Formal Catalytic Mechanism of Ascorbate Oxidase
Ascorbate oxidase (EC 18.104.22.168), a copper enzyme widely distributed in the plant kingdom, catalyzes the oxidation of ascorbate by oxygen to the final products dehydroascorbate and water . Yama-zaki and Piette [2,3] showed that the catalyzed reaction proceeds first to an ascorbyl radical, which dismutes spontaneously to ascorbate and dehydroascorbate. They also found that the enzyme has the same maximal velocity with reductate or ascorbate as donor substrate . Nakamura et al.  have shown that double reciprocal plots for ascorbate oxidase with ascorbate as the varied substrate at different oxygen concentrations are a family of parallel straight lines. This pattern implies that the points of entry of the reduc-tant and oxidant substrates into the catalytic cycle are separated by a reaction that is irreversible under initial velocity conditions. The results of stopped flow experiments by Nakamura and Ogura [5,6] have been cited as evidence that oxidation of the reduced enzyme by molecular oxygen is intrinsically a faster process than reduction of the oxidized enzyme by donor substrates . Gerwin et al.  showed that the active form of the reductant substrate is the monoanion.
KeywordsFormal Mechanism Maximal Velocity Catalytic Cycle Enzymic Form Double Reciprocal Plot
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