Inner-sphere mechanism phenol

The mnlticopper oxidases couple the one-electron or two-electron oxidation of their substrates to the four-electron rednction of dioxygen to water (36). The reaction with substrate can proceed via an onter-sphere or an inner-sphere mechanism, and as a resnlt, the snbstrate specificity varies substantially among the enzymes. The best-characterized enzymes are laccase, ascorbate oxidase, and cernloplasmin. Radical phenol and amine species formed by laccase and ascorbate oxidase... 

Rates of ligand exchange depend quite strongly on the coordina-tive environment of the metal center. The water exchange rate of Fe(H2O)5(OH)is almost three orders of magnitude higher than that of Fe(H20)g+, and follows a dissociative, rather than an associative exchange mechanism (20). Fe(1120)5(OH)has also been shown to form inner-sphere complexes with phenols (27), catechols (28), and a-hydroxycarboxylic acids (29) much more quickly than Fe(H20) +. The mechanism for complex formation with phenolate anion (A-) is shown below (27) ... 

Figure 8.1. Reduction of tervalent metal oxide surface sites by phenol (HA) showing inner-sphere and outer-sphere mechanisms. [From Stone (1986), with permission.]...

Fig. 21. Proposed catalytic mechanism for substrate oxidation by galactose oxidase. (A) Substrate binding displaces Tyr-495 phenolate which serves as a general base for abstracting the hydroxylic proton. (B) Stererospecihc pro- hydrogen abstraction by the Tyr-Cys phenoxyl radical. (C) Inner sphere electron transfer reducing Cu(II) to Cu(I). (D) Dissociation of the aldehyde product.

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