Molybdoenzyme

Molybdenum trisulfide, 3, 1431 Molybdoarsenates, 3, 1041 non-Keggin, 3, 1042 Molybdoarsenic acid liquid-liquid extraction, 1, 548 Molybdoborates, 3,1042 Molybdocerophosphoric add determination, 1, 548 Molybdoenzymes, 3, 1352 molybdenum site, 3,1391 Molybdogermanates, 3,1038 Molybdogermanic acid liquid-liquid extraction, 1, 548 Molybdoniobic acid... 

Wan J, TK Tokunaga, E Brodie, Z Wang, Z Zheng, D Herman, TC Hazen, MK Firestone, SR Sutton (2005) Reoxidation of bioreduced uranium under reducing conditions. Environ Sci Technol 39 6162-6169. Weiner JH, DP Macisaac, RE Bishop, PT Bilous (1988) Purification and properties of Escherichia coli dimethyl sulfoxide reductase, an iron-sulfur molybdoenzyme with broad substrate specificity J Bacterial 170 1505-1510. 

Nagel M, JR Andreesen (1990) Purification and characterization of the molybdoenzymes nicotinate dehydrogenase and 6-hydroxynicotinate dehydrogenase from Bacillus niacini. Arch Microbiol 154 605-613. 

Self, W. T., Grunden, A. M., Hasona, A., and Shanmugam, K. T. (1999). Transcriptional regulation of molybdoenzyme synthesis in Escherichia coli in response to molybdenum ModE-molybdate, a repressor of the modABCD (molybdate transport) operon is a secondary transcriptional activator for the hyc and nar operons. Microbiology 145, 41-55. 

Figure 1 Overview of specific use of seienium in bioiogical systems. Selenium can be incorporated into macromolecules in at least three separate pathways. From the reduced form of selenide, selenium is activated to selenophosphate by the action of the enzyme selenophosphate synthetase (SPS or SelD). This activated form is then used as a substrate for pathway-specific enzymes that lead to (1) insertion as selenocysteine into proteins during translation (selenoproteins), (2) incorporation into tRNA molecules as mnm Se U or Se U, and (3) insertion into a unique class of molybdoenzymes as a labile, but required, cofactor. The need for activation to selenophosphate has been demonstrated in all cases at the genetic and biochemical level, with the exception of the labile selenoenzymes, where activation of selenium has only been proposed based on proximity of genes within an operon encoding SPS and a molybdoenzyme. ...

The formal transfer of an oxygen atom is one way of describing the function of the Mo site in molybdoenzymes.5 The formation of dinuclear reduction products is a complication that causes difficulty in trying to model the mononuclear site.5,165 This difficulty can be overcome by the use of sterically demanding ligands that prevent the formation of the dinuclear complex,73 79,125 176 177 For example, the cycle shown in Scheme 3 can be effected without dimerization. Further, in this case DMSO and the enzyme substrate biotin sulfoxide, can serve as the oxo donor to form the Movl dioxo complex during the catalytic cycle.79,177 The Movl complex involved is discussed structurally above (Figure 7). 

Molybdoenzymes other than the nitrogenases are usually termed oxomolybdoenzymes. This prefix relates to the nature of the catalysis effected, i.e. the net effect of the conversion (xanthine to uric acid, sulfite to sulfate, nitrate to nitrite, or aldehyde to carboxylate) corresponds to the transfer of one oxygen atom to or from the substrate. Furthermore, molybdenum X-edge EXAFS studies have established that this metal is coordinated to one or more terminal oxo groups in each enzyme studied by this technique.204... 

Two other families of molybdoenzymes are the sulfite oxidase family6463 13 and the dimethylsulfoxide reductase family.632 641 Nitrogenase (Chapter 24) constitutes a fourth family. Sulfite oxidase (Eq. 16-60) is an essential human liver enzyme (see also Chapter... 

The recognition that the Mo in the molybdoproteins exists in organic cofactor forms came from studies of mutants of Aspergillus and Neurospora.650 In 1964, Pateman and associates discovered mutants that lacked both nitrate reductase and xanthine dehydrogenase. Later, it was shown that acid-treated molybdoenzymes released a material that would restore activity to the inactived nitrate reductase from the mutant organisms. This new coenzyme, a phosphate ester of molybdopterin (Fig. 15-17), was characterized by Rajagopalan and coworkers.650 651 A more complex form of the coenzyme, molybdopterin cytosine dinucleotide... 

Figure 16-32 Structures surrounding molybdenum in three families of molybdoenzymes. See Hille.632...

Although several of the reactions catalyzed by molybdoenzymes are classified as dehydrogenases, all of them except nitrogenase involve H20 as either a reactant or a product. The EXAFS spectra suggest that the Mo(VI)02 unit is converted to Mo(IV)0 during reaction with a substrate Sub (Eq. 16-65, step a). Reaction of the Mo(IV)0 with water (step b) completes the catalysis. 

A,TV-Dimethyl-A -phenylthiourea has been shown to coordinate to Rh111 as an N—S bidentate involving four-membered chelate ring formation.154 N-Substituted thioamides also may bond in this manner.155 156 l-Amidino-2-thioureas (44) may behave either as N—S or as N—N bidentates, with this donor choice being dependent mainly on pH and the nature of the metal ion.157 As N—S donors they are known to stabilize lower oxidation states.158 As part of a study on Mo—S-containing complexes as models for redox-active molybdoenzymes, Dilworth et al. have shown that some p-(substituted)phenylhydrazines may coordinate as N—S bidentates in three different ways to one metal atom.159 The three diazenido, diazene and hydrazonido forms vary in their degree of deprotonation and therefore their anionic nature. 

A molybdenum cofactor has been isolated from Proteus mirabilis 047, and has a molecular weight greater than 1000. The molybdoenzymes of E. coli, in addition to the formate dehydrogenases described above and the nitrate reductase (Section 62.1.9.6), also include the membrane-bound trimethylamine oxidase1044 and the soluble biotin sulfoxide reductase.1045... 

The results described in the previous sections have demonstrated the versatility of molybdenum as a reaction centre in biology. The molybdenum cofactor stands at the centre of an important network of cellular functions that are all catalyzed by molybdoenzymes. The similarities and differences in these reactions are of great interest, and relate clearly to the detailed arrangement of terminal sulfur and oxygen atoms. However, the unexpected results found for carbon monoxide oxidase and the requirement for selenium in some cases indicate that other factors are also important. 

An organism such as E. coli has at least five molybdoenzymes. It will be of great interest to look at the synthesis, availability and cellular distribution of the molybdenum cofactor and its relationship to the function of these molybdoenzymes at different stages of the cell cycle. The study of chlorate-resistant1057 and nitrate reductase-deficient1058 mutants of E. coli, which are... 

E. coli uses nitrate as a terminal electron acceptor through a respiratory, dissimilatory nitrate reductase whose synthesis is induced when nitrate is provided, and which is repressed by oxygen. Nitrate reductase is discussed with other molybdoenzymes in Section 62.1.9, and catalyzes the reduction of nitrate to nitrite. The enzyme is isolated from the cytoplasmic membrane of E. coli, and contains three subunits (a, j8 and y) although the y-subunit may be absent in some preparations. The -y-subunit is a b-type cytochrome, and the a-subunit is reported to be the catalytic subunit. The enzyme contains a number of iron-sulfur clusters, including a HiPIP and at least two ferredoxins.1054,1437... 

This has been isolated from a range of nitrogen-fixing organisms, and appears to be identical in all cases except when an inactive form is produced by mutants deficient in certain of the nif genes. It differs from the molybdenum cofactor found in a number of molybdoenzymes (Section 62.1.9). 

Both assimilatory and dissimilatory nitrate reductases are molybdoenzymes, which bind nitrate at the molybdenum. EXAFS studies1050 have shown that there are structural differences between the assimilatory nitrate reductase from Chlorella vulgaris and the dissimilatory enzyme from E. coli. The Chlorella enzyme strongly resembles sulfite oxidase1050,1053 and shuttles between mon-and di-oxo forms, suggesting an oxo-transfer mechanism for reduction of nitrate. This does not appear to be the case for the E. coli enzyme, for which an oxo-transfer mechanism seems to be unlikely. The E. coli enzyme probably involves an electron transfer and protonation mechanism for the reduction of nitrate.1056 It is noteworthy that the EXAFS study on the E. coli nitrate reductase showed a long-distance interaction with what could be an electron-transfer subunit. 

With the exception of tetrathionate reductase [78] and poly sulfide reductase [79], the reducing molybdoenzymes are involved in the removal of an oxygen atom from either an arsenic, sulfur-, selenium-, nitrogen-, carbon-, or chlorine-centered... 

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