Ascorbate peroxidase


Ascorbate peroxidase is an enzyme that catalyzes the chemical reaction
It 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.
This enzyme belongs to the family of oxidoreductases, specifically those acting on a peroxide as acceptor. The systematic name of this enzyme class is L-ascorbate:hydrogen-peroxide oxidoreductase. Other names in common use include L-ascorbic acid peroxidase, L-ascorbic acid-specific peroxidase, ascorbate peroxidase, and ascorbic acid peroxidase. This enzyme participates in the ascorbate and aldarate metabolism. APXs are important in cellular antioxidant networks in photosynthetic organisms; they are the primary component of the ascorbate-glutathione cycle and are important for peroxide scavenging and redox signaling.

Reaction

In the catalytic cycle, the immediate one-electron oxidized product is monodehydroascorbate. MDHA is either enzymatically reduced back to ascorbate by monodehydroascorbate reductase or two MDHA molecules disproportionate to ascorbate and dehydroascorbate.

Overview

Ascorbate-dependent peroxidase activity was first reported in 1979, more than 150 years after the first observation of peroxidase activity in horseradish plants and almost 40 years after the discovery of the closely related cytochrome c peroxidase enzyme.
Peroxidases have been classified into three types : ascorbate peroxidases is a class I peroxidase enzyme. APXs catalyze 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 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 an electron donor, but most APXs will also oxidise other organic substrates that are more characteristic of the class III peroxidases, 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. Some proteins from the APX family lack the ascorbate-binding amino acid residues suggesting that they might oxidize other molecules than ascorbate.

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.
APX follows the typical heme-peroxidase mechanism with high-valent iron intermediates:
  1. Formation of Compound I: APX reacts with H2O2 to form Compound I - where the heme is oxidized to Fe4+ = O. This produces a porphyrin pi-organic cation radical.
  2. # APX + H2O2 → Compound I + H2O
  3. Formation of Compound II: Through a one electron reduction, Compound I is reduced by substrate to form Compound II; Compound II accepts a second electron from ascorbate to regenerate the ferric resting state. This is s sequential single-electron transfer steps.
  4. # Compound I + HS → Compound II + S•
  5. # Compound II + HS → APX + S• + H2O
In ascorbate peroxidase, Compound I is a transient species and contains a high-valent iron species and a porphyrin pi-cation radical, as found in horseradish peroxidase. Compound II contains only the ferryl heme. Spectroscopic and kinetic work on plant APXs supports these intermediates and sequential one electron transfers.

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.
As of late 2007, 12 structures have been solved for this class of enzymes, with PDB accession codes,,,,,,,,,,, and.