The Structure of Formaldehyde-Inhibited Xanthine Oxidase Determined by 35 GHz 2H ENDOR Spectroscopy.

The formaldehyde-inhibited Mo(V) state of xanthine oxidase (I) has been studied for four decades, yet it has not proven possible to distinguish unequivocally among the several structures proposed for this form. The uniquely large isotropic hyperfine coupling for 13C from CH2O led to the intriguing suggestion of a direct Mo-C bond for the active site of I. This suggestion was supported by the recent crystal structures of glycoland glycerol-inhibited forms of aldehyde oxidoreductase, a member of the xanthine oxidase family. 1H and 2H ENDOR spectra of I(C1,2H2O) in H2O/D2O buffer now have unambiguously revealed that the active-site structure of I contains a CH2O adduct of Mo(V) in the form of a four-membered ring with S and 0 linking the C to Mo and have ruled out a direct Mo-C bond. Density functional theory computations are consistent with this conclusion. We interpret the large 13C coupling as resulting from a `transannular hyperfine interaction". [ABSTRACT FROM AUTHOR]