The Xanthine Oxidase Family

FIGURE 17.3 (a) Ribbon diagram of the XDH subunit with its three major domains and two connecting loops. From N- to C-terminus, the 

The mechanism by which xanthine oxidase brings about hydroxylation must take into account the fact that water, rather than O2 is the ultimate source of oxygen incorporated into the product. In a single-turnover experiment using H2 0, the radioisotope is not incorporated into the product, whereas, when the enzyme from that experiment is incubated with substrate in unlabelled water, 8- 0-uric acid is produced. It follows that 

FIGURE 17.4 The topology of xanthine oxidoreductase. Schematic representation of the homologies that exist among the molybdenum 

FIGURE 17.5 The active site structure of xanthine oxidoreductase. (A) The LMoOS(OU) core, with an approximately square-pyramidal coordination geometry, with the Mo=0 group in the apical position. L represents the bidentate enedithiolate ligand contributed by the pterin cofactor, and lies in the equatorial plane along with the Mo=S and Mo—OH groups. (From Hille, 2005. Copyright 2005 with permission from Elsevier.) 

FIGURE 17.6 A reaction mechanism for the aldehyde oxidase group of molybdenum hydroxylases. As with hydroxylation of heterocycles, the conversion of aldehydes to the corresponding carboxylic acids has been proposed to proceed via bases-assisted nucleophilic attack of the Mo—OH on the substrate carbonyl, with concomitant hydride transfer to the Mo=S. (From Hille, 2005. Copyright 2005 with permission from Elsevier.) 

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Romao MJ, Huber R (1998) Structure and Function of the Xanthine-Oxidase Family of Molybdenum Enzymes. 90 69-96 Rosenzweig A, see Penneman RA (1973) 13 1-52... 

The three known crystal structures of molybdopterin-containing enzymes are from members of the first two families the aldehyde oxido-reductase from D. gigas (MOP) belongs to the xanthine oxidase family (199, 200), whereas the DMSO reductases from Rhodobacter (R.) cap-sulatus (201) and from/ , sphaeroides (202) and the formate dehydrogenase from E. coli (203) are all members of the second family of enzymes. There is a preliminary report of the X-ray structure for enzymes of the sulfite oxidase family (204). 

The aldehyde oxidoreductase from Desulfovibrio gigas shows 52% sequence identity with xanthine oxidase (199, 212) and is, so far, the single representative of the xanthine oxidase family. The 3D structure of MOP was analyzed at 1.8 A resolution in several states oxidized, reduced, desulfo and sulfo forms, and alcohol-bound (200), which has allowed more precise definition of the metal coordination site and contributed to the understanding of its role in catalysis. The overall structure, composed of a single polypeptide of 907 amino acid residues, is organized into four domains two N-terminus smaller domains, which bind the two types of [2Fe-2S] centers and two much larger domains, which harbor the molybdopterin cofactor, deeply buried in the molecule (Fig. 10). The pterin cofactor is present as a cytosine dinucleotide (MCD) and is 15 A away from the molecular surface,... 

Huber R, Hof P, Duarte RO, et al. A structure-based catalytic mechanism for the xanthine oxidase family of molybdenum enzymes. Proc Natl Acad Sci USA 1996 93( 17) 8846—8851. 

Brondino, C.D., Romao, M.J., Moura, I. and Moura, J J.G. (2006) Molybdenum and tungsten enzymes the xanthine oxidase family, Curr. Opin. Chem. Biol., 10, 109-114. 

It has long been a synthetic challenge to prepare mono(oxido) mono(sulfido) molybdenum centers that are analogous to the Mo(VI) state of enzymes in the xanthine oxidase family. Although studies in this area have met with limited success, it seems possible that the sulfido linkage of these centers could be stabilized by the 1,2-enedithiolate or by a molybdenum-bound thiolate ligand. 

Members of the xanthine oxidase family have MPT coordinated to a fac-MoOY(H20) (Y = O or S) center with no amino acid residue bound. 

A wide range of soluble redox enzymes contain one or more intrinsic [2Fe-2S]2+ +, [3Fe-4S]+ , or [4Fe S]2+ + clusters that function in electron transport chains to transfer electrons to or from nonheme Fe, Moco/Wco, corrinoid, flavin, thiamine pyrophosphate (TPP), Fe S cluster containing, or NiFe active sites. Many have been structurally and spectroscopically characterized and only a few of the most recent examples of each type are summarized here. Dioxygenases that function in the dihydroxylation of aromatics such as benzene, toluene, benzoate, naphthalene, and phthalate contain a Rieske-type [2Fe-2S] + + cluster that serves as the immediate electron donor to the monomeric nonheme Fe active site see Iron Proteins with Mononuclear Active Sites). The xanthine oxidase family of molybdoenzymes see Molybdenum MPT-containing Enzymes) contain two [2Fe-2S] + + clusters that mediate electron transfer between the Moco active site and the Other soluble molybdoen-... 

D. J. Lowe, Enzymes of the Xanthine Oxidase Family The Role of Molybdenum, in Metal Ions in Biological Systems , eds. A. Sigel and H. Sigel, Marcel Dekker, New York, 2002, Vol. 39, p. 455. 

Members of the xanthine oxidase family (i.e., the true hydroxylases) have molybdenum centers which consist of a single cofactor dithiolene ligand coordinated to a fac-MoOS-(H20) unit. Members of the sulfite oxidase family are likely to possess a single cofactor dithiolene coordinated to a cis M0O2 unit (additional coordination positions may be taken up by water and/or a cysteine residue that is conserved within the... 

Xanthine Oxidase Family Members of the xanthine oxidase family generally catalyze hydroxylation reactions of the following type ... 

The Xanthine Oxidase Family (LMoOS-Possessing Enzymes)... 

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