Molybdoenzymes reaction

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. 

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. 

Caradonna, J. P., Reddy, P. R., and Holm, R. H., 1988, Kinetics, mechanisms, and catalysis of oxygen atom transfer reactions of S Oxide and pyridine N Oxide substrates with molybdenum (IV,VI) complexes relevance to molybdoenzymes, J. Am. Chem. Soc. 110 2139n2144. 

The control that the ene-dithiolate ligand has upon the first ionization energy of these complexes illustrates the importance of this ligation to the reactivity found for molybdoenzymes. The ene-dithiolate ligand acts as an electronic buffer , effectively dampening the harsh electronic changes that would otherwise be expected to take place with changes in the metal formal oxidation states and atom transfer reactions at the active site in these enzymes. 

The biosynthesis of Moco starts with GTP, which is converted in a GTP cyclohydrolase-independent reaction to the first stable intermediate precursor Z, a cyclic pyranopterin monophosphate. Precursor Z is converted into MPT by the insertion of two sulfur atoms, forming the dithiolene group which coordinates the molybdenum atom. After insertion of molybdate, Moco is formed that can be further modified and is inserted into various molybdoenzymes. ... 

Singular value decomposition (SVD) analysis on spectro-photometric data obtained from an oxygen atom transfer (OAT) reaction involving a molybdoenzyme model system is reported. Specifically, the rate of solvolysis reaction of a phosphoryl intermediate complex has been compared with independent measurements. The SVD derived reaction rates are consistent with other measurements. This generalized approach is applicable in examining other bioinorganic reactions, and data processing. 

Here we described the application of SVD toward understanding the OAT reactivity of a well-defined molybdoenzyme model system. The SVD algorithm produces rate constants that are comparable with the single wavelength measurement or NMR methods, provided that each step in the reaction occurs on a similar time scale. The algorithm does exceptionally well at the prediction of the pure component spectra and detection of key absorption bands. 

As an essential constituent of several important molybdoenzymes. Mo has specific roles in the metabolism of N and S in crop plants. Further studies are needed to determine the detailed structures of these molybdoenzymes and the involvement of different valence states of Mo in their reaction mechanisms. Furthermore, Mo has a wide range of nonspecific effects involving the regulation of many enzymes other than molybdoenzymes, carbohydrate metabolism, reproductive physiology, anion balance, root exudation, plant water relations, and disease incidence in plants. In the future, attempts must be made to understand the precise roles of Mo in DNA stability, protein synthesis, carbohydrate metabolism, plant stress, and disease resistance. Research is also needed on the effects of Mo nutrition on the quality of food grains and vegetable and fruit crops. 

It was more than a decade later that Holm (1986) put such reactivity into the context of molybdoenzymes. He exploited PPhj as a model substrate and its oxo-acceptor nature for a reaction with the synthesized complex [Mo02(NS)2] demonstrating, hence, that a synthetic complex may respond to the presence of a suitable substrate by an oxo-transfer reaction similar to the molybdoenzymes. This reactivity was extended to the backward reaction where DMSO was used as the second substrate to regenerate the oxidized [Mo02(-NS)2] from the reduced [MoO(-NS)2] species (Figure 3.5). 

Figure 3.5 The first demonstration of two-directional oxo-transfer reaction in reference to molybdoenzymes.

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