Computational exploration of reactive fragment for mechanism-based inhibition of xanthine oxidase

The xanthine oxidase (XO) family is a group of molybdenum-containing enzymes that catalyze the transfer of an oxygen atom from water to a substrate, such as xanthine or aldehyde. The proposed mechanism for the reductive half-reaction of xanthine oxidase involves nucleophilic addition of Mo-bound hydroxide to the substrate and hydride transfer from the substrate to sulfido group (Mo=S) at the molybdenum cofactor (Moco) reaction center. A DFT study of a mechanism involving the near-by glutamic acid (Glu) residue for the oxidation of several model substrates found that the direct involvement of the Glu residue side chain significantly increases the reactivity of the cofactor. The reaction is stepwise with a hydride transfer as the rate determining step. On the basis of mechanistic studies, a QM model was established to search the potential reactive fragments for mechanism-based inhibitors. Two important parameters were designed to evaluate each fragment: the activation enthalpy (ΔH≠) which estimates how effectively an inhibitor competes with xanthine to react with the Moco site, and the enthalpy associated with release of the product (ΔH) which estimates how easily the product in product complex (PRO) could be replaced by a water molecule. The computational findings add to our understanding of the catalytic mechanism of xanthine oxidase, and also assist the screening of mechanism-based inhibitors for this important drug target.