Uric acid, from xanthine structure

Ureides are a class of cyclic or acyclic nitrogenous organic compounds derived from or structurally related to urea. Representatives of this class which have been found in plants include allantoin, allantoic acid, citrulline, uric acid, hypoxanthine, xanthine, caffeine, hydroxycitruUine, and albizziine. The structures of allantoin (ALN), allantoic acid (ALA), and citrulline (CIT), ureides which occur throughout the plant kingdom, are presented in Fig. 1. These ureido compounds play an essential role in the assimilation, metabolism, transport, and storage of nitrogen in plants. In this chapter, attention will be focused on the occurrence, function, synthesis, and metabolism of these key metabolites. 

Xanthine dehydrogenase that mediates the conversion of hypoxanthine into xanthine and uric acid has been studied extensively since it is readily available from cow s milk. It has also been studied (Leimkiihler et al. 2004) in the anaerobic phototroph Rhodobacter capsulatus, and the crystal structures of both enzymes have been solved. Xanthine dehydrogenase is a complex flavoprotein containing Mo, FAD, and [2Fe-2S] redox centers, and the reactions may be rationalized (Hille and Sprecher 1987) ... 

XOR accelerates the hydroxylation of purines, pyrimidines, pterins and aldehydes [132]. In humans, the enzyme catalyzes the last two steps of purine catabolism the oxidation of hypoxanthine to xanthine and of the latter to uric acid. An unusual property of this, but not aU XOR enzymes [133], is its interconversion between xanthine dehydrogenase and xanthine oxidase activities which implies a switch between NAD" and molecular oxygen being used as the final electron acceptor [134]. Structural studies suggest that this switch, that can be irreversibly induced by proteolysis [135], results from conformational changes that lead to both restricted access to the NAD cofactor to its binding site and changes in the redox potential of the FAD cofactor [136],... 

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