Aminocyclopropanecarboxylate oxidase explained

Aminocyclopropanecarboxylate oxidase
Ec Number:	1.14.17.4

In enzymology, an aminocyclopropanecarboxylate oxidase is an enzyme that catalyzes the chemical reaction

1-aminocyclopropane-1-carboxylate + ascorbate + O₂

\rightleftharpoons ethylene + cyanide + dehydroascorbate + CO₂ + 2 H₂O

The 3 substrates of this enzyme are 1-aminocyclopropane-1-carboxylate, ascorbate, and O₂, whereas its 5 products are ethylene, cyanide, 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

• Zhang Z, Schofield CJ, Baldwin JE, Thomas P, John P . Expression, purification and characterization of 1-aminocyclopropane-1-carboxylate oxidase from tomato in Escherichia coli . Biochem. J. . 307 . 77 - 85 . 7717997 . 1136747 . Pt 1 . 1995 . 10.1042/bj3070077 .
• Zhang Z, Barlow JN, Baldwin JE, Schofield CJ . 1997 . Metal-catalyzed oxidation and mutagenesis studies on the iron(II) binding site of 1-aminocyclopropane-1-carboxylate oxidase . Biochemistry . 36 . 15999 - 6007 . 9398335 . 10.1021/bi971823c . 50 .
• Pirrung MC . 1999 . Ethylene biosynthesis from 1-aminocyclopropanecarboxylic acid . Acc. Chem. Res. . 32 . 8 . 711 - 718 . 10.1021/ar960003 .
• Charng YY, Chou SJ, Jiaang WT, Chen ST, Yang SF . 2001 . The catalytic mechanism of 1-aminocyclopropane-1-carboxylic acid oxidase . Arch. Biochem. Biophys. . 385 . 179 - 85 . 11361015 . 10.1006/abbi.2000.2138 . 1 .
• Thrower JS, ((Blalock R III)), Klinman JP . 2001 . Steady-state kinetics of substrate binding and iron release in tomato ACC oxidase . Biochemistry . 40 . 9717 - 24 . 11583172 . 10.1021/bi010329c . 32 .
• Pirrung MC, Kaiser LM, Chen J . Purification and properties of the apple fruit ethylene-forming enzyme . Biochemistry . 32 . 29 . 7445–50 . July 1993 . 8338842 . 10.1021/bi00080a015 .
• Murphy LJ, Robertson KN, Harroun SG, Brosseau CL, Werner-Zwanziger U, Moilanen J, Tuononen HM, Clyburne JA . A simple complex on the verge of breakdown: isolation of the elusive cyanoformate ion . Science . 344 . 6179 . 75–8 . 2014 . 24700853 . 10.1126/science.1250808 . 2014Sci...344...75M . 37086909 .