ABSTRACT
Hydrolysis of the product Schiff base liberates the aldehyde product and gener ates the aminoquinol form of the cofactor (Fig. 6C), which must then be reoxidized (Fig. 6D). Dooley and co-workers [177] demonstrated a temperature-dependent equi librium between the Cu(II)/aminoquinol and Cu(I)/TPQ semiquinone forms of the resultant species, whose forward rate constant has been measured to be 75 and 20,000 s_1 for two different CAOs [178,179]. These rates are much greater than the catalytic rate-limiting step, suggesting, but not proving, that the Cu(I)/semiquinone form could be an intermediate in catalysis. In chemical terms, Cu(I) would be a natural site for binding molecular oxygen as the first step in the oxidative half cycle. However, Su and Klinman [180] present evidence that the rate-limiting step in the oxidative half cycle is the initial electron transfer to dioxygen, which chemical precedent would suggest to be fast if binding of dioxygen was to Cu(I). It is concluded that the rate-limiting electron transfer step is directly from the aminoquinol form of the cofactor to dioxygen, with the copper remaining divalent. Su and Klinman [180] further discuss the nature of subsequent electron transfer and proton transfer steps in the oxidative half cycle which lead to generation of the hydrogen peroxide product. They envisage that the aminoquinol form of the cofactor acts as a transducer storing two electrons and two protons for later delivery to dioxygen.
