User:Adam Mirando/Sandbox 1: Difference between revisions

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⁺ 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="gluarg" /><ref name="conver" />.  Crystallographic data have show that residues 535 and 992 are capable of forming disulfide bonds<ref name="gluarg" />. Due to a distance of 15.7 Ǻ between the α-carbons of Cys535 and 992, the formation of a bond would require a substantial conformational change <ref name="structure" />. Further structural analyses reveal a peptide cluster composed of Arg426, Arg334, Trp335, and Phe549 that are tightly packed in the XDH but dispersed following disulfide bond formation. This modification is then transmitted to a loop consisting of residues 422-432 in the FAD domain, restricting NAD⁺ from binding while opening a channel accessible for O₂ in the now dispersed peptide cluster<ref name="gluarg" />.