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In enzymology, a L-ascorbate oxidase is an enzyme that catalyzes the chemical reaction


 * 2 L-ascorbate + O2 $$\rightleftharpoons$$ 2 dehydroascorbate + 2 H2O

Thus, the two substrates of this enzyme are L-ascorbate and O2, whereas its two products are dehydroascorbate and H2O.

Function
This enzyme belongs to the family of oxidoreductases, specifically those acting on diphenols and related substances as donor with oxygen as acceptor. This enzyme participates in ascorbate metabolism. It employs one cofactor, copper.

Nomenclature
The systematic name of this enzyme class is L-ascorbate:oxygen oxidoreductase. Other names in common use include ascorbase, ascorbic acid oxidase, ascorbate oxidase, ascorbic oxidase, ascorbate dehydrogenase, L-ascorbic acid oxidase, AAO, L-ascorbate:O2 oxidoreductase, and AA oxidase.

L-ascorbate Oxidase

 * multi-copper oxidase, part of "blue" oxidase group
 * found in many plants and animals, reported instances in microorganisms
 * catalyzes the oxidation of ascorbic acid (vitamin C), defence against oxidative stress
 * plants have two times enzyme activity in leaves compared to stem
 * Enzyme can be isolated from many plant sources with extensive reports of isolation from zucchini and squash (ease of characterization)
 * Very stong Cu-protein bond at physiological pH
 * catalytic cycle: single electron transfer from ascorbate ion reducing agent at one site to dioxygen
 * intramolecular electron transfer from T1[Cu(I)] to T3[Cu(II)] essential for total of 4 electron electron transfer to dioxygen producing two water molecules
 * Important ligands in mechanism: Cys 507 (thiolate at T1 site), His 506, and His 508 (imidazole chains at T3 site)
 * dioxygen binding and reduction site at three copper nuclear centre of enzyme
 * after transport of ascorbic acid to apoplast of plant cells, AO reduces AA to monodehydrascorbate (MDHA)
 * MDHA unstable radical disproportionates to form DHA (in above chem rxn given)
 * AO believed to play a role in cell elongation due to DHA production (DHA loosens cell walls and correl. to growth of cells)