The nature of O2 activation by the ethylene-forming enzyme 1-aminocyclopropane-1-carboxylic acid oxidase.
Ethylene is a plant hormone important in many aspects of plant growth and development such as germination, fruit ripening, and senescence. 1-Aminocyclopropane-1-carboxylic acid (ACC) oxidase (ACCO), an O2-activating ascorbate-dependent nonheme iron enzyme, catalyzes the last step in ethylene biosynthesis. The O2 activation process by ACCO was investigated using steady-state kinetics, solvent isotope effects (SIEs), and competitive oxygen kinetic isotope effects (18O KIEs) to provide insights into the nature of the activated oxygen species formed at the active-site iron center and its dependence on ascorbic acid. The observed large 18O KIE of 1.0215 +/- 0.0005 strongly supports a rate-determining step formation of an Fe(IV) O species, which acts as the reactive intermediate in substrate oxidation. The large SIE on kcat/Km(O2) of 5.0 +/- 0.9 suggests that formation of this Fe(IV) O species is linked to a rate-limiting proton or hydrogen atom transfer step. Based on the observed decrease in SIE and 18O KIE values for ACCO at limiting ascorbate concentrations, ascorbate is proposed to bind in a random manner, depending on its concentration. We conclude that ascorbate is not essential for initial O2 binding and activation but is required for rapid Fe(IV) O formation under catalytic turnover. Similar studies can be performed for other nonheme iron enzymes, with the 18O KIEs providing a kinetic probe into the chemical nature of Fe/O2 intermediates formed in the first irreversible step of the O2 activation.
Mirica
Liviu M
LM
Department of Chemistry, University of California, Berkeley, CA 94720, USA.
Klinman
Judith P
JP
D002384:Catalysis; D005030:Ethylenes / Q000096:biosynthesis*; D005971:Glutamates / Q000378:metabolism*; D007700:Kinetics; D010100:Oxygen / Q000378:metabolism*; D010937:Plant Growth Regulators / Q000096:biosynthesis*; D010944:Plants / Q000201:enzymology
D005030:Ethylenes; D005971:Glutamates; D010937:Plant Growth Regulators; C066302:2,4-methanoglutamate; C036216:ethylene; D010100:Oxygen