Crystal structures of the active and alloxanthine-inhibited forms of xanthine dehydrogenase from Rhodobacter capsulatus

Xanthine dehydrogenase (XDH), a complex molybdo/iron-sulfur/flavoprotein, catalyzes the oxidation of hypoxanthine to xanthine followed by oxidation of xanthine to uric acid with concomitant reduction of NAD(+). The 2.7 Angstrom resolution structure of Rhodobacter capsulatus XDH reveals that the bacterial and bovine XDH have highly similar folds despite differences in subunit composition. The NAD(+) binding pocket of the bacterial XDH resembles that of the dehydrogenase form of the bovine enzyme rather than that of the oxidase form, which reduces O-2 instead of NAD(+). The drug allopurinol is used to treat XDH-catalyzed uric acid build-up occurring in gout or during cancer chemotherapy. As a hypoxanthine analog, it is oxidized to alloxanthine, which cannot be further oxidized but acts as a tight binding inhibitor of XDH. The 3.0 Angstrom resolution structure of the XDH-alloxanthine complex shows direct coordination of alloxanthine to the molybdenum via a nitrogen atom. These results provide a starting point for the rational design of new XDH inhibitors.