Xanthine Oxidoreductase XOR

Xanthine oxidoreductase (XOR) is a molybdenum-containing complex homodimeric 300-kDa cytosolic enzyme. Each subunit contains a molybdopterin cofactor, two nonidentical iron-sulfur centers, and FAD (89). The enzyme has an important physiologic role in the oxidative metabolism of purines, e.g., it catalyzes the sequence of reactions that convert hypoxanthine to xanthine then to uric acid (Fig. 4.36). 

Notable exceptions occur in the formate dehydrogenases and acetylene hydratase, and possibly in bacterial YedY " and the mitochondrial ami-doxime reducing component (mARC) enzyme. A markedly different type of reaction is catalyzed by members of the xanthine oxidoreductase (XOR) family of enzymes and involves the formal insertion of an oxygen atom into a substrate C-H bond. Thus, these enzymes are often categorized as hydroxylases.However, the reactivity patterns of the XORs differ substantially... 

Xanthine dehydrogenase, XDH [E.C. 1.1.1.204], and the closely related isoenzyme xanthine oxidoreductase, XOR [E.C. 1.1.3.22], are molybdopterin oxidoreductive enzymes. The enzymes catalyze oxidation of a C-substrate atom by two electrons. In human body the enzymes convert the nucleic acid metabolite hypoxanthine into xanthine and xanthine into uric acid. Fig. 10.6 The reductive rate constant of XDH,... 

In terms of the reaction catalyzed, molybdopterin-containing enzymes can be divided in two groups those that mediate oxygen atom transfer, such as dimethyl sulfoxide (DMSO) reductase and sulfite oxidase (SO), and those that catalyze hydroxylation reactions of aromatic heterocyclic compounds and aldehydes [116], for instance xanthine oxidoreductase (XOR) and aldehyde oxidoreductase (AOR). However, this functional classification does not coincide with structural properties that suggest that the enzymes should be grouped into five families, whose most representative members are (1) DMSO reductase (2) XOR (3) SO (4) aldehyde-... 

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