The Xanthine Oxidase Cycle

Xanthine oxidase and related hydroxylase enzymes exhibit broad substrate specificity and an apparently complex catalytic cycle (109, 237). The important centers identified by EXAFS and EPR studies have 

The complications that arise when substrate is in excess (observation of the rapid signal) can be attributed to the effects of substrate binding prior to product release during the Mo(IV) to Mo(VI) one-electron transformations (109, 234). Under these conditions cycle B, involving the Rapid Type 2 center 27b and the Rapid Type 1 center 27a, may pertain. The Rapid centers, whose EPR spectra exhibit two strongly coupled protons, are proposed to differ (primarily) according to the purine bound at the noncovalent binding site. 

Consistent with McWhirter and Hille s suggestion (234), 27a is cata-lytically incompetent until oxidized to 51, the Michaelis complex. The present proposal accommodates a number of other aspects of xanthine 

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A Model for Enzymes with [MoOj] Oxidized Centers The Xanthine Oxidase Cycle... 

The chapter consists of nine sections. Sections II through VII deal with the pterin-containing molybdenum enzymes. Biochemical and model studies of molybdopterin, Mo-co, and related species are described in Section II. In Section III, we briefly survey physical and spectroscopic techniques employed in the study of the enzymes, and consider their impact upon the current understanding of the coordination about the molybdenum atom in sulfite oxidase and xanthine oxidase. Model studies are described in Sections IV and V. Section IV concentrates on structural and spectroscopic models, whereas Section V considers aspects of the reactivity of model and enzyme systems. The xanthine oxidase cycle (Section VI) and facets of intramolecular electron transfer in molybdenum enzymes (Section VII) are then treated. Section VIII describes the pterin-containing tungsten enzymes and the evolving model chemistry thereof Future directions are addressed in Section IX. 

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