Xanthine oxidase

The enzyme xanthine oxidase, or XO, (bovine milk enzyme is PDB 1FIQ, EC catalyzes the oxidation of hypoxanthine to xanthine and can further catalyze the oxidation of xanthine to uric acid


  • hypoxanthine + O2 + H2O ↔ xanthine + H2O2
  • xanthine + O2 + H2O ↔ uric acid + H2O2  

Protein structure

The protein is large, having a molecular weight of 270,000, and has 2 flavin molecules (bound as FAD), 2 molybdenum atoms, and 8 iron atoms bound per enzymatic unit. The molybdenum atoms are contained as molybdopterin cofactors and are the active sites of the enzyme. The iron atoms are part of [2Fe-2S] ferredoxin iron-sulfur clusters and participate in electron transfer reactions.

Active site and mechanism of xanthine oxidase

The active site of XO is composed of a molybdopterin unit with the molybdenum atom also coordinated by terminal oxygen (oxo), sulfur atoms and a terminal hydroxide. In the reaction with xanthine to form uric acid, an oxygen atom is transferred from molybdenum to xanthine. The reformation of the active molybdenum center occurs by the addition of water. Like other known molybdenum-containing oxidoreductases, the oxygen atom introduced to the substrate by XO originates from water rather than from dioxygen (O2).

 \(O_O)/ HUH?

Sorry, I came across this thing when browsing random articles on Wikipedia....
My birthday is today (well, 1420hrs GMT actually, or the early morning of the 6th in parts of Australia appearently)


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