Molybdenum enzymes 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. 

Sulfite oxidase contains an oxo-molybdenum center and a 6-type cytochrome. The proposed catalytic sequence (254-256) for the enzyme is shown in Fig. 16. Oxidation of sulfite to sulfate, a two-electron process, occurs at the molybdenum center with concomitant reduction of the molybdenum from VI to IV. Electrons are removed from the enzyme by interactions of the heme of the 6-type cytochrome with exogenous cytochrome c, a one-electron process. Thus, the proposed mechanism of Fig. 16 involves two separate intramolecular electron transfers be-... 

A corollary to the assumption that the enzymes cycle between Mo(VI) and Mo(IV) is that Mo(V), the valence state with which this chapter is primarily concerned, will appear in the systems only incidentally. If intramolecular electron transfer to and from the other centers of the enzymes is fast, then Mo(V) will appear as electrons are transferred to Mo(VI) or from Mo(IV) by these centers (Section 3). This hypothesis is capable of explaining the apparent stability of the nitrate complex of Mo(V) in nitrate reductase. If nitrate can undergo only two-electron reduction at the molybdenum center, then we must regard this signal-giving species itself not as an unexpectedly stable Michaelis complex, but as no more than a dead-end complex. 

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. 

These kinetics data are consistent with a preequilibrium dissociation of dmf from the molybdenum center to form a reactive five-coordinate species that rapidly reduces the Fe(III) center via an inner sphere (halogen transfer) reaction. Other one-electron atom transfer reactions are known in oxo-molybdenum chemistry (262). An innersphere (atom transfer) mechanism is not a viable model for intramolecular transfer in sulfite oxidase because in the enzyme the Mo and Fe centers are almost certainly held too far apart by the protein framework. Moreover, the 65-type heme center of sulfite oxidase is six-coordinate with axial histidine ligands from the protein and hence cannot participate in atom transfer reactions. 

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