The nature of O2 activation by the ethylene-forming enzyme 1-aminocyclopropane-1-carboxylic acid oxidase

Liviu M. Mirica, Judith P. Klinman

Research output: Contribution to journalArticlepeer-review

Abstract

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 O 2-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 FeIV=O species, which acts as the reactive intermediate in substrate oxidation. The large SIE on k cat/Km(O2) of 5.0 ± 0.9 suggests that formation of this FeIV=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 O 2 binding and activation but is required for rapid FeIV=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.

Original languageEnglish (US)
Pages (from-to)1814-1819
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Volume105
Issue number6
DOIs
StatePublished - Feb 12 2008
Externally publishedYes

Keywords

  • 2 His, 1-Asp proteins
  • High-valent iron oxo species
  • Nonheme iron enzymes
  • Oxygen-18 kinetic isotope effects

ASJC Scopus subject areas

  • General

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