Dioxomolybdenum(VI) complexes of the hydrotris(3,5-dimethyl-1-pyrazolyl)borate ligand. Synthesis and oxygen atom transfer reactions

Roberts, S.A.; Young, Charles G.; Kipke, Cary A.; Cleland, W. E.; Yamanouchi, Kuniko; Carducci, Michael D.; Enemark, John H.

doi:10.1021/ic00344a007

Cited by 117 publications

(129 citation statements)

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“…This orbital control exerted by the oxido ligand on electron transfer via the dithiolene has been termed "oxo-gate hypothesis" [62,63]. Trispyrazolylborato molybdenum complexes Mo VI (O) 2 (Tp R )X turned out to be especially successful in OAT and ET/PT chemistry [Scheme 4d, Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate, Tp iPr = hydrotris(3-isopropylpyrazolyl)borate] [64][65][66][67][68][69]. Indeed, OAT to phosphanes was successful leading to the oxido phos- the catalytic cycle [66,67].…”

Section: Important Bioinspired Model Systemsmentioning

confidence: 99%

Bioinspired functional analogs of the active site of molybdenum enzymes: Intermediates and mechanisms

Heinze

2015

Coordination Chemistry Reviews

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Section: Important Bioinspired Model Systemsmentioning

confidence: 99%

Bioinspired functional analogs of the active site of molybdenum enzymes: Intermediates and mechanisms

Heinze

2015

Coordination Chemistry Reviews

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“…The reducing equivalents are then singly transferred to the heme center and ultimately passed to cytochrome c in the oxidative half-cycle to reestablish the resting Mo(VI) state. Studies of the chemistry of inorganic model compounds (64)(65)(66)(67)(68)(69)(70) have helped to improve our understanding of the reaction catalyzed by sulfite oxidase. All model compounds have the composition of L n MoO 2 , in which L is a bulky ligand that prevents the formation of µ-oxo bridged Mo 2 clusters.…”

Section: Mechanistic Implicationsmentioning

confidence: 99%

“…This reaction shows that although the Mo=O bond is known to be stable, an oxygen atom transfer in solution is possible. Another model compound based on a trispyrazolborate ligand (66) reacts in the same way as described above. The significance of this complex is the reaction with water in the presence of an oxidant to obtain the starting compound (68), because this reaction should resemble the mechanism by which the Mo(VI) state of sulfite oxidase is regenerated.…”

Section: Mechanistic Implicationsmentioning

confidence: 99%

MOLYBDENUM-COFACTOR–CONTAINING ENZYMES: Structure and Mechanism

Kisker

Schindelin

Rees

1997

Annu. Rev. Biochem.

492

370

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Molybdenum-containing enzymes catalyze basic metabolic reactions in the nitrogen, sulfur, and carbon cycles. With the exception of the nitrogenase cofactor, molybdenum is incorporated into proteins as the molybdenum cofactor that contains a mononuclear molybdenum atom coordinated to the sulfur atoms of a pterin derivative named molybdopterin. Certain microorganisms can also utilize tungsten in a similar fashion. Molybdenum-cofactor-containing enzymes catalyze the transfer of an oxygen atom, ultimately derived from or incorporated into water, to or from a substrate in a two-electron redox reaction. On the basis of sequence alignments and spectroscopic properties, four families of molybdenum-cofactorcontaining enzymes have been identified. The available crystallographic structures for members of these families are discussed within the framework of the active site structure and catalytic mechanisms of molybdenum-cofactor-containing enzymes. Although the function of the molybdopterin ligand has not yet been conclusively established, interactions of this ligand with the coordinated metal are sensitive to the oxidation state, indicating that the molybdopterin may be directly involved in the enzymatic mechanism. CONTENTS

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“…Although the oxygen atom incorporated into product is ultimately derived from water, it has been known for some time that the proximal oxygen atom donor is a catalytically labile site on the enzyme, which, in the course of a single turnover, transfers its oxygen to substrate, to be regenerated subsequently by oxygen derived from solvent prior to a second turnover (6). In light of the known precedence for oxo transfer in the literature of small inorganic complexes of molybdenum (7)(8)(9)(10)(11)(12)(13)(14)(15)(16), it was originally thought most likely that the catalytically labile site of the enzyme was the MoϭO group. More recently, it has been suggested that the catalytically labile site might instead be a metal-coordinated hydroxide (17)(18)(19).…”

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confidence: 99%

The Reductive Half-reaction of Xanthine Oxidase

Xia

Dempski

Hille

1999

Journal of Biological Chemistry

100

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The kinetics of xanthine oxidase has been investigated with the aim of addressing several outstanding questions concerning the reaction mechanism of the enzyme. Steady-state and rapid kinetic studies with the substrate 2,5-dihydroxybenzaldehyde demonstrated that (k cat /K m ) app and k red /K d exhibit comparable bellshaped pH dependence with pK a values of 6.4 ؎ 0.2 and 8.4 ؎ 0.2, with the lower pK a assigned to an active-site residue of xanthine oxidase (possibly Glu-1261, by analogy to Glu-869 in the crystallographically known aldehyde oxidase from Desulfovibrio gigas) and the higher pK a to substrate. Early steps in the catalytic sequence have been investigated by following the reaction of the oxidized enzyme with a second aldehyde substrate, 2-aminopteridine-6-aldehyde. The absence of a well defined acid limb in this pH profile and other data indicate that this complex represents an E ox ⅐S rather than E red ⅐P complex (i.e. no chemistry requiring the active-site base has taken place in forming the long wavelengthabsorbing complex seen with this substrate). It appears that xanthine oxidase (and by inference, the closely related aldehyde oxidases) hydroxylates both aromatic heterocycles and aldehydes by a mechanism involving base-assisted catalysis. Single-turnover experiments following incorporation of 17 O into the molybdenum center of the enzyme demonstrated that a single oxygen atom is incorporated at a site that gives rise to strong hyperfine coupling to the unpaired electron spin of the metal in the Mo V oxidation state. By analogy to the hyperfine interactions seen in a homologous series of molybdenum model compounds, we conclude that this strongly coupled, catalytically labile site represents a metalcoordinated hydroxide rather than the Mo‫؍‬O group and that this Mo-OH represents the oxygen that is incorporated into product in the course of catalysis.Xanthine oxidase from cow's milk is a homodimer with a molecular mass of 300 kDa, with each catalytically independent subunit possessing four prosthetic groups: one molybdenum center, one FAD, and two Fe 2 S 2 (Cys) 4 iron-sulfur centers. Physiologically, the enzyme catalyzes the oxidative hydroxylation of hypoxanthine to xanthine and the subsequent hydroxylation of xanthine to uric acid, the final two steps of purine metabolism in mammals. Substrate hydroxylation takes place at the molybdenum center of the enzyme, which becomes reduced from Mo VI to Mo IV in the process (1). The reducing equivalents introduced at the molybdenum center are subsequently passed via intramolecular electron transfer to the flavin center, where reaction with O 2 takes place to give peroxide or superoxide, depending on the level of enzyme reduction (2, 3).Xanthine oxidase exhibits an unusually broad specificity toward reducing substrates. It can hydroxylate a wide variety of purines, pteridines, and related aromatic heterocycles and also a range of both aliphatic and aromatic aldehydes, taking these to the corresponding carboxylic acid. Indeed, xanthine oxidase belongs to the ...

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Dioxomolybdenum(VI) complexes of the hydrotris(3,5-dimethyl-1-pyrazolyl)borate ligand. Synthesis and oxygen atom transfer reactions

Cited by 117 publications

References 0 publications

Bioinspired functional analogs of the active site of molybdenum enzymes: Intermediates and mechanisms

Bioinspired functional analogs of the active site of molybdenum enzymes: Intermediates and mechanisms

MOLYBDENUM-COFACTOR–CONTAINING ENZYMES: Structure and Mechanism

The Reductive Half-reaction of Xanthine Oxidase

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