Xanthine oxidase desulfo

Xanthine oxidase Xanthine oxidase Xanthine oxidase Xanthine oxidase (desulfo)... 

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,... 

It is usually believed that NO inhibits enzymes by reacting with heme or nonheme iron or copper or via the S-nitrosilation or oxidation of sulfhydryl groups, although precise mechanisms are not always evident. By the use of ESR spectroscopy, Ichimori et al. [76] has showed that NO reacts with the sulfur atom coordinated to the xanthine oxidase molybdenum center, converting xanthine oxidase into a desulfo-type enzyme. Similarly, Sommer et al. [79] proposed that nitric oxide and superoxide inhibited calcineurin, one of the major serine and threonine phosphatases, by oxidation of metal ions or thiols. 

Fe2S2] clusters are part of the molybdenum containing hydroxylases. Typically, apart from molybdenum and two EPR-distinct iron-sulfur centres there can be FAD as additional cofactor. In Chlostridium purinolyticum a selenium-dependent purine hydroxylase has been characterized as molybdenum hydroxylase. The EPR of the respective desulfo molybdenum (V) signal indicated that the Mo-ligands should differ from those of the well known mammalian corollary xanthine oxidase.197 For the bacterial molybdenum hydroxylase quinoline oxidoreductase from Pseudomonas putida an expression system was developed in order to be able to construct protein mutants for detailed analysis. EPR was used to control the correct insertion of the cofactors, specifically of the two [Fe2S2] clusters.198... 

EXAFS studies120 have shown that the Mo02+ core is readily identifiable in a variety of known compounds. These have been used120 to calibrate the EXAFS technique which was subsequently used to identify this core in sulfite oxidase and in desulfo xanthine oxidase.82... 

Figure 16-31 (A) Structure of molybdopterin cytosine dinucleotide complexed with an atom of molybdenum. (B) Stereoscopic ribbon drawing of the structure of one subunit of the xanthine oxidase-related aldehyde oxidoreductase from Desulfo-vibrio gigas. Each 907-residue subunit of the homodimeric protein contains two Fe2S2 clusters visible at the top and the molybdenum-molybdopterin coenzyme buried in the center. (C) Alpha-carbon plot of portions of the protein surrounding the molybdenum-molybdopterin cytosine dinucleotide and (at the top) the two plant-ferredoxin-like Fe2S2 clusters. Each of these is held by a separate structural domain of the protein. Two additional domains bind the molybdopterin coenzyme and there is also an intermediate connecting domain. In xanthine oxidase the latter presumably has the FAD binding site which is lacking in the D. gigas enzyme. From Romao et al.633 Courtesy of R. Huber.

The first hint of an essential role of molybdenum in metabolism came from the discovery that animals raised on a diet deficient in molybdenum had decreased liver xanthine oxidase activity. There is no evidence that xanthine oxidase is essential for all life, but a human genetic deficiency of sulfite oxidase or of its molybdopterin coenzyme can be lethal.646,646a,b The conversion of molybdate into the molybdopterin cofactor in E. coli depends upon at least five genes.677 In Drosophila the addition of the cyanolyzable sulfur (Eq. 16-64) is the final step in formation of xanthine dehydrogenase.678 It is of interest that sulfur (S°) can be transferred from rhodanese (see Eq. 24-45), or from a related mercaptopyruvate sulfurtransferase679 into the desulfo form of xanthine oxidase to generate an active enzyme.680... 

Figure 15 The coordination geometry around molybdenum as suggested from the EXAFS of xanthine oxidase (mammalian) (a) oxidized form (b) reduced form (c) desulfo oxidized form (d) desulfo reduced form [133,135,146-151],...

As with xanthine oxidase, the sulfido ligand of the active form of aldehyde oxidoreductase is readily replaced by an oxido ligand to yield a cofactor with a structure that resembles that of oxidized sulfite oxidase and assimilatoiy nitrate reductase. Both x-ray and EXAFS data are available for the bis(oxido) form, and, with the exception of the oxido replaced sulfido ligand, few changes are obvious in the overall structure of the oxidized form of the desulfo cofactor. Upon reduction of the enzyme the oxido ligand is presumably reduced to hydroxido, an observation that is supported by EPR data for the Mov state. 

The reduction of (tpb )Movl(0)2SPh has been shown first to generate [(tpb )Mo-v(0)2SPh]-, which is subsequently protonated to (tpb )Mov(0)(OH)SPh. Although the source of the proton in these studies in unclear, the observed A( H) = 13.1 x 10 4 cm 1 is similar to that observed in desulfo xanthine oxidase [204— 206],... 

General Considerations. Much experimental information is available concerning the role of molybdenum in xanthine oxidase (19, 20). In early work (prior to 1970), there was much confusion in the literature because of the presence of various inactive forms of the enzyme. It is now known that both demolybdo and desulfo forms of xanthine oxidase were present in most early preparations and remain present in many current preparations as well (20, 64). 

Bordas, J., Bray, R. C., Gamer, C. D., Gutteridge, S., and Hasnain, S. S., 1980, X-ray absorption spectroscopy of xanthine oxidase the molybdenum centres of the functional and the desulfo forms, Biochem. J. 191 499n508. 

Deactivation of oxidized xanthine oxidase by cyanide produces thiocyanate and an inactive or desulfo form of the enzyme. The oxidized desulfo enzyme contains a dioxo-Mo(VI) center. Since it is generally believed that a terminal thio ligand on Mo is removed in the cyanolysis reaction, the interaction of oxo-thio-Mo(VI) complexes with CN is worthy of investigation. In general, reactions of thio-Mo( VI) complexes (typically thiomolybdates) with cyanide leads to the formation of SCN" by formal sulfur atom transfer and reduction of the metal, e.q., Eq. (25). 

---