Environmental Engineering Reference
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2.1.4 Reaction Mechanism
The structural, spectroscopic, and computational investigations agree with the
following mechanism for Cu,Mo-CODHs (Figure
4
). Catalytic CO oxidation starts
with the Mo(VI)/Cu(I) state, where CO binds to Cu(I) generating a Cu(I)-CO species
[
56
]. Binding to the electron-rich Cu(I) populates the
* orbital of CO, thereby
activating it for the nucleophilic attack of the equatorial Mo
π
O oxygen on the carbon
of Cu-bound CO. Based on the crystal structure of an isocyanide-inhibited state of
Cu,Mo-CODH the possibility of a thiocarbonate intermediate was discussed
[
50
]. However, the formation of a C-S bond with CO in the catalytic cycle is most
likely thermodynamically unfavorable, as indicated by computational studies [
60
,
61
].
Thus, CO oxidation likely leaves the Mo-S-Cu moiety intact and involves a
Mo(VI)-S-Cu(I)-C-O- metallacycle in the next step [
67
], which breaks down to
form CO
2
and Mo(IV). Electron transfer from the active site to external acceptors,
closes the catalytic cycle. Electrons generated by CO or H
2
oxidation are in a first step
takenupbyMoreducingMo(VI)toMo(IV).Electronsarethentransferredviathetwo
different [2Fe2S] clusters to the FAD, where they are donated to external electron
acceptors, regenerating the enzyme for another turnover.
¼
Figure 4 Mechanism of CO oxidation by Cu,Mo-CODHs. (1) The Mo(VI)-Cu(I) state is ready
for CO activation. (2) Cu is binding CO, activating it for a nucleophilic attack by the Mo-bound
oxo ligand. (3) The intermediary metallacycle rearranges and water binding may support CO
2
liberation. (4) The Mo(IV)-Cu(I) active site releases two electrons and a proton to regenerate the
Mo(VI)-Cu(I) state 1.
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