Febuxostat blockade of substrate access to the active site of both reduced and oxidized xanthine oxidase. Space-taking representation of the interaction of febuxostat and amino acid residues of xanthine oxidase. Febuxostat, represented in green, occupies the channel accessing the Mo-pterin moiety in the active site of xanthine oxidase. In contrast to allopurinol and oxypurinol, which bind directly to the active site and result in competitive inhibition of enzyme activity, febuxostat blocks substrate access to the channel resulting in a pattern of mixed inhibition of enzyme activity. Reprinted with permission from . © 2006 American Society for Biochemistry and Molecular Biology.