Molybdenum enzymes model systems

J. T. Bolin and co-workers, in E. I. Stiefel, D. Coucouvanis, and W. E. Newton, eds.. Molybdenum Enzymes, Cofaetors and Model Systems American... 

Stifel, E. L Coucouvanis, D. Newton, W. E., Eds. Molybdenum Enzymes, Cofactors and Model Systems, ACS Symposium Series 535 American Chemical Society Washington, DC, 1993 Chapters 10-23. 

Although oxo transfer allows the oxidation of enzyme substrates in biological model systems [196,198,231], the ultimate source of the oxygen incorporated by molybdenum or tungsten enzymes is water (Eq. 19). As such, in the absence of an oxygen atom donor,... 

Alternatively, it is well known (74) that oxo group removal coupled with reduction is facilitated by the presence of acid. Protonation of oxo groups produces hydroxo or aquo ligands which are superior leaving groups. The hydrolysis of ATP near pH 7 produces protons, and this hydrolysis may be enzymically controlled to produce these protons near the oxo group which would facilitate the coupled reduction and oxo removal on the molybdenum site. In either case, the role of ATP would appear to be the production of an open site on molybdenum and in some model systems (104,105), ATP may also be serving this function. Further experimental elaboration of the ATP reactions is desirable in both enzyme and model systems. 

Bolen, J.T., Cambasso, N., Muchmore, S.W., Morgan, T.V., and Mortenson, L. E. (1993) Structure and Environment of metal clusters of the nitrogenase molybdenum iron protein from Clostridium pasterianum, in Stiefel, E.I., Coucouvanis, D., and ewton, W.E. (eds.), Molibdenum Enzymes, Cofactors, and Model Systems, Am. Chem. Soc., Wahington, DC. 

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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