User:Adam Mirando/Sandbox 1: Difference between revisions

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'''Xanthine oxidoreductase''' (XOR) is an extensively studied metalloflavoprotein that is found in a variety of different organisms, ranging from bacteria to eukaryotes <ref>PMID:11848841</ref>. XORs are dimeric enzymes typically around 280 kDa in size with two interconvertible forms: xanthine dehydrogenase (XDH) [1.17.1.4] and xanthine oxidase (XO) [1.17.3.2]. Conversion between the two forms is mediated through the reversible oxidation of three cysteine residues or irreversible [http://en.wikipedia.org/wiki/Trypsin trypsin] truncation <ref name="structure" />. XOR is involved in purine catabolism, catalyzing the oxidation of [http://en.wikipedia.org/wiki/Hypoxanthine hypoxanthine] and [http://en.wikipedia.org/wiki/Xanthine xanthine] to [http://en.wikipedia.org/wiki/Urate urate] through the extraction of two electrons <ref name="gluarg" />. The transport of these electrons is facilitated by the molybdenum of the <scene name='User:Adam_Mirando/Sandbox_1/Mo_pterin_domain/3'>molybdopterin cofactor</scene>, two  
'''Xanthine oxidoreductase''' (XOR) is an extensively studied metalloflavoprotein that is found in a variety of different organisms, ranging from bacteria to eukaryotes <ref>PMID:11848841</ref>. XORs are dimeric enzymes typically around 280 kDa in size with two interconvertible forms: xanthine dehydrogenase (XDH) [1.17.1.4] and xanthine oxidase (XO) [1.17.3.2]. Conversion between the two forms is mediated through the reversible oxidation of several cysteine residues or irreversible [http://en.wikipedia.org/wiki/Trypsin trypsin] truncation <ref name="structure" />. XOR is involved in purine catabolism, catalyzing the oxidation of [http://en.wikipedia.org/wiki/Hypoxanthine hypoxanthine] and [http://en.wikipedia.org/wiki/Xanthine xanthine] to [http://en.wikipedia.org/wiki/Urate urate] through the extraction of two electrons <ref name="gluarg" />. The transport of these electrons is facilitated by the molybdenum of the <scene name='User:Adam_Mirando/Sandbox_1/Mo_pterin_domain/3'>molybdopterin cofactor</scene>, two  
<scene name='User:Adam_Mirando/Sandbox_1/Fes_clusters/2'>iron sulfur centers</scene>, and a bound  
<scene name='User:Adam_Mirando/Sandbox_1/Fes_clusters/2'>iron sulfur centers</scene>, and a bound  
<scene name='User:Adam_Mirando/Sandbox_1/Fad_domain/4'>FAD</scene> coenzyme. In XDH the electrons are then passed preferentially from the reduced flavin to a final NAD+ acceptor, creating NADH <ref name="thermo" />. Apart from NADH, XDH may also use O<sub>2</sub> as a final electron acceptor. In contrast, conversion to the XO form precludes NAD<sup>+</sup> from binding, permitting only the use of O<sub>2</sub>. Consequently, the reduction of O<sub>2</sub> produces substantial amounts of H<sub>2</sub>O<sub>2</sub> and superoxide as byproducts <ref name="conver">PMID:15878860</ref><ref name="gluarg" />.  The prodution of these oxidative species has been implicated in the innate immune response <ref>PMID:12967676</ref> and cardiovascular disease, such as [http://en.wikipedia.org/wiki/Atherosclerosis atherosclerosis] <ref>PMID:12958034</ref>, [http://en.wikipedia.org/wiki/Reperfusion_injury ischemia-reperfusion injury], and chronic heart failure <ref>PMID:14694147</ref> <ref>PMID:12105162</ref>.
<scene name='User:Adam_Mirando/Sandbox_1/Fad_domain/4'>FAD</scene> coenzyme. In XDH the electrons are then passed preferentially from the reduced flavin to a final NAD+ acceptor, creating NADH <ref name="thermo" />. Apart from NADH, XDH may also use O<sub>2</sub> as a final electron acceptor. In contrast, conversion to the XO form precludes NAD<sup>+</sup> from binding, permitting only the use of O<sub>2</sub>. Consequently, the reduction of O<sub>2</sub> produces substantial amounts of H<sub>2</sub>O<sub>2</sub> and superoxide as byproducts <ref name="conver">PMID:15878860</ref><ref name="gluarg" />.  The prodution of these oxidative species has been implicated in the innate immune response <ref>PMID:12967676</ref> and cardiovascular disease, such as [http://en.wikipedia.org/wiki/Atherosclerosis atherosclerosis] <ref>PMID:12958034</ref>, [http://en.wikipedia.org/wiki/Reperfusion_injury ischemia-reperfusion injury], and chronic heart failure <ref>PMID:14694147</ref> <ref>PMID:12105162</ref>.
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===Xanthine Dehydrogenase/Xanthine Oxidase Conversion===
===Xanthine Dehydrogenase/Xanthine Oxidase Conversion===


Xanthine oxidoreductase has two functional forms: xanthine dehydrogenase and xanthine oxidase. This conversion is controlled by the oxidation state of Cys535, Cys992, and Cys1316. When these residues are reduced, the enzyme functions as a dehydrogenase, using NAD<sup>+</sup> as its final receptor. Following chemical modification (ie fluorodinitrobenzene) or oxidation (ie 4,4'-dithiopridine) the oxidase form is favored. Once oxidized, incubation with a reducing agent (ie dithiothreitol) will restore the enzyme to the the dehydrogenase form <ref name="conver" />. Studies involving the C535A/C992R/C1316S triple mutant, however, were unable to convert to the oxidase form. Consequently, crystal structures of this mutant revealed a monomeric structure, in contrast to the normally homodimeric wild type enzyme <ref name="conver" />.  
Xanthine oxidoreductase has two functional forms: xanthine dehydrogenase and xanthine oxidase. This conversion is controlled by the oxidation state of Cys535, Cys992, Cys1316, and Cys1324. When these residues are reduced, the enzyme functions as a dehydrogenase, using NAD<sup>+</sup> as its final receptor. Following chemical modification (ie fluorodinitrobenzene) or oxidation (ie 4,4'-dithiopridine) the oxidase form is favored. Once oxidized, incubation with a reducing agent (ie dithiothreitol) will restore the enzyme to the the dehydrogenase form <ref name="conver" />. Studies involving the C535A/C992R/C1316S triple mutant, however, were unable to convert to the oxidase form. Consequently, crystal structures of this mutant revealed a monomeric structure, in contrast to the normally homodimeric wild type enzyme <ref name="conver" />. The mechanism of this conversion is thought to be the formation of disulfide bridges between Cys535 and Cys992 and Cys1316 and Cys1324<ref name="conver" /><ref name="gluarg" />. A distance of 15.7 Ǻ between the α-carbons of Cys535 and 992 suggests that the formation of a bond would require a substantial conformational change(enroth). Crystallographic data containing bonded residues 535 and 992 further supports the contribution of this disulfide to the conversion <ref name="gluarg" />.  


== Mechanism ==
== Mechanism ==
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