Xanthine oxidoreductase family

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,... 

Xanthine Oxidase Family Xanthine oxidoreductase Cow s milk 2, 290 MoOS(OH)(MPT) xanthine —uric acid IFIQ... 

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... 

The most intensively studied enzyme from this family is xanthine oxidoreductase, comprising the pairing of xanthine oxidase (XO) and xanthine dehydrogenase (XDH). These enzymes oxidize the heterocycles hypoxanthine... 

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-... 

Canne and co-workers have presented EPR studies of three prokaryotic enzymes of the xanthine oxidase family, namely quinoline 2-oxidoreductase, quinaldine 4-oxidase, and isoquinoline l-oxidoreductaseJ In quinoline 2-oxidoreductase a neutral flavin radical was observed, while in quinaldine 4-oxidase an anionic radical was detected. The rapid Mo(V) signal was observed in all three enzymes with only small differences in magnetic parameters. From spectra simulations of Mo (/ = 5/2) substituted quinoline 2-oxidoreductase, a deviation of 25° between the maximal g and Mo-hfc tensor component was derived. The Mo(V) species was detected in small amounts upon reduction with substrates in quinoline 2-oxidoreductase and quinaldine 4-oxidase, but showed a different kinetic behaviour with an intense EPR signal in isoquinoline 1-oxidoreductase. The two [2Fe-2S] clusters produced different EPR signals in all three enzymes and, in isoquinoline 1-oxidoreductase, revealed a dipolar interaction, from which a maximum distance of 15 A was estimated. 

Upon purification of the XDH from C. purinolyticum, a separate Se-labeled peak appeared eluting from a DEAE sepharose column. This second peak also appeared to contain a flavin based on UV-visible spectrum. This peak did not use xanthine as a substrate for the reduction of artificial electron acceptors (2,6 dichlor-oindophenol, DCIP), and based on this altered specificity this fraction was further studied. Subsequent purification and analysis showed the enzyme complex consisted of four subunits, and contained molybdenum, iron selenium, and FAD. The most unique property of this enzyme lies in its substrate specificity. Purine, hypoxanthine (6-OH purine), and 2-OH purine were all found to serve as reductants in the presence of DCIP, yet xanthine was not a substrate at any concentration tested. The enzyme was named purine hydroxylase to differentiate it from similar enzymes that use xanthine as a substrate. To date, this is the only enzyme in the molybdenum hydroxylase family (including aldehyde oxidoreductases) that does not hydroxylate the 8-position of the purine ring. This unique substrate specificity, coupled with the studies of Andreesen on purine fermentation pathways, suggests that xanthine is the key intermediate that is broken down in a selenium-dependent purine fermentation pathway. ... 

Figure 16. Consensus oxidized active-site structures of the xanthine oxidase (XO), sulfite oxidase (SO), and DMSO reductase (DMSOR), and aldehyde oxidoreductase (AOR) families of mononuclear molybdenum and tungsten enzymes and the structure of the common ppd cofactor (41, 42). The question mark in the AOR structure indicates uncertainty in the presence of a coordinated water molecule.

Of the three members of the XO family, xanthine oxidase/dehydrogenases and aldehyde oxidoreductases catalyse the hydroxylation of carbon centres, whereas the third family member, the CO dehydrogenase from Oligotropha carboxidovorans, converts CO to CO2. This latter enzyme is a structural exception, in that it has a dinuclear heterometal [CUSM0O2H] cluster, with the sulfido ligand coordinated by the Cu centre (for the structure of this, see Chapter 4, Figure 4.12). 

Figure 11. Comparison of the EPR powder spectra (20 K) of the two reduced [2Fe2S] clusters FeSI and FeSII of various molybdenum hydroxylases of the xanthine oxidase (XO) family showing a pronounced variation of EPR parameters of the centers. The principal g-tensor components of both clusters are indicated. The horizontal arrows mark appearance of dipolar interaction between the clusters. The dashed rectangle covers the field range of sizeable contributions from paramagnetic Mo(V) species. Qor quinoline oxidoreductase Qox qui-naldine oxidase lor isoquinoline oxidoreductase Mop aldehyde oxidoreductase.

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