Molybdenum centers

These enzymes may contain other redox-active sites (iron-sulfur centers, hemes, and/or flavins), either in distinct domains of a single polypeptide or bound in separate subunits. These additional cofactors perform electron transfer from the molybdenum center to an external electron acceptor/donor. 

In the first family, the metal is coordinated by one molecule of the pterin cofactor, while in the second, it is coordinated to two pterin molecules (both in the guanine dinucleotide form, with the two dinucleotides extending from the active site in opposite directions). Some enzymes also contain FejSj clusters (one or more), which do not seem to be directly linked to the Mo centers. The molybdenum hydroxylases invariably possess redox-active sites in addition to the molybdenum center and are found with two basic types of polypeptide architecture. The enzymes metabolizing quinoline-related compounds, and derivatives of nicotinic acid form a separate groups, in which each of the redox active centers are found in separate subunits. Those enzymes possessing flavin subunits are organized as a2jS2A2, with a pair of 2Fe-2S centers in the (3 subunit, the flavin in the (3 subunit, and the molybdenum in the y subunit. 

XOD is one of the most complex flavoproteins and is composed of two identical and catalytically independent subunits each subunit contains one molybdenium center, two iron sulfur centers, and flavine adenine dinucleotide. The enzyme activity is due to a complicated interaction of FAD, molybdenium, iron, and labile sulfur moieties at or near the active site [260], It can be used to detect xanthine and hypoxanthine by immobilizing xanthine oxidase on a glassy carbon paste electrode [261], The elements are based on the chronoamperometric monitoring of the current that occurs due to the oxidation of the hydrogen peroxide which liberates during the enzymatic reaction. The biosensor showed linear dependence in the concentration range between 5.0 X 10 7 and 4.0 X 10-5M for xanthine and 2.0 X 10 5 and 8.0 X 10 5M for hypoxanthine, respectively. The detection limit values were estimated as 1.0 X 10 7 M for xanthine and 5.3 X 10-6M for hypoxanthine, respectively. Li used DNA to embed xanthine oxidase and obtained the electrochemical response of FAD and molybdenum center of xanthine oxidase [262], Moreover, the enzyme keeps its native catalytic activity to hypoxanthine in the DNA film. So the biosensor for hypoxanthine can be based on... 

Fig. 9. X-ray structure of [Mo(N2)2(dppe)(pyNP2)] in which the fac-coordinating pyNP2 ligand is bonded only in a bidentate fashion to the molybdenum center (45).

Sulfite oxidase is a dimetallic enzyme that mediates the two-electron oxidation of sulfite by the one-electron reduction of cytochrome c. This reaction is physiologically essential as the terminal step in oxidative degradation of sulfur compounds. The enzyme contains a heme cofactor in the 10 kDa N-terminal domain and a molybdenum center in the 42 kDa C-terminal domain. The catalytic cycle is depicted in Fig. 9. 

Structural basis of biological nitrogen fixation, Phil. Trans. R. Soc., 363, 971-984. Schrock, R.R. (2005) Catalytic reduction of dinitrogen to ammonia at a single molybdenum center,... 

NAH is composed of four subunits (SDS-PAGE) and contains a molybdenum cofactor (Dilworth 1983). Analysis of the electron paramagnetic resonance (EPR) spectra of the molybdenum center of NAH revealed a coordination of molybdenum to selenium (Gladyshev et al. 1994b). Apparently NAH is much like other selenium-dependent molybdenum hydroxylases such as XDH from C. barkeri and other purinolytic Clostridia. Whether or not the selenium is present as a ligand of molybdenum or is coordinated to molybdenum while being bound to another molecule (e.g., sulfur of cysteine) is still not known. The nature of the selenium cofactor and the mechanism of its incorporation into NAH are most likely similar to XDH and thus also require more study. 

Coupled electron-proton transfer, reactions of oxo-molybdenum centers, 40 57-59 Coupled-ring nitrides molecular structure of, 15 401-6 preparation of, 15 400 properties and reactions of, 15 402-403 purification of, 15 400-401 Covalent bonding... 

Electronic absorption spectroscopy charge transfer transitions, 19 71 d-d transitions, 19 70, 71 flavocytochrome b, 36 269-271 intraligand transitions, 19 71-80 of organometallics, 19 69-80 Electronic coupling, between donor and acceptor wave functions, 41 278 Electronic nuclear double resonance spectroscopy, molybdenum center probes, 40 13... 

M-F coupling constants in, 4 245 Mo, molybdenum center probes, 40 16 multinuclear, tetracyano complexes containing oxo or nitrido ligands, 40 303-304... 

One-dimensional electrical conductors, platinum complexes, 26 235-268 band theory, 26 237-241 charge density waves, 26 239-240 Kahn-Teller effect, 26 239-240 and superconductivity, 26 240-241 One-electron reactions, oxo-molybdenum centers, 40 56-57... 

Oxo-molybdenum centers, 40 48-61 coupled electron-proton transfer reactions, 40 57-59... 

Similarly, catechin polymers formed upon horseradish peroxidase-catalyzed oxidation of catechin or polycondensation of catechin with aldehydes prove much more efficient than catechin (at identical monomer concentration) at inhibiting XO and superoxide formation. A more detailed investigation with the catechin-acetaldehyde polycondensate (which is expected to form in wine because of the microbial oxidation of ethanol to acetaldehyde) shows that inhibition is noncompetitive to xanthine and likely occurs via binding to the FAD or Fe/S redox centers involved in electron transfers from the reduced molybdenum center to dioxygen with simultaneous production of superoxide. 

A similar effect conceivably accounts for the higher Pi value (—0.74 V) (weaker net electron-donation) of the cyanide ligand estimated [15] at trans- FeH(dppe)2 with the strong donor trans-hydride, in comparison with that (—l.OV) [10] obtained at Cr(CO)5 and also proposed at trans- MoL(dppe)2) (L = CO, N2) with the strong net electron-acceptor L ligand, in spite of the lower electron-richness of the former Fe site Eg = 1.04 V) relatively to the latter (Eg = —0.11 or —0.13 V) molybdenum centers. 

In the earlier examples, the binuclear complexes with a M02S3 core clearly cannot be seen as modeling the metal sites in FeMo—co since it is known to contain a single molybdenum center [74]. However, what is suggested by these studies is that several steps (if not all) of the reduction of dinitrogen to ammonia by Mo-nitrogenase could take place at a... 

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