Oxo-molybdenum centers

Coupled electron-proton transfer, reactions of oxo-molybdenum centers, 40 57-59 Coupled-ring nitrides molecular structure of, 15 401-6 preparation of, 15 400 properties and reactions of, 15 402-403 purification of, 15 400-401 Covalent bonding... 

One-dimensional electrical conductors, platinum complexes, 26 235-268 band theory, 26 237-241 charge density waves, 26 239-240 Kahn-Teller effect, 26 239-240 and superconductivity, 26 240-241 One-electron reactions, oxo-molybdenum centers, 40 56-57... 

Oxo-molybdenum centers, 40 48-61 coupled electron-proton transfer reactions, 40 57-59... 

Although oxo-molybdenum centers coordinated by a single dithiolene ligand constitute the redox-independent core of the active sites of both the xanthine... 

C. Models of Enzymes Containing [MoOS] Oxidized Centers Reactions of Oxo-Molybdenum Centers... 

Physical studies on oxidized and reduced enzymes show that all of the enzymes studied to date possess an oxo-molybdenum center that cycles between the Mo(VI) and the Mo(IV) states during catalysis and that the Mo(V) state can be detected as a transient species by EPR spectroscopy. Scheme 3 shows a simple cycle of reactions that describes the oxidation (or in reverse, reduction) of a substrate at an oxo-molybdenum center, such as that present in sulfite oxidase. 

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-... 

There is very little information concerning one-electron reactions at oxo-molybdenum centers because of the tendency for oxo-molybdenum complexes to form dinuclear systems (37 and 38). To our knowledge, (L-N3)MoO,jX3, j complexes constitute the only family of mononuclear... 

F.E. Inscore, R. McNaughton, B.L. Westcott, M.E. Helton, R. Jones, I.K. Dhawan, J.H. Enemark, and M.L. Kirk, Spectroscopic evidence for a unique bonding interaction in oxo-molybdenum dithiolate complexes implications for sigma electron transfer pathways in the pyranopterin dithiolate centers of enzymes. Inorg. Chem. 38, 1401-1410 (1999). 

M-F coupling constants in, 4 245 Mo, molybdenum center probes, 40 16 multinuclear, tetracyano complexes containing oxo or nitrido ligands, 40 303-304... 

Model studies clearly demonstrate that oxo transfer is a viable mechanism for many of the enzyme reactions shown in Table 2d. However, primarily because of difficulties in labeling studies, it has not yet proved possible to validate oxo transfer as a physiologically relevant enzymatic mechanism. Although it has been possible to oxidize and reduce molybdenum centers using certain oxygen atom donors or acceptors, these experiments serve only to demonstrate that such processes are possible and not that they are part of the physiologically relevant pathway [231,233],... 

Photoelectron spectroscopy (PES) has been shown to provide a convenient probe of metal ion effective nuclear charge and the nature of the metal-ligand bond via the energy of valence-electron photoionizations (16, 20, 22, 284, 285, 312, 332-334). Recently, PES spectroscopy has been employed in the study of oxo-molybdenum compounds of the type (L-A5)MoE(X,Y) [E = O, S, NO X, Y = halide, alkoxide, or thiolate] in order to evaluate the synergy between the axial (E) and equatorial (X,Y) donors in affecting the ionization energy of the HOMO localized on the Mo center (16, 284, 334). These studies have conclusively shown that equatorial dithiolene coordination electronically bulfers the Mo center in (L-A pMoEttdt) (Fig. 13) from the severe electronic perturbations associated with the enormous variation in the Ji-donor/acceptor properties... 

There are now three different proteins of the XDH/XO family whose structures have been determined by X-ray protein crystallography. The structure of aldehyde oxidoreductase from the bacterium Desulfovibrio gigas was the first X-ray structure determined for an oxo-molybdenum enzyme (17) and has been followed by stmctures of XO/XDH (10) and carbon monoxide dehydrogenase (CODH) (19, 21). Although the three enzymes are placed in the same family, there are structural differences among them at the active site and at the phosphate remote from the Mo center. 

This chemistry may be relevant to the nature and function of the molybdenum center of E. coli formate dehydrogenase. The Mo K-edge EXAFS of both the oxidized and reduced form of this enzyme were found to be very similar, each inolving a des-oxo-molybdenum site with four Mo S bonds at 2.35 A, (probably) one Mo O bond at 2.1 A, and one Mo—Se interaction at 2.62 A. The Se K-edge EXAFS showed clear evidence for a S Se contact of 2.19 A, presumably indicative of a novel seleno-sulfide ligand to the molybdenum (121). 

The reaction mechanism given in Figure 10 is consistent with the bulk of the presently available information, but raises the interesting question as to how, in the absence of a second (spectator) oxo group, the Mo=0 bond of the oxidized enzyme is made labile. Indeed one possibility is that the oxidized site does possess a spectator oxo (McAlpine et al., 1997 McAlpine et al., 1997), although this presently appears to be a minority opinion in the field. In the context of the mechanism given in Figure 10, the Mo=0 bond of the molybdenum center must be sufficiently labile that... 

The photoelectron spectra of model complexes for these molybdenum enzyme active sites have been investigated to gain a better understanding of their basic electronic structure and the role of the ene-dithiolate hgand. For example, the metal coordination of model complexes such as Tp MoO(tdt) are similar to the molybdenum center of sulfite oxidase, which possesses the basic structural core of a terminal oxo group cis to a 1,2-dithiolate. 

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. 

XAS has played a vital role in defining the chemical nature of these molybdenum centers and how they respond to changes in the oxidation level of the protein and/or to the presence of substrates, substrate analogs, or inhibitors of enzymatic activity (9, 13, 15). The prefix oxo for this latter group of enzymes is appropriate. Thus, not only does each enzyme catalyze a conversion, the net result of which can be represented as oxygen atom transfer, but also XAS studies have confirmed the presence of at least one terminal oxo ligand (Mo=0) for molybdenum in each of the enzymes. 

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