X-ray absorption spectroscopy of xanthine oxidase. The molybdenum centres of the functional and the desulpho forms

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Abstract
X-ray absorption spectra were recorded for the Mo K-edge region of xanthine oxidase. Both the absorption edge and the extended fine structure (e.x.a.f.s.) regions were investigated. Spectra were obtained for samples of the desulfo enzyme as well as for mixtures of this with the active enzyme. The spectrum of the pure active form was obtained by difference. The desulfo enzyme shows a pronounced step in the absorption edge, of a type previously associated with terminal O ligands. In the active enzyme this step has decreased markedly. Satisfactory simulations of the e.x.a.f.s. spectrum of the desulfo enzyme could be obtained by assuming the Mo to be bonded to 2 terminal O atoms (Mo = O about 0.175 nm), 2 S atoms (presumably from cysteine residues, Mo-S about 0.250 nm) and 1 S atom (presumably from a methionine residue, Mo-S about 0.290 nm). E.x.a.f.s. of the active enzyme differed appreciably from this. The spectrum of the active enzyme could be simulated if a S atom at about 0.225 nm (i.e., presumably a terminal S atom) replaced one of the terminal O atoms of the desulfo form, with small changes in the other bond distances. Validity of the interpretative procedures, which involved phase shift and amplitude calculations ab initio, was demonstrated by using low MW compounds of known structure.