In
enzymology
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. A ...
, an aminocyclopropanecarboxylate oxidase () is an
enzyme that
catalyzes
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
the
chemical reaction
:1-aminocyclopropane-1-carboxylate + ascorbate + O
2 ethylene + cyanide + dehydroascorbate + CO
2 + 2 H
2O
The 3
substrates of this enzyme are
1-aminocyclopropane-1-carboxylate
1-Aminocyclopropane-1-carboxylic acid (ACC) is a disubstituted cyclic α- amino acid in which a cyclopropane ring is fused to the C atom of the amino acid. It is a white solid. Many cyclopropane-substituted amino acids are known, but this one oc ...
,
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
O2, whereas its 5
products are
ethylene
Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon-carbon double bonds).
Ethylene i ...
,
cyanide
Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms.
In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of a ...
,
dehydroascorbate,
CO2, and
H2O.
This enzyme belongs to the family of
oxidoreductases, specifically those acting on paired donors, with O2 as oxidant and incorporation or reduction of oxygen. The oxygen incorporated need not be derived from O2 with reduced ascorbate as one donor, and incorporation of one atom of oxygen into the other donor. The
systematic name of this enzyme class is 1-aminocyclopropane-1-carboxylate oxygenase (ethylene-forming). Other names in common use include ACC oxidase, and ethylene-forming enzyme.
Structural studies
As of late 2007, two
structures have been solved for this class of enzymes, with
PDB accession codes and .
Reaction Mechanism
Mechanistic and structural studies support binding of ACC and oxygen to an iron center located in the active site of ACC oxidase. The ring-opening of bound ACC is believed to result in the elimination of ethylene together with an unstable intermediate, cyanoformate ion, which then decomposes to cyanide ion and carbon dioxide. Cyanide ion is a known deactivating agent for iron-containing enzymes, but the cyanoformate ion intermediate is believed to play a vital role to carry potentially toxic cyanide away from the active site of ACC oxidase. Cyanoformate was recently identified in condensed media as a tetraphenylphosphonium salt with a weak carbon-carbon bond.
References
*
*
*
*
*
*
*
EC 1.14.17
Enzymes of known structure
{{1.14-enzyme-stub