Aminocyclopropanecarboxylate oxidase

In enzymology, an aminocyclopropanecarboxylate oxidase (EC 1.14.17.4) is an enzyme that catalyzes the chemical reaction

1-aminocyclopropane-1-carboxylate + ascorbate + O₂ 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 1W9Y and 1WA6.

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 (Pt 1): 77–85. PMC 1136747. PMID 7717997. 
• 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 (50): 15999–6007. doi:10.1021/bi971823c. PMID 9398335. 
• Pirrung MC (1999). "Ethylene biosynthesis from 1-aminocyclopropanecarboxylic acid". Acc. Chem. Res. 32: 711–718. doi: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 (1): 179–85. doi:10.1006/abbi.2000.2138. PMID 11361015. 
• Thrower JS, Blalock R 3rd Klinman JP (2001). "Steady-state kinetics of substrate binding and iron release in tomato ACC oxidase". Biochemistry. 40 (32): 9717–24. doi:10.1021/bi010329c. PMID 11583172. 
• Pirrung MC, Kaiser LM, Chen J (July 1993). "Purification and properties of the apple fruit ethylene-forming enzyme". Biochemistry 32 (29): 7445–50. doi:10.1021/bi00080a015. PMID 8338842. 
• Murphy LJ, Robertson KN, Harroun SG, Brosseau CL, Werner-Zwanziger U, Moilanen J, Tuononen HM, Clyburne JA (2014). "A simple complex on the verge of breakdown: isolation of the elusive cyanoformate ion". Science 344 (6179): 75–8. doi:10.1126/science.1250808. PMID 24700853.