Overview for MACiE Entry M0139
EC Number: 22.214.171.124 (A member of the Oxidoreductases, Acting on CH or CH2 groups, With NAD+ or NADP+ as acceptor)
Enzyme Name: xanthine dehydrogenase
Biological Species: Bos taurus (Cow)
Catalytic Chain UniprotKB Accession Codes:
Representative PDB Code: 1v97 - CRYSTAL STRUCTURE OF BOVINE MILK XANTHINE DEHYDROGENASE FYX-051 BOUND FORM (Resolution = 1.94 Å).
Catalytic CATH Codes:
Display structure information
Overall Comment: Xanthine dehydrogenase (EC 126.96.36.199) and xanthine oxidase (EC 188.8.131.52) are two variants of the same gene product. The former prefers NAD+ as the oxidising substrate whereas the latter uses dioxygen exclusively . The mechanism shown in MACiE strongly supports all the available evidence. The pH dependence of the reaction strongly suggests that the active site base (Glu1261A) is initially in its unprotonated state and that the enzyme only works on the substrate in its neutral state . The mechanism shown in here is supported by mutational spectroscopic computational and crystallographic studies [1,2,3,4]. There has been some debate as to whether the catalytically labile oxygen is from the Mo=O or Mo-OH groups. Isotope labelling experiments of solvent shows that the catalytically labile group is the Mo-OH and that it is replenished from solvent at the end of the catalytic cycle [1 2].
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Stepwise Description of the Reaction
|Step 1||Glu1261 deprotonates the MTE bound hydroxide, activating it for nucleophilic attack upon the xanthine substrate, resulting in a nucleophilic addition of the substrate to the cofactor. Xanthine then transfers a hydride to the sulfur bound to the molybdenum ion (Mo=S) resulting in a two electron reduction of the Mo(VI) to Mo(IV) and a thiol bound to molybdenum.|
|Step 2||Water deprotonates the molybdenum bound thiol, reforming the Mo=S species and a single electron transfer from the Mo(IV) through the rest of the cofactor and two iron-sulfur clusters to a bound FAD cofactor, forming Mo(V) and a radical on the FAD cofactor.|
|Step 3||A free hydroxide ion attacks the Mo(V), releasing the urate product (oxidised xanthine), which is re-protonated from the Glu1261, in a nucleophilic substitution reaction. This also results in the second electron transfer from the Mo(V) through the rest of the cofactor and two iron-sulfur clusters to a bound FAD cofactor, which then deprotonates a hydroxonium ion.|
|Step 4||FAD is regenerated through a hydroxide transfer from the FAD cofactor to the substrate NAD|
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Catalytic Residues Involved
||Location of Function
Organic Cofactors for M0139
Metal Cofactors for M0139
- K. Okamoto et al. (2004), Proc. Natl Acad. Sci. USA, 101, 7931-7936. The crystal structure of xanthine oxidoreductase during catalysis: implications for reaction mechanism and enzyme inhibition.
- R. Hille (2005), Arch. Biochem. Biophys., 433, 107-116. Molybdenum-containing hydroxylases.
- E.-Y. Choi et al. (2004), J. Inorg. Biochem., 98, 841-848. Studies on the mechanism of action of xanthine oxidase.
- S. Leimkuhler et al. (2004), J. Biol. Chem., 279, 40437-40444. The role of active site glutamate residues in catalysis of Rhodobacter capsulatus xanthine dehydrogenase.
- X.-H. Zhang et al. (2005), Inorg. Chem., 44, 1466-1471. A theoretical study on the mechanism of the reductive half-reaction of xanthine oxidase.
Homologue information for M0139 (1v97)
MACiE Homologues (within the PDB)
MACiE Homologues (within UniprotKB/SwissProt)
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