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Animal SOs possess two redox-active domains and are ideal for studying IET in relation to the overall conformation of the protein and the molecular dynamics of the motion of the two domains relative to one another. The procedures for preparing and purifying recombinant hSO [34, 44] have been further optimized in our laboratory [4, 5, 29], so that a wide range of mutant proteins in 50–100 mg quantities can be prepared for comprehensive kinetic, mechanistic, and spectroscopic studies. The ready accessibility of recombinant hSO has made it possible to investigate the effects of modifying amino acid residues of interest to better understand the long range IET process in hSO and to obtain insight into biochemical basis of pathological mutations of hSO. Experimental and theoretical studies agree that domain movement is necessary for SO to achieve a productive conformation during electron transfer, and that the length and composition of the interdomain tether is an important factor in the rates of IET. However, the interactions such as hydrogen bonding and hydrophobic effects involved in mediating the correct mutual orientation and the distance following the initial orientation are unknown. Studies of site-specific mutants reveal a great deal about the IET complex in hSO. Mutating several aromatic residues that extend from the pterin to the surface of the protein suggest that alternative ET pathways may be in operation in hSO. The profound effects of mutating residues surrounding the Mo active site (e. g. R160Q and Y343F) have been demonstrated [28, 45], and extension to mutations of other positively charged residues surrounding the active site, such as Lys200, is warranted. Finally, all of the discussions presented here and elsewhere for IET in hSO rest upon the single structure of intact native cSO. It is clear that X-ray crystal structures of hSO and various mutants are needed to better understand the unexpected properties of this fascinating biomedically important protein.

Animal SOs possess two redox-active domains and are ideal for studying IET in relation to the overall conformation of the protein and the molecular dynamics of the motion of the two domains relative to one another. The procedures for preparing and purifying recombinant hSO [34, 44] have been further optimized in our laboratory [4, 5, 29], so that a wide range of mutant proteins in 50–100 mg quantities can be prepared for comprehensive kinetic, mechanistic, and spectroscopic studies. The ready accessibility of recombinant hSO has made it possible to investigate the effects of modifying amino acid residues of interest to better understand the long range IET process in hSO and to obtain insight into biochemical basis of pathological mutations of hSO. Experimental and theoretical studies agree that domain movement is necessary for SO to achieve a productive conformation during electron transfer, and that the length and composition of the interdomain tether is an important factor in the rates of IET. However, the interactions such as hydrogen bonding and hydrophobic effects involved in mediating the correct mutual orientation and the distance following the initial orientation are unknown. Studies of site-specific mutants reveal a great deal about the IET complex in hSO. Mutating several aromatic residues that extend from the pterin to the surface of the protein suggest that alternative ET pathways may be in operation in hSO. The profound effects of mutating residues surrounding the Mo active site (e. g. R160Q and Y343F) have been demonstrated [28, 45], and extension to mutations of other positively charged residues surrounding the active site, such as Lys200, is warranted. Finally, all of the discussions presented here and elsewhere for IET in hSO rest upon the single structure of intact native cSO. It is clear that X-ray crystal structures of hSO and various mutants are needed to better understand the unexpected properties of this fascinating biomedically important protein.

Publisher Summary This chapter presents a discussion on the bioinorganic chemistry of pterin-containing molybdenum and tungsten enzymes. The chapter focuses on the current results and the interplay of model and enzyme chemistry. Attention is directed to sulfite oxidase and xanthine oxidase, the archetypal examples of molybdenum enzymes, containing, respectively, dioxo-Mo(VI) and oxo-thio-Mo(VI) oxidized centers. Biochemical and model studies of molybdopterin, Mo-co, and related species are described in this chapter. A brief survey on the physical and spectroscopic techniques employed in the study of the enzymes is presented and their impact on the current understanding of the coordination about the molybdenum atom in sulfite oxidase and xanthine oxidase is discussed. Structural and spectroscopic models are also presented in the chapter. The chapter also explains the xanthine oxidase cycle and facets of intramolecular electron transfer in molybdenum enzymes. The pterin-containing tungsten enzymes and the evolving model chemistry are discussed. Current descriptions of the coordination environment of the molybdenum centers of the enzymes rest primarily upon the comparisons of the spectra of the enzymes with the spectra of well-characterized molybdenum complexes. The unexpected discovery that several hyperthermophilic organisms possess pterin-containing tungsten enzymes promises to stimulate the development of oxo-thio-tungsten chemistry.

A series of mononuclear (diolato)- or (dithiolato) oxomolybdenum(V) complexes, LMoO[E-(CH 2 ) n -E] (L=hydrotris(3,5-dimethyl-1-pyrazolyl) borate; E=O, S; n=2-4), and bis(alkoxo) complexes, LMoO(OR) 2 (R=Me, Et, n Pr), have been synthesized and characterized by elemental analysis, mass spectrometry, electron paramagnetic resonance spectroscopy (EPR), cyclic voltammetry, and IR and electronic absorption spectroscopy

Preparation de MoO 2 (dttd), MoOCl(dttd) et MoO(PPh 2 Et)(dttd). On donne la structure des complexes prepares et on les etudie electrochimiquement. Spectre RPE de MoOCl(dttd). dttd=le dibenzotetrathiadecane et PPh 2 Et=la diphenylethylphosphine

The synthesis, crystal structure, spectroscopic and electrochemical properties of tris(quinoxaline-2,3-dithiolato)molybdate(IV), [Mo(qdt)3]2−, are reported. This complex undergoes reversible protonation on the ligand nitrogen atoms and the triply protonated species [Mo(Hqdt)3]+ has also been isolated and characterized as its chloride salt.

Le complexe du titre avec E=O cristallise dans le systeme monoclinique avec le groupe d'espace P2 1 /c (R=0,037). Mo est dans un environnement de coordination octaedrique deforme. Le complexe avec E=S et solvate par CH 2 Cl 2 cristallise dans le systeme orthorhombique avec le groupe d'espace P2 1 2 1 2 1 (R=0,035). La geometrie de coordination est analogue a celle du precedent

Synthese de nouveaux complexes MoO 2 LH 2 avec LH 4 =N,N'-bis(hydroxy-2 t-butyl-3 benzyl) ethylenediamine. Etude de la reduction coulometrique. Etude RX, RPE

Preparation des complexes MoL 2 (L=N(hydroxy-2 benzyl)-mercapto-2 ou hydroxy-2 aniline). Comportement electrochimique reversible. Formation des complexes MoO 2 (LH 2 ). Spectres RPE

Spectre RMN de 95 Mo de plusieurs complexes contenant le groupe cis- [Mo(NO) 2 ] 2+ . La valeur des deplacements chimiques diminue lorsque la possibilite des coordinats a delocaliser les electrons de l'atome metallique augmente. Les largeurs de raies dependent de la viscosite du solvant