Ascorbate peroxidase (or L-ascorbate peroxidase, APX) () is an

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...

that catalyzes the

chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking ...

:L-ascorbate + H₂O₂

\rightleftharpoons

dehydroascorbate + 2 H₂O It is a member of the family of

heme Heme, or haem (pronounced / hi:m/ ), is a precursor to hemoglobin, which is necessary to bind oxygen in the bloodstream. Heme is biosynthesized in both the bone marrow and the liver. In biochemical terms, heme is a coordination complex "consis ...

-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

oxidoreductase In biochemistry, an oxidoreductase is an enzyme that catalyzes the transfer of electrons from one molecule, the reductant, also called the electron donor, to another, the oxidant, also called the electron acceptor. This group of enzymes usually ...

s, specifically those acting on a peroxide as acceptor (peroxidases). The

systematic name A systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance, out of a specific population or collection. Systematic names are usually part of a nomenclature. A semisystematic name or semitrivial ...

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

ascorbate Vitamin C (also known as ascorbic acid and ascorbate) is a water-soluble vitamin found in citrus and other fruits and vegetables, also sold as a dietary supplement and as a topical 'serum' ingredient to treat melasma (dark pigment spots) and ...

and aldarate metabolism.

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 Cytochrome ''c'' peroxidase, or CCP, is a water-soluble heme-containing enzyme of the peroxidase family that takes reducing equivalents from cytochrome ''c'' and reduces hydrogen peroxide to water: :CCP + H2O2 + 2 ferrocytochrome ''c'' + 2H+ � ...

enzyme. Peroxidases have been classified into three types (class I, class II and class III): ascorbate peroxidases is a class I peroxidase enzyme. APXs catalyse the H₂O₂-dependent oxidation of

in plants, algae and certain cyanobacteria. APX has high sequence identity to

, 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

. 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 (such as

horseradish peroxidase The enzyme horseradish peroxidase (HRP), found in the roots of horseradish, is used extensively in biochemistry applications. It is a metalloenzyme with many isoforms, of which the most studied type is C. It catalyzes the oxidation of various o ...

), 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 (equations �� where HS = substrate and S^• = one electron oxidised form of substrate). :APX + H₂O₂ → Compound I + H₂O :Compound I + HS → Compound II + S^• :Compound II + HS → APX + S^• + H₂O In ascorbate peroxidase, Compound I is a transient (green) species and contains a

high-valent iron High-valent iron commonly denotes compounds and intermediates in which iron is found in a formal oxidation state > 3 that show a number of bonds > 6 with a coordination number ≤ 6. The term is rather uncommon for hepta-coordinate compounds of ...

species (known as ferryl heme, Fe^IV) and a

porphyrin Porphyrins ( ) are a group of heterocyclic macrocycle organic compounds, composed of four modified pyrrole subunits interconnected at their α carbon atoms via methine bridges (=CH−). The parent of porphyrin is porphine, a rare chemical com ...

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

Applications in cellular imaging

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

References

External links

EC 1.11.1.11

L-ascorbate peroxidase (EC-Number 1.11.1.11 )
{{Portal bar, Biology, border=no EC 1.11.1 Enzymes of known structure