Studies on the mechanism of action of xanthine oxidase

Recent studies of the reaction mechanism of the molybdenum-containing enzyme xanthine oxidase are presented. The pH-dependence of both the steady-state and rapid reaction kinetics of the enzyme exhibits is bell-shaped, with pK(a)s for the acid and alkaline limbs of 6.6 and 7.4, respectively. These are assigned to ionizations of an active site base and substrate, respectively, with the implication that enzyme acts on the neutral rather than monoanionic form of the purine substrate. A computational study provides evidence that in the course of the reaction tautomerization of substrate occurs, with a proton moving from N-3 to N-9 in the course of the reaction - enzyme facilitation of this tautomerization may contribute as much as 24 kcal/mol in transition state stabilization for the reaction. Electron spin echo (ESEEM) and electron-nuclear double resonance (ENDOR) studies of the so-called "very rapid" Mo(V) intermediate of the reaction, the latter work using a newly synthesized form of the substrate 2-hydroxy-6-methylpurine that has been selectively isotopically labeled at C-8, indicates that product is bound to the molybdenum of the active site in a simple, end-on fashion, consistent with a reaction mechanism involving nucleophilic attack of a (deprotonated) Mo-OH on the C-8 position of substrate. A kinetic study using a series of purines has failed to identify a correlation between the one-electron reduction potential for substrate and catalytic effectiveness, indicating that a reaction mechanism initiated by one-electron, outer-sphere electron transfer is unlikely. Finally, a consideration of the active site structure in the context of the above work suggests specific amino acid residues to target for site-directed mutagenesis studies. Preliminary experiments with two such mutants are entirely consistent with the proposed catalytic roles of two active site glutamate residues.