The nature of O ₂ activation by the ethylene-forming enzyme 1-aminocyclopropane-1-carboxylic acid oxidase

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 O 2 activation process by ACCO was investigated using steady-state kinetics, solvent isotope effects (SIEs), and competitive oxygen kinetic isotope effects ( 18 O 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 18 2 His, 1-Asp proteins ͉ high-valent iron oxo species ͉ oxygen-18 kinetic isotope effects ͉ nonheme iron enzymes E thylene is a major plant hormone important in many aspects of plant growth and development such as germination, fruit ripening, and senescence (1). The ability to control ethylene formation in a time-dependent manner would have far-reaching economic, agricultural, and environmental implications. The last step in the biosynthesis of ethylene, the two electron oxidation of 1-aminocyclopropane-1-carboxylic acid (ACC) to ethylene, CO 2 , and HCN, is catalyzed by ACC oxidase (ACCO) (2-4). This reaction also requires the concomitant reduction of O 2 to water and the oxidation of ascorbate to dehydroascorbate (Fig. 1), whereas CO 2 (or bicarbonate) has been shown to act as an activator for ethylene formation (5).ACCO belongs to the family of O 2 -activating nonheme iron enzymes. Although the sequence homology among these enzymes is not high, all their active sites contain a single ferrous ion bound in a tridendate ligand arrangement referred to as a ''2-His-1-carboxylate facial triad'' (4). A crystal structure of ACCO has been reported (6), revealing a solvent-exposed active site and confirming the ligation of the iron center (His-177, Asp-179, and His-234) inferred from mutagenesis studies (7), and the general jellyroll motif found in other nonheme iron enzymes (8,9).What distinguishes ACCO from the other enzymes of this family is the two-electron donating cosubstrate needed for reducing O 2 to water: ascorbate in the case of ACCO (10-13) and ␣-ketoglutarate (␣KG) in almost all other known examples (4). The mechanism of ␣KG-dependent enzymes has been studied extensively, and it generally is accepted that O 2 activation at the iron center is linked to oxidative decarboxylation of the ␣KG, ultimately forming an Fe IV AO species as the reactive intermediate (14)(15)(16)(17). This high-valent Fe/O 2 species functions as a ''generic'' oxidant for a wide range of oxidation/oxygenation reactions.In contrast to the ␣KG-dependent enzymes, where ␣KG rather than substrate binds to the metal center, spectroscopic studies of ACCO have shown that ACC coordinates to the iron center via both its amino and carboxylate groups (10,18).† This finding implies a distinctly different mode for reductant interaction with ACCO than for most other nonheme iron enzymes. Magnetic circular dichroism ...

A Simple Complex on the Verge of Breakdown: Isolation of the Elusive Cyanoformate Ion

“…It is not definitively known whether this decomposition occurs at the active site or elsewhere (5). Bicarbonate has been identified as an essential mediator for proton transfer in the process (6)(7)(8). It remains unclear as to how the iron-containing active site of ACC oxidase is protected from the cyanide by-product, a known deactivating agent of iron-containing enzymes in general and Fe(III) in particular (9).…”

De novo transcriptome assembly of sugarcane leaves submitted to prolonged water-deficit stress

“…Number 1.14.17.4 refers to the enzyme aminocyclopropanecarboxylate oxidase. of ethylene, a hormone that plays an important role in different stages of plant growth and development such as the germination, maturation, and senescence of fruits (Mirica and Klinman, 2008). In tomato, ACC oxidase accumulates in various tissues during different stages of development and during leaf, fruit, and flower senescence (Barry et al, 1996).…”

Cloning and Identification of an ACO Gene Involved in Ethylene Production Pathway from Ginkgo biloba

“…The Fe2OG dioxygenases catalyze a variety of twoelectron oxidations, including hydroxylations, desaturations, and oxidative ring closures (Costas et al, 2004). Although the sequence homology among these enzymes is not high, all their active sites contain a single ferrous ion bound in a tridentate ligand arrangement referred to as a "2-His-1-carboxylate facial triad" (Mirica and Klinman, 2008). Secondary structure analysis of GbACO protein by SOMPA revealed that GbACO consists of α-helix (40.43%), β-turn (10.33%), extended strand (13.07%), and random coil (36.17%).…”

“…Under conditions of high ethylene production, the pathway can also be regulated at the level of conversion of ACC into ethylene by ACO (Vanderstraeten and Van Der Straeten, 2017). ACO enzyme belongs to the family of oxidoreductases which utilize Fe (II) as a cofactor and 2-oxoglutarases (2OG) as cosubstrate, although ACO uses ascorbate as a co-substrate (Prescott, 1993;Ryle and Hanusinger, 2002;Zhang et al, 2004;Mirica and Klinman, 2008). ACO needs bicarbonate as an activator and catalyzes the oxidation of ACC to give ethylene, CO 2 , and HCN.…”