of the molybdenum cofactor biosynthesis protein MobA from Escherichia coli at near-atomic resolution, Structure, 2000, 8 , 1115-1125.
Molybdopterin synthase
Molybdopterin is inserted into xanthine dehydrogenase only after molybdenum chelation, and both metal chelation and molybdopterin insertion can occur only under high molybdate concentrations. In the biosynthesis of the molybdenum cofactor the biosynthesis of molybdopterin and molybdopterin guanine dinucleotide are split at a stage when the molybdenun atom is added to molybdopterin.
Leimkuhler, S., Angermuller, S., Schwarz, G., Mendel, R.R., Klipp,W., Activity of the molybdopterin-containing xanthine dehydrogenase of Rhodobacter capsulatus can be restored by high molybdenum concentrations in a moeA mutant defective in molybdenum cofactor biosynthesis, Journal Of Bacteriology , 1999, 181 , 19, 5930-5939.
Molybdenum cofactor (Moco) biosynthesis is an evolutionarily conserved pathway present in eubacteria, archaea and eukaryotes, including humans. Genetic deficiencies of enzymes involved in Moco biosynthesis in humans lead to a severe and usually fatal disease. Moco contains a tricyclic pyranopterin, termed molybdopterin, that bears the cis-dithiolene group responsible for molybdenum ligation. The dithiolene group of molybdopterin is generated by molybdopterin synthase, which consists of a large and small subunits. The 1.45 Angstrom resolution crystal structure of molybdopterin synthase reveals a heterotetrameric protein in which the C-terminus of each small subunit is inserted into a large subunit to form the active site. In the activated form of the enzyme this C-terminus is present as a thiocarboxylate. In the structure of a covalent complex of molybdopterin synthase, an isopeptide bond is present between the C-terminus of the small subunit and a Lys side chain in the large subunit. The strong structural similarity between the small subunit of molybdopterin synthase and ubiquitin provides evidence for the evolutionary antecedence of the Moco biosynthetic pathway to the ubiquitin dependent protein degradation pathway.
Rudolph, M.J., Wuebbens, M. M., Rajagopalan, K. V., and Schindelin, H., Crystal structure of molybdopterin synthase and its evolutionary relationship to ubiquitin activation, Nature Structural Biology, 2001, 8, 42-46.
See also
Schrag, J.D., Huang, W. J., Sivaraman, J., Smith, C., Plamondon, J., Larocque, R., Matte, A., and Cygler, M., The crystal structure of Escherichia coli MoeA, a protein from the molybdopterin synthesis pathway, Journal of Molecular Biology , 2001, 310, 419-431.
Molybdopterin is a pyranopterin with a unique dithiolene group coordinating molybdenum or tungsten in all molybdenum- and tungsten-enzymes except nitrogenase. In Escherichia coli, molybdopterin is formed by incorporation of two sulfur atoms into a precursor, which is catalyzed by the molybdopterin synthase enzyme. A two-step reaction of molybdopterin synthesis is proposed where the dithiolene is generated by two thiocarboxylates derived from a single tetrameric molybdopterin synthase.
Gutzke, G., Fischer, B., Mendel, R. R., and Schwarz, G., Thiocarboxylation of molybdopterin synthase provides evidence for the mechanism of dithiolene formation in metal-binding pterins, Journal of Biological Chemistry, 2001, 276, 36268-36274.
See also
Leimkuhler, S., Wuebbens, M. M., and Rajagopalan, K. V., Characterization of Escherichia coli MoeB and its involvement in the activation of molybdopterin synthase for the biosynthesis of the molybdenum cofactor, Journal of Biological Chemistry, 2001, 276, 34695-34701.
Krepinsky, K. and Leimkuhler, S., Site-directed mutagenesis of the active site loop of the rhodanese-like domain of the human molybdopterin synthase sulfurase MOCS3 - Major differences in substrate specificity between eukaryotic and bacterial homologs, Febs Journal, 2007, 274, 2778-2787.
Nitrate reductase
In plants and some animals the first stage in the reduction of nitrate is to nitrite. The reduction is catalysed by nitrate reductase, a flavoprotein enzyme which has molybdenum as the only metal requirement. Molybdenum acts as an electron acceptor from reduced FAD in the enzyme. The molybdenum cofactor is an oxomolybdenum sulfur species with a pterin ligand.
Berks, B. C., Ferguson, S. J., Moir, J. W. B. and Richardson, D. J., Biochim. Biophys. Acta - Bioenergetics , 1995, 1232, 97.
Campbell, W. H., Plant Physiology , 1996, 111 , 355.
Collison, D., Garner, C. D. and Joule, J. A., Chem. Soc. Rev. , 1996, 25 , 25.
Although NR always catalyzes the conversion of nitrate to nitrite, its location in the cell, structure, and function are organism-dependent. Protein sequencing is used to determine phylogenetic relationships and to examine similarities in structure and function. Conserved binding sites for molybdenum and pterin cofactors are found.
Stolz, J.F. and Basu, P., Evolution of nitrate reductase: Molecular and structural variations on a common function, Chembiochem, 2002, 3, 198-206.
Gonzalez, P. J., Correia, C., Moura, I., Brondino, C. D., and Moura, J. J. G., Bacterial nitrate reductases: Molecular and biological aspects of nitrate reduction, Journal of Inorganic Biochemistry, 2006, 100, 1015-1023.
Nitrogenase
Nitrogenase is the complex metalloenzyme responsible for biological dinitrogen reduction. The mechanism of nitrogen fixation in enzymes and in model systems in vitro has been extensively investigated.
Burris, R. H. and Roberts, G. P., Ann. Rev. Nutrition , 1993, 13 , 317.
Sellman, D., Agnew. Chem. Int. Ed. , 1993, 105 , 64.
The microorganisms which fix molecular nitrogen fall into two classes: (a) the symbiotic microorganisms which fix nitrogen in association with plants, e.g., Rhizobium; (b) asymbiotic microorganisms which are free-living and include Azotobacter vinelandii and Clostridium Pasteurianum. From cell-free extracts of C. Pasteurianum, two metalloproteins have been obtained. The hydrogen donating system, azoferredoxin, which contains iron and sulfide and the nitrogen activating system, molybdoferredoxin, which contains molybdenum, iron, and sulfide. The structure of the active centre has been shown by X-ray crystallography to be a Fe-Mo-S cluster.
Kim, J., Woo, D. and Rees, D. C., Biochemistry ,1993 , 32 ,7104.
Chen, J., Christiansen, J., Campbasso, N., Bolin, J. T., Tittsworth, B. C., Hales, B. J., Rehr, J. J. and Cramer, S. P., Angew. Chem. Int. Edn. ,1993, 32,1592.
The mechanism of the N2 fixing enzyme nitrogenase has been reviewed including a discussion of why low molecular weight complexes have not yet been able to copy nitrogenase catalyzed reactions or to act as competitive catalysts with nitrogenase-like activity.
Sellmann, D., Utz, J., Blum, N., Heinemann, F.W., On the function of nitrogenase FeMo cofactors and competitive catalysts: chemical principles, structural blue-prints, and the relevance of iron sulfur complexes for N-2 fixation, Coordination Chemistry Reviews , 1999, 192 , 607-627.
Recent developments in understanding the mechanism of the Mo-based nitrogenase are reviewed: how nucleotide binding and hydrolysis are coupled to electron transfer and substrate reduction, how electrons are accumulated and transferred within the MoFe-protein, and how substrates bind and are reduced at the active site metal cluster.
Christiansen, J., Dean, D. R., and Seefeldt, L. C., Mechanistic features of the Mo-containing nitrogenase, Annual Review of Plant Physiology and Plant Molecular Biology, 2001, 52, 269.
The Mo-site and its ligand environment of the FeMo-cofactor (FeMo-co) were studied using the hybrid density functional method B3LYP.
Szilagyi, R.K., Musaev, D. G., and Morokuma, K., Theoretical studies of biological nitrogen fixation. I. Density functional modeling of the Mo-site of the FeMo-cofactor, Inorganic Chemistry , 2001, 40, 766-775
The MoFe protein of the nitrogenase enzyme from Azotobacter vinelandii was altered by substituting the alpha Gly(69) residue by serine. Then both CO and acetylene become competitive inhibitors of dinitrogen reduction. CO is also converted from a non-competitive inhibitor of the wild type enzyme to a competitive inhibitor of acetylene, nitrous oxide, and azide reduction. These results are interpreted in terms of a two-site model with both sites located in close proximity within a specific 4Fe-4S face of FeMo cofactor. Site 1 is a high affinity acetylene-binding site to which CO also binds; but dinitrogen, azide, and nitrous oxide do not bind. Site 2 is a low affinity acetylene-binding site to which CO, dinitrogen, azide, and nitrous oxide also bind.
Christiansen, J., Seefeldt, L. C., and Dean, D. R., Competitive substrate and inhibitor interactions at the physiologically relevant active site of nitrogenase, Journal of Biological Chemistry , 2000, 275 , 36104-36107.
The role of molybdenum in nitrogenase has been reviewed with particular reference to theoretical modeling of the molybdenum centre.
Barriere, F., Modeling of the molybdenum center in the nitrogenase FeMo- cofactor, Coordination Chemistry Reviews, 2003, 236, 71-89.
See also
Bell, J., Dunford, A. J., Hollis, E., and Henderson, R. A., The role of Mo atoms in nitrogen fixation: Balancing substrate reduction and dihydrogen production, Angewandte Chemie-International Edition, 2003, 42 , 1149-1152.
Benton, P.M.C., Laryukhin, M., Mayer, S. M., Hoffman, B. M., Dean, D. R., and Seefeldt, L. C., Localization of a substrate binding site on the FeMo-cofactor in nitrogenase: Trapping propargyl alcohol with an alpha-70- substituted MoFe protein, Biochemistry, 2003, 42, 9102-9109.
Hinnemann, B. and Norskov, J. K., Modeling a central ligand in the nitrogenase FeMo cofactor, Journal of the American Chemical Society, 2003, 125, 1466-1467.
Barney, B. M., Lee, H. I., Dos Santos, P. C., Hoffmann, B. M., Dean, D. R., and Seefeldt, L. C., Breaking the N-2 triple bond: insights into the nitrogenase mechanism, Dalton Transactions, 2006, 2277-2284.
Selenate and nitrate
Some of the common themes and variations between the different classes of nitrate and selenate reductases are reviewed.
Watts, C.A., Ridley, H., Dridge, E. J., Leaver, J. T., Reilly, A. J., Richardson, D. J., and Butler, C. S., Microbial reduction of selenate and nitrate: common themes and variations, Biochemical Society Transactions, 2005, 33, 173-175.
Sulfite oxidase
See also below: Molybdenum cofactor deficiency in humans: neurological consequences of sulfite oxidase deficiency.
Sulfite is the main intermediate in the oxidation of sulfur compounds to sulfate, the major product of most dissimilatory sulfur-oxidizing prokaryotes. Two pathways of sulfite oxidation are known: (1) direct oxidation to sulfate catalyzed by a sulfite: acceptor oxidoreductase, which is thought to be a molybdenum-containing enzyme; (2) indirect oxidation under the involvement of the enzymes adenylylsulfate (APS) reductase and ATP sulfurylase and/or adenylylsulfate phosphate adenylyltransferase with APS as an intermediate. Direct oxidation appears to have a wider distribution. In many pro- and also eukaryotes sulfite is formed as a degradative product from molecules containing sulfur as a heteroatom. In these organisms detoxification of sulfite is generally achieved by direct oxidation to sulfate.
Kappler, U. and Dahl, C., Enzymology and molecular biology of prokaryotic sulfite oxidation, Fems Microbiology Letters, 2001, 203 , 1-9.
Sulfite oxidase which was deficient in molybdopterin was reconstituted in vitro with the molybdenum cofactor (Moco) synthesized de novo from precursor and molybdate. In vitro reconstitution of the purified apoprotein was achieved using an incubation mixture containing purified precursor, purified molybdopterin synthase, and sodium molybdate.
Leimkuhler, S. and Rajagopalan, K. V., In vitro incorporation of nascent molybdenum cofactor into human sulfite oxidase, Journal of Biological Chemistry, 2001, 276, 1837-1844.
The electronic (charge-transfer) spectrum of the enzyme sulfite oxidase has been probed by temperature-dependent magnetic circular dichroism (MCD) spectroscopy. The enzyme was poised in the catalytically relevant [ Mo(V):Fe(II)] state by anaerobic reduction with sulfite in the absence of cytochrome c. A feature at 22 250 cm-1 in the MCD is assigned as the cysteine S(sigma)-->Mo d(xy) charge transfer. The primary role of the co-ordinated cysteine is to decrease the effective nuclear charge on Mo by charge donation to the metal, statically poising the active site at more negative reduction potentials during electron transfer regeneration.
Helton, M.E., Pacheco, A., McMaster, J., Enemark, J. H., and Kirk, M. L., An MCD spectroscopic study of the molybdenum active site in sulfite oxidase: insight into the role of co-ordinated cysteine, Journal of Inorganic Biochemistry , 2000, 80 , 227-233.
Molybdenum is an essential constituent of the enzyme hepatic sulfite oxidase. The reduced enzyme, like xanthine oxidase, gives an electron paramagnetic resonance signal of molybdenum(V).
Sheep and cows, develop adverse reactions to feed containing 2-30 ppm molybdenum; horses and pigs tolerate feed with concentrations > 1000 ppm molybdenum [Smyth, 1956].
Cohen, H. J., Fridovich, I. and Rajagopalan, K. V., J. Biol. Chem., 1971, 246 , 374.
Smyth, H.E., Hygienic standard for daily inhalation. Ind Hyg Q, 1956, 17 ,129-185.
Molybdenum K-edge X-ray absorption studies of the oxidized and reduced active sites of the sulfite dehydrogenase from Starkeya novella showed that the molybdenum atom of the oxidized enzyme is bound by two Mo=O ligands at 1.73 angstrom and three thiolate Mo-S ligands at 2.42 angstrom, whereas the reduced enzyme has one oxo at 1.74 angstrom, one long oxygen at 2.19 angstrom (characteristic of Mo-OH2), and three Mo-S ligands at 2.40 angstrom
Doonan, C. J., Kappler, U., and George, G. N., Structure of the active site of sulfite dehydrogenase from Starkeya novella, Inorganic Chemistry, 2006, 45, 7488-7492.
Enemark, J. H., Astashkin, A. V., and Raitsimring, A. M., Investigation of the coordination structures of the molybdenum(v) sites of sulfite oxidizing enzymes by pulsed EPR spectroscopy, Dalton Transactions, 2006, 3501-3514.
Hemann, C., Hood, B. L., Fulton, M., Hansch, R., Schwarz, G., Mendel, R. R., Kirk, M. L., and Hille, R., Spectroscopic and kinetic studies of Arabidopsis thaliana sulfite oxidase: Nature of the redox-active orbital and electronic structure contributions to catalysis, Journal of the American Chemical Society, 2005, 127, 16567-16577.
The recent developments in our understanding of sulfite oxidizing enzyme mechanisms that are driven by a combination of molecular biology, rapid kinetics, pulsed electron paramagnetic resonance (EPR), and computational techniques are the subject of this review.
Feng, C. J., Tollin, G., and Enernark, J. H., Sulfite oxidizing enzymes, Biochimica et Biophysica Acta-Proteins and Proteomics, 2007, 1774, 527-539.
Xanthine dehydrogenase and xanthine oxidase
Xanthine oxidase (EC 1.1.3.22) and xanthine dehydrogenase (EC 1.1.1.204) belong to the molybdenum hydroxylase flavoprotein family; they represent different forms of the same gene product. Xanthine oxidoreductase activity is rate-limiting in purine catabolism. The enzymes can metabolize xenobiotics, including a number of anticancer compounds, to their active metabolites.
Xanthine oxidoreductase is implicated in the pathophysiology of inflammatory diseases and atherosclerosis and in ischemia- reperfusion injury.
Pritsos , C.A. , Cellular distribution, metabolism and regulation of the xanthine oxidoreductase enzyme system, Chem.Biol.Interact. , 12-1-2000 , 129 , 195-208.
The role of molybdenum in xanthine oxidase has been extensively studied [Collison et al., 1996; Peive, 1968]. During the enzyme-catalysed reaction the oxidation state of molybdenum changes and so molybdenum is involved in the electron-transfer pathway. The substrate interacts directly with molybdenum. The molybdenum cofactor is an oxomolybdenum pterin complex similar to the cofactor of nitrate reductase [Berks et al., 1995; Campbell , 1996; Collison et al., 1996; Peive, 1968].
Collison, D., Garner, C. D. and Joule, J. A., Chem. Soc. Rev. , 1996, 25 , 25.
Peive, Ya. V.,(ed.), Biol. Rol Molibdena, Sb. Tr. Simp. 1968 (Publ. 1972), Nauka, Moscow , U.S.S.R., 207, 235.
Berks, B. C., Ferguson, S. J., Moir, J. W. B. and Richardson, D. J., Biochim. Biophys. Acta - Bioenergetics , 1995, 1232, 97.
Campbell, W. H., Plant Physiology , 1996, 111 , 355.
Collison, D., Garner, C. D. and Joule, J. A., Chem. Soc. Rev. , 1996, 25 , 25.
Xanthine oxidizing enzymes isolated from leaves of leguminous plants did not react with molecular oxygen at a significant rate, indicating that all of them are xanthine dehydrogenase . The visible absorption spectrum of the pure protein was assigned: 312 - 390 nm, related to the molybdopterin component of the enzyme; 420 - 510 nm, flavin chromophores; 550 nm, iron-sulphur centres. The fluorescence excitation spectrum showed peaks at 274 and 368 nm, similar to pterin and xanthopterin. The fluorescence emission spectrum was characterized by two maxima at about 400 and 460 nm, typical of flavin chromophores.
Montalbini, P., Xanthine dehydrogenase from leaves of leguminous plants: Purification, characterization and properties of the enzyme, Journal of Plant Physiology , 2000, 156 , 3-16.
Xanthine dehydrogenase (xanthine dehydrogenase ) is a member of the molybdenum hydroxylase family of enzymes catalyzing the oxidation of hypoxanthine and xanthine to uric acid. The enzyme is also required for the production of one of the major Drosophila eye pigments, drosopterin. The xanthine dehydrogenase gene has been isolated.
Pitts, R.J. and Zwiebel, L. J., Isolation and characterization of the xanthine dehydrogenase gene of the Mediterranean fruit fly, Ceratitis capitata, Genetics , 2001, 158, 1645-1655.
The compound 1-(3, 4- dimethoxy-2-chlorobenzylideneamino)-3-hydroxyguanidine (PR5) is a member of a novel class of compounds, xanthine oxidase electron acceptor-inhibitor drugs. They have potential use in the prevention of free radical mediated tissue damage in organ ischemia- reperfusion diseases.
PR5 acts as an alternative electron acceptor in xanthine oxidase catalysed oxidation of xanthine. The action of PR5 is associated with the inhibition of superoxide radical formation. It is suggested that PR5 binds and becomes reduced at the molybdenum centre of the xanthine oxidase.
Dambrova, M., Baumane, L., Kiuru, A., Kalvinsh, I., and Wikberg, J. E., N-Hydroxyguanidine compound 1-(3,4-dimethoxy- 2-chlorobenzylideneamino)- 3-hydroxyguanidine inhibits the xanthine oxidase mediated generation of superoxide radical, Arch.Biochem.Biophys. , 5-1-2000, 377 , 101-108.
The drug allopurinol is used to treat xanthine dehydrogenase -catalyzed uric acid build-up occurring in gout or during cancer chemotherapy. As a hypoxanthine analog, it is oxidized to alloxanthine, which cannot be further oxidized but acts as a tight binding inhibitor of xanthine dehydrogenase . The 3.0 Angstrom resolution structure of the xanthine dehydrogenase -alloxanthine complex shows direct coordination of alloxanthine to the molybdenum via a nitrogen atom. These results provide a starting point for the rational design of new xanthine dehydrogenase inhibitors
Truglio, J.J., Theis, K., Leimkuhler, S., Rappa, R., Rajagopalan, K. V., and Kisker, C., Crystal structures of the active and alloxanthine-inhibited forms of xanthine dehydrogenase from Rhodobacter capsulatus, Structure , 2002, 10, 115-125.
Reactive oxygen species, either superoxide anion radical or hydrogen peroxide, are generated when xanthine oxidase catalyses the hydroxylation of purines. Xanthine oxidase serum levels are increased in various pathological states: hepatitis, inflammation, ischemia-reperfusion, carcinogenesis and aging. The reactive oxygen species are involved in oxidative damage. The inhibition of this enzymatic pathway might be beneficial. Excess of uric acid, the metabolic product of xanthine oxidase, can lead to gout. Allopurinol is a clinically useful inhibitor used in the treatment of gout.The structure and mechanism of the enzyme, associated pathological states, and the development of new xanthine oxidase inhibitors are reviewed.
Borges, F., Fernandes, E., and Roleira, F. , Progress towards the discovery of xanthine oxidase inhibitors, Current Medicinal Chemistry, 2002, 9, 195-217.
An overview of the current state of our understanding of the reaction mechanism of the molybdenum-containing enzyme xanthine oxidoreductase is presented, with an emphasis on work done in the past five years. Recent studies of the biosynthesis of the pterin cofactor bound to the metal in the active site are also reviewed, as is crystallographic work that has clarified the coordination geometry of the molybdenum center. This structural work provides the context in which to understand recent mechanistic studies of the enzyme, in particular those aimed at elucidating the role of specific amino acid residues in the active site of the enzyme
Hille, R., Structure and function of xanthine oxidoreductase, European Journal of Inorganic Chemistry, 2006, 1913-1926.
Acute lung injury represents a wide spectrum of pathologic processes, the most severe being the acute respiratory distress syndrome. Reactive oxygen intermediates have been implicated in the pathobiochemistry of acute lung injury. The endogenous sources that contribute to the generation of reactive oxygen intermediates in acute lung injury probably include, inter alia, the molybdenum hydroxylases. Gene expression of xanthine dehydrogenase /XO is regulated in a cell-specific manner and is markedly affected by inflammatory cytokines, steroids, and physiologic events such as hypoxia. Posttranslational processing is also important in regulating xanthine dehydrogenase /XO activity.
Hoidal, JR, Xu, P, Huecksteadt, T, Sanders, KA, Pfeffer, K, Sturrock, AB, Lung injury and oxidoreductases, Environmental Health Perspectives , 1998, 106 , 1235-1239.
Copper inhibition of xanthine oxidase
Milk xanthine oxidase forms optically observable complexes with Cu2+ ion. Cu2+ ion binds to milk xanthine oxidase with sulfur and nitrogenous ligands. Two Cu2+ bound milk xanthine oxidase complexes are formed at two different time scales of the interaction, earlier than 5 ms and at around 20 s. The second complex may be responsible for the inhibition of the enzyme activity.
Sau, A.K., Mondal, M. S., and Mitra, S., Interaction of Cu2+ ion with milk xanthine oxidase, Biochimica Et Biophysica Acta-Protein Structure and Molecular Enzymology, 2001, 1544, 89-95.
Xanthine oxidase (XO)-catalyses nitrite reduction to nitric oxide under anaerobic conditions. The XO reducing substrates xanthine, NADH, and 2,3 -dihydroxybenzaldehyde triggered nitrite reduction to NO, and the molybdenum-binding XO inhibitor oxypurinol inhibited this NO formation, indicating that nitrite reduction occurs at the molybdenum site. However, at higher xanthine concentrations, partial inhibition was seen, suggesting the formation of a substrate-bound reduced enzyme complex with xanthine blocking the molybdenum site. Studies of the pH dependence of NO formation indicated that XO-mediated nitrite reduction occurred via an acid-catalyzed mechanism, Nitrite and reducing substrate concentrations were important regulators of XO-catalyzed NO generation. XO-catalyzed nitrite reduction can be an important source of NO generation under ischemic conditions
Li, H.T., Samouilov, A., Liu, X. P., and Zweier, J. L., Characterization of the magnitude and kinetics of xanthine oxidase-catalyzed nitrite reduction - Evaluation of its role in nitric oxide generation in anoxic tissues, Journal of Biological Chemistry, 2001, 276, 24482-24489.
Xanthine oxidase and purine metabolism
Xanthine oxidase is a molybdoflavoprotein enzyme with the composition 2FAD:8Fe:2Mo [Bray et al., 1996; Collison et al., 1996; Bray, 1963; Bray and Swann, 1972; Peive,1968]. The enzyme has a low specificity and will catalyse oxidation of many purines and aldehydes with very different rates of reaction. The oxidation of xanthine to uric acid is an essential stage in the catabolism of purine bases in some animals. For example, in parts of New Zealand where the concentration of molybdenum in the soil was low (0.03 ppm compared with 0.4 ppm in other areas) serious losses of sheep occurred owing to the formation of xanthine calculi in the kidneys of the sheep [Underwood, 1962]. Because of the low molybdenum concentration in the pasture xanthine oxidase activity was low and the conversion of xanthine to uric acid was hindered with the consequence that xanthine was precipitated in the kidneys. Raising the molybdenum level in the pasture prevented the development of renal calculi. The relationship between molybdenum concentration and xanthine oxidase activity is not, however, quite straightforward. At low concentrations molybdenum may stimulate the activity of xanthine oxidase; but at high concentrations molybdenum may actually reduce the activity of the enzyme. The activity of xanthine oxidase in cows' milk is proportional to the molybdenum content and both depend on the quantity of ingested molybdenum [Kiermeir and Capellani, 1957]. Addition of molybdenum to the diet causes an increase in the milk molybdenum but does not affect xanthine oxidase activity if this is high before feeding additional molybdenum.
The effect of molybdenum on xanthine oxidase activity in rats has been studied but the results are conflicting. Thus in one study increased dietary molybdenum (0-800 ppm), although causing an increase in liver molybdenum, caused a decrease in liver xanthine oxidase activity and in the concentration of uric acid in the blood [Cox et al., 1960]. With rats fed 1.0 mg Mo/kg body weight daily there were no changes in purine metabolism but 20 mg Mo daily caused increased xanthine oxidase activity in the liver and kidneys and in the blood urate and urea levels [Grigoryan et al., 1969]. According to another report rats receiving added dietary molybdenum (50-500 ug) showed increased xanthine oxidase activity and increased uric acid concentrations in the blood and urine, but 5 ug/day decreased xanthine oxidase activity [Peive, 1968; Gusev, 1969]. Xanthine oxidase activity rises markedly during virus multiplication and it is possible that the enzyme has a controlling effect on the pattern of nucleic acid synthesis in vivo [Bray, 1963].
Bray, R. C., in The Enzymes , eds Boyer, P. D., Hardy, L. and Myrback, K., Academic Press, New York , 2nd Edn., 1963, 7, 533.
Bray, R. C., Knowles, P. F. and Meriwether, L. S., in Magnetic Resonance in Biological Systems , ed. Ehrenberg, A., Malstrom, B. G. and Vanngard, T., Pergamon, 1967, 249.
Bray, R. C. and Swann, J. C., Structure and Bonding, 1972, 11 , 107.
Bray, R. C., in Proceedings of the Climax International Conference on the Chemistry and Uses of Molybdenum, ed. Mitchell, P. C. H., Climax Molybdenum Co. Ltd, London and Ann Arbor , 1973, 216.
Bray, R. C., Bennett, B.m Burke, J. F., Chovnick, A., Doyle, W. A., Howes, B. D., Lowe, D. J., Richards, R. L., Turner, N. A., Ventom, A. and Whittle, J. R-S, Biochem. Soc. Trans. , 1996, 24, 99.
Collison, D., Garner, C. D. and Joule, J. A., Chem. Soc. Rev. , 1996, 25 , 25.
Peive, Ya. V.,(ed.), Biol. Rol Molibdena, Sb. Tr. Simp. 1968 (Publ. 1972), Nauka, Moscow , U.S.S.R., 207, 235.
Underwood, J. E., Trace Elements in Human and Animal Nutrition, Academic Press, London , 2nd Ed., 1962, 100.
Kiermeir, F. and Capellani, K., Naturwiss., 1957, 44 , 69.
Peive, Ya. V.,(ed.), Biol. Rol Molibdena, Sb. Tr. Simp. 1968 (Publ. 1972), Nauka, Moscow , U.S.S.R., 207, 235.
Cox, D. H., Davis, G. K., Shirley, R. L.and Jack, F. H., J. Nutr., 1960, 70 , 63.
Grigoryan, M. S. and Brutyan, A. S., Tr. Erevan. Zootekh.-Vet. Inst., 1968, 29 , 57, 61.
Grigoryan, M. S., Tatevosyan-Markaryan, L. G. and Asmangulyan,M. S. Grigoryan, L. G. Tatevosyan-Markaryan, and Asmangulyan, T. A., Biol. Zh. Arm., 1969, 22 , 102.
The function in health and disease of the Mo-based enzymes sulfite oxidase, xanthine oxidase and aldehyde oxidase has been discussed. Xanthine oxidase, which occurs in the liver of humans and catalyses the formation of uric acid, may be anticarcinogenic through the development of protective systems against oxygen radicals.
Moriwaki, Y., Yamamoto, T., Higashino, K., Distribution And Pathophysiologic Role Of Molybdenum-Containing Enzymes, Histology And Histopathology , 1997,12, 513-524.
According to a molecular modelling study of the interaction of xanthine and hypoxanthine with xanthine oxidase correct positioning of the carbonyl group in the active site cavity is essential for a productive interaction with XO. The dimensions of the active site are mapped starting from the superimposition of the physiological substrates.
Rastelli, G., Costantino, L., Albasini, A., Model of the interaction of substrates and inhibitors with xanthine oxidase, Journal of the American Chemical Society , 1997, 119 , 13, 3007-3016.
Doonan, C. J., Nielsen, D. J., Smith, P. D., White, J. M., George, G. N., and Young, C. G., Models for the molybdenum hydroxylases: Synthesis, characterization and reactivity of cis-oxosulfido-Mo(VI) complexes, Journal of the American Chemical Society, 2006, 128, 305-316.
A model system for molybdenum oxotransferases provides evidence for all biologically realistic intermediates, namely mononuclear Mo-VI, Mo-V and Mo-IV species. Dinucleation to EPR-silent [(Mo2O3)-O-V] species prevailing in homogeneous solution is suppressed by immobilising the active species to a polymeric support by two-point attachment.
Heinze, K. and Fischer, A., Polymer-supported dioxido-Mo-VI complexes as truly functional molybdenum oxotransferase model systems, European Journal of Inorganic Chemistry, 2007, 1020-1026.
Enzyme inhibition by molybdate
Both arsenate and sulfate reduction were inhibited by molybdate. Arsenate-reducing bacterium, strain OREX-4, grew on lactate, with either arsenate or sulfate serving as the electron acceptor
Newman, D.K., Kennedy, E.K., Coates, J.D., Ahmann, D., Ellis, D.J., Lovley, D.R., Morel, F.M.M. ,Dissimilatory arsenate and sulfate reduction in Desulfotomaculum auripigmentum sp. nov. Archives Of Microbiology, 1997, 168 ,.380-388.
Reduction of dimethylsulfoxide was not inhibited by molybdate. In Desulfovibrio desulfuricans strain PA2805, DMSO reduction occurred simultaneously-with sulfate reduction and was not effectively inhibited by molybdate, a specific inhibitor of sulfate reduction.
Jonkers, H.M., Vandermaarel, M.J.E.C., Vangemerden, H., Hansen, T.A. ,Dimethylsulfoxide Reduction By Marine Sulfate-Reducing Bacteria, Fems Microbiology Letters, 1996, 136 , 283-287.
Townsend G.T., Ramanand K., Suflita J.M., Reductive dehalogenation and mineralization of 3-chlorobenzoate in the presence of sulfate by microorganisms from a methanogenic aquifer, Applied And Environmental Microbiology , 1997, 63 , 2785-2791.
Polycyclic aromatic hydrocarbons, naphthalene and phenanthrene, were oxidized to carbon dioxide under sulfate-reducing conditions. Hydrocarbon oxidation was sulfate dependent. Molybdate, a specific inhibitor of sulfate reduction, inhibited hexadecane oxidation.
Coates, J.D., Woodward, J., Allen, J., Philp, P., Lovley, D.R., Anaerobic degradation of polycyclic aromatic hydrocarbons and alkanes in petroleum-contaminated marine harbor sediments, Applied And Environmental Microbiology ,1997, 63 , 3589-3593.
In anaerobic treatment of wastewater molybdate inhibited both sulfidogenesis and benzoate degradation. Molybdate inhibition increased progressively with concentration. Biogranules lost 50% of their methanogenic activity when treating waste water containing 48 mg/l of molybdate. In continuous experiments the molybdate toxicity had a threshold level. Below 0.5-0.8 mM molybdate bacterial activities were unaffected. The molybdate toxicity was not permanent. Biogranules were able gradually to regain their bioactivities once molybdate was removed from the waste water.
Liu, Y., Fang, H.H.P. Effects of molybdate on sulfate reduction and benzoate degradation, Journal Of Environmental Engineering-Asce , 1997,123 ,.949-953.
Liu, S.M., Kuo, C.L. Anaerobic biotransformation of pyridine in estuarine sediments. Chemosphere , 1997, 35 , 2255-2268.
Molybdate through inhibition of sulfate-reducing bacteria inhibits mercury methylation . Group VI anions, MO42-, added to sediment slurries inhibited mercury methylation: tellurate (> 50 nM )> selenate ((> 270 nM )> molybdate (100 mu M)> tungstate (700 mu M). The inhibition of mercury methylation is due to the inhibition of sulfate-reducing bacteria, which are responsible for Hg methylation. In the concentration ranges encountered in most natural aquatic environments, the inhibition of MeHg production by Group VI anions is unlikely.