Reactions of Oxo-Molybdenum Centers

The intimate mechanisms of the molybdenum enzymes have yet to be fully elucidated. Current evidence supports the transfer of an oxygen atom between Mo(IV/VI) and substrate (189) and the regeneration of the active site by two one-electron processes, the first of which generates transient Mo(V) centers (17). In view of the reactions taking place at the Mo center (Eq. (13)) and in an overall sense (Eq. (14)), chemical modeling has naturally concentrated on  

It appears likely that the mechanism adopted by the enzymes may depend on the conditions of catalysis and that under physiological 

The possible role of oxygen atom transfer in molybdenum enzyme catalysis was recognized in the early 1970s (190-194). In the ensuing years, a wealth of chemistry has established molybdenum as the premier exponent of such reactions (7, 195). Importantly, related dioxo-Mo(VI) and oxo-Mo(IV) complexes are interconverted by oxygen atom transfer reactions (Eq. (13)). These reactions are effected by reductants (X) such as tertiary alkyl and aryl compounds of the group 15 elements (especially phosphines) and oxidants (XO) such as S- and N-oxides. In many cases, however, the Mo(VI) and Mo(IV) compounds participate in a comproportionation reaction yielding dinuclear Mo(V) complexes (Eq. (15)). 

The comproportionation reaction (15) is biologically unimportant and must be prevented in meaningful enzyme models. Accordingly, much effort has been invested in ligand design aimed to sterically inhibit the formation of dinuclear species several successful and significant 

Rate Constants for the Reaction of Mo(VI) Complexes with PPho 

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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... 

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

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... 

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... 

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. 

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],... 

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. 

Oxomolybdenum centers are present in a series of enzymes known as molybdenum hydroxylases [4] which catalyze oxygen atom transfer reactions of a variety of small molecules. Examples include the oxidation of xanthine to uric acid and the reduction of N03 to N02"- The reactions are equivalent to two-electron transfers, and the molybdenum atom cycles between oxidation states VI and IV during these processes. Molybdenum (VI) typically contains two multiply bonded oxo (or sulfido) groups, and molybdenum (IV) contains one. Therefore, it is believed that Mo02 and MoO +centers intercovert by coupled transfer of protons and electrons over a narrow potential interval during turnover of these enzymes. However, other explanations are possible. 

Substitutions at triangular trinuclear oxo-carboxylato complexes which M3(/i3-0) cores, which may be simple or may be mixed metal or mixed valence species, are often slow. The third step in water replacement in Ru(III)30(OH2)3 derivatives by acetate has a half-life of several minutes at ambient temperatures, while replacement of water by CDjOD in [Ru2Rh ( 3-0)(0Ac)g(0H2)3] has rate constants of 6.0x10 and 1.2x10 for reaction at the Ru and Rh, respectively, at 21 °C in CD30D. Further examples of kinetic studies at trinuclear centers M3O where the metal atoms are molybdenum or tungsten can be found in Section 8.2.2 earlier in this chapter. 

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