Xanthine oxidase intramolecular electron transfer

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. 

With the exception of the recently reported DMSO reductases from bacteria (71,72), all of the enzymes of Table I contain additional redox active prosthetic groups besides Mo-co. Substrate oxidation (reduction) occurs at the molybdenum center, and electrons are removed (added) via one of the other prosthetic groups. These two processes are coupled by intramolecular electron transfer between the molybdenum center and the other redox centers of the enzyme. Results for xanthine oxidase and sulfite oxidase and approaches to modeling the coupling in sulfite oxidase are summarized below. 

The catalytic cycle is completed with re-oxidation of the molybdenum first to Mo(v), and then to Mo(vi), by intramolecular electron transfer to the cytochrome with cytochrome c serving as the external electron acceptor. Like xanthine oxidase, sulfite oxidase was among the very earliest to which XAS... 

The possible mechanism of intramolecular electron transfer over this distance between the two centers within the molecule and the nature of the chemical groupings on which it depends is of considerable interest. However, we can do no more than speculate about the problems at present. A line of experimental work which might prove valuable would be to study further the phenomenon (Bray et al, 1975) of diminished rates of intramolecular electron transfer in chemically modified forms of xanthine oxidase, in comparison with the native form. 

Xanthine is converted to uric acid at the molybdenum center of the enzyme, and the electrons are removed from the enzyme by oxidation of the flavin center. From early reductive titrations of xanthine oxidase with sodium dithionite, it was proposed that reducing equivalents were equilibrated among the four redox-active centers (Mo-co, two separate Fe2S2 centers, flavin) at a rate that was rapid relative to the overall catalytic rate of substrate turnover (243). Under such conditions, the flux of reducing equivalents through the enzyme should be influenced by the relative reduction potentials of the redox centers involved (244). Any effects of pH and temperature on the reduction potentials of individual redox components would affect the apparent rates of intramolecular transfer of the enzyme. 

With the exception of reduced -nicotinamide adenine dinucleotide (NADH), substrates interact at the Mo centre and two electrons are transferred from the substrate to Mo(VI), reducing the metal to Mo(IV). The substrate residue reacts with an oxo ligand of Mo and a proton also reduces a terminal sulphide ligand of Mo. Hydrolysis of the Mo-substrate complex releases oxidized product, while the Mo(IV) is reoxidized by intramolecular transfer to other redox centres. The catalytic mechanism as described by Bray is depicted below [23], Aldehyde oxidase and xanthine oxidase can each take up to six... 

---