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Ascorbate peroxidase

Ascorbate peroxidase (or APX) is a member of the family of heme-containing peroxidases. Heme peroxidases catalyse the H2O2-dependent oxidation of a wide range of different, usually organic, substrates in biology.

Overview

Ascorbate-dependent peroxidase activity was first reported in 1979,, more than 150 years after the first observation of peroxidase activity in horseradish plantsPlanche, LA. (1810) Bull. Pharm., 2, 578 and almost 40 years after the discovery of the closely related cytochrome c peroxidase enzyme. Peroxidases have been classified into three type (class I, class II and class III): ascorbate peroxidases is a class I peroxidase enzyme. APXs catalyse the H2O2-dependent oxidation of ascorbate in plants, algae and certain cyanobacteria. APX has high sequence identity to cytochrome c peroxidase, which is also a class I peroxidase enzyme. Under physiological conditions, the immediate product of the reaction, the monodehydroascorbate radical, is reduced back to ascorbate by a monodehydroascorbate reductase ( monodehydroascorbate reductase (NADH)) enzyme. In the absence of a reductase, two monodehydroascorbate radicals disproportionate rapidly to dehydroascorbic acid and ascorbate. APX is an integral component of the glutathione-ascorbate cycle.

Substrate specificity

APX enzymes show high specificity for ascorbate as electron donor, but most APXs will also oxidise other organic substrates that are more characteristic of the class III peroxidases (such as horseradish peroxidase), in some cases at rates comparable to that of ascorbate itself. This means that defining an enzyme as an APX is not straightforward, but is usually applied when the specific activity for ascorbate is higher than that for other substrates.

Mechanism

Most of the information on mechanism comes from work on the pea cytosolic and soybean cytosolic enzymes. The mechanism of oxidation of ascorbate is achieved by means of an oxidized Compound I intermediate, which is subsequently reduced by substrate in two, sequential single electron transfer steps (equations 13, where HS = substrate and S• = one electron oxidised form of substrate). APX + H2O2 → Compound I + H2O 1 Compound I + HS → Compound II + S• 2 Compound II + HS → APX + S• + H2O 3 In ascorbate peroxidase, Compound I is a transient (green) species and contains a high-valent iron species (known as ferryl heme, FeIV) and a porphyrin pi-cation radical,, as found in horseradish peroxidase. Compound II contains only the ferryl heme.

Structural information

The structure of pea cytosolic APX was reported in 1995. The binding interaction of soybean cytosolic APX with its physiological substrate, ascorbate, and with a number of other substrates are also known.

Applications in cellular imaging

Both pea APX and soybean APX have been used in electron microscopy studies for cellular imaging.

See also

References

External links

"green air" © 2007 - Ingo Malchow, Webdesign Neustrelitz
This article based upon the http://en.wikipedia.org/wiki/Ascorbate_peroxidase, the free encyclopaedia Wikipedia and is licensed under the GNU Free Documentation License.
Further informations available on the list of authors and history: http://en.wikipedia.org/w/index.php?title=Ascorbate_peroxidase&action=history
presented by: Ingo Malchow, Mirower Bogen 22, 17235 Neustrelitz, Germany