 Xanthine

Xanthine is a purine nucleobase with two oxo groups at positions2 and6 of the purine framework. The neutral compound has formula ${\mathrm{C}}_5{\mathrm{H}}_4{\mathrm{N}}_4{\mathrm{O}}_2$ and can occur in different forms related by tautomerism. The general chemical name therefore need not select the same hydrogen placement as a particular structural drawing.^[ref]

A metabolic example is the oxidation of hypoxanthine through xanthine toward uric acid and urate. The accepted xanthine dehydrogenase reaction uses NAD+ as an electron acceptor; the related xanthine oxidase form uses oxygen. The identity of the electron acceptor matters even though both names involve xanthine oxidation.^[ref]

For an atom-and-charge bookkeeping example, writing the product in its neutral uric acid form gives the formal oxidation half-reactionBoth sides contain five carbons, four nitrogens, three oxygens and six hydrogens, with total charge zero. This represents a two-electron oxidation and addition of oxygen to the framework. It is a balanced half-reaction, not a claim that the enzyme transfers two free electrons directly into solution. Subsequent uric acid/urate protonation is a distinct equilibrium.