Oxidative reactions molybdenum hydroxylases

In addition to these classical aromatic ring hydroxylations, many nitrogen heterocycles are substrates for molybdenum-containing enzymes, such as xanthine oxidase and aldehyde oxidase, which are present in the hepatic cytosolic fractions from various animal species. The molybdenum hydroxylases (B-75MI10902) catalyze the oxidation of electron-deficient carbons in aromatic nitrogen heterocycles. The reactions catalyzed by these enzymes are generally represented by equations (2) and (3). 

Molybdenum hydroxylases (i.e., AO and XO) are flavoproteins that contain in addition to a FAD, a pterine cofactor coordinated to a molybdenum atom, and an iron sulfur center for their catalytic activity. They catalyze the two-electron oxidation of substrates with transfer to molecular oxygen to produce H2O2, and insert an atom of oxygen from water into a wide range of N-hctcrocycies and aldehydes via two-electron redox reaction as shown in equation 1.4 ... 

In this section are described the important chemical features of those substrates which are oxidized by the molybdenum hydroxylases. Although these enzymes, particularly aldehyde oxidase, also catalyse numerous reductive reactions under anaerobic conditions in vitro, it has not yet been established whether they occur under physiological conditions and there are as yet insufficient examples of any one reduction reaction to permit any conclusions regarding the structure of substrates. Thus, such reactions will not be discussed here (see [11] and references therein). Properties of those inhibitors which bind at the Mo centre and are also substrate analogues will also be included. However, the interaction of inhibitors such as cyanide and arsenite with the molybdenum hydroxylases and the mechanism of action of the specific xanthine oxidase inhibitor, allo-purinol, have been comprehensively described elsewhere [8, 12, 14, 157]. 

In spite of the close structural relationship of the molybdenum hydroxylases, including a tendency for hydrophobic substrate/enzyme interaction, there is a very significant difference in the substrate specificity of the two enzymes. Not only is there considerable variation in the affinities for substrates and inhibitors, but there is often a difference in the position of oxidative attack. As both enzymes catalyse apparently similar nucleophilic reactions, this difference cannot be explained solely from electronic considerations and is probably due, to a great extent, to the differential response of each enzyme to steric factors. 

Hypoxanthine is oxidized at carbon 2 by both molybdenum hydroxylases, although xanthine oxidase is much more effective as a catalyst in this reaction [ 10]. A methyl substituent in this position prevents oxidation by either enzyme. Introduction of A-methyl substituents into the hypoxanthine nucleus produces dramatic effects on enzymic oxidation rates and also gives some insight into the productive modes of binding to each enzyme. Thus, it has been proposed that hypoxanthine tautomerizes in the xanthine oxidase-substrate complex to the 3-NH-form with a simultaneous shift of the NH-group in the imidazole ring from position 9 to 7 [ 198,200]. In support of this hypothesis, when tautomerism in the imidazole ring is prevented by substitution at N-7 or N-9, such compounds are almost refractory to oxidation (see Table 3.9)... 

Oxomolybdenum centers are present in a series of enzymes known as molybdenum hydroxylases [4] which catalyze oxygen atom transfer reactions of a variety of small molecules. Examples include the oxidation of xanthine to uric acid and the reduction of N03 to N02"- The reactions are equivalent to two-electron transfers, and the molybdenum atom cycles between oxidation states VI and IV during these processes. Molybdenum (VI) typically contains two multiply bonded oxo (or sulfido) groups, and molybdenum (IV) contains one. Therefore, it is believed that Mo02 and MoO +centers intercovert by coupled transfer of protons and electrons over a narrow potential interval during turnover of these enzymes. However, other explanations are possible. 

As indicated in Fig. 25-18, free adenine released from catabolism of nucleic acids can be deaminated hydrolytically to hypoxanthine, and guanine can be deaminated to xanthine.328 The molybdenum-containing xanthine oxidase (Chapter 16) oxidizes hypoxanthine to xanthine and the latter on to uric acid. Some Clostridia convert purine or hypoxanthine to xanthine by the action of a selenium-containing purine hydroxylase.3283 Another reaction of xanthine occurring in some plants is conversion to the trimethylated derivative caffeine. 328b One of the physiological effects of caffeine in animals is inhibition of pyrimidine synthesis.329 However, the effect most sought by coffee drinkers may be an increase in blood pressure caused by occupancy of adenosine receptors by caffeine.330... 

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