Ascorbate oxidase catalytic cycle

A tentative catalytic mechanism of ascorbate oxidase has been proposed based on the refined X-ray structure and on spectroscopic and mechanistic studies of ascorbate oxidase and the related laccase. The results of these studies have been discussed in detail (74). The X-ray structure determinations of the fully reduced and peroxide derivatives define two important intermediate states during the catalytic cycle. A proposal for the catalytic mechanism incorporating this new information is given in Messerschmidt et al. (150) and presented in Fig. 14. This scheme should be valid in principle also for laccase due to the close similarities of both blue oxidases. 

In the case of laccase and ascorbate oxidase, the observed ET rates for the reduction of the type-3 coppers (see Table VIII) are lower than the observed turnover number. This can be explained only by the possibility that the enzymes are in a resting form under the experimental conditions. A considerable reorganization energy seems to be necessary to get to the reduced state of the type-3 coppers (release of the bridging OH" and movement of the copper GU2 and GU3). From these data it cannot be decided what the rate-limiting step is in the catalytic cycle, either this intramolecular ET or the reaction of the dioxygen at the trinuclear copper site. 

X-ray structures of functional derivatives of ascorbate oxidase provided pictures of intermediate states, which will probably be passed during the catalytic cycle. A catalytic mechanism that is based on the available mechanistic data and these new results has been proposed. 

Intramolecular ET between distinct copper centers is part of the catalytic cycles of many copper-containing redox enzymes, such as the multicopper oxidases, ascorbate oxidase, and ceruloplasmin, as well as the copper-containing nitrite reductases. Examination of internal LRET in these proteins is of considerable interest as it may also provide insights into the evolution of selected ET pathways in particular, whether and how the enzymes have evolved in order to optimize catalytic functions. With the increase in the number of known high-resolution 3D structures of transition metal containing redox enzymes, studies of structure-reactivity relationships have become feasible and indeed many have been carried out during the last two decades. 

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