Stereospecific Conversion of 1-Aminocyclopropanecarboxylic Acid to Ethylene by Plant Tissues

Abstract

Inasmuch as the molecule of 1-aminocyclopropanecarboxylic acid (ACC) possesses reflective symmetry but lacks rotational symmetry, the two chemically alike methylene groups can be distinguished by a stereospecific enzyme. To determine whether ACC conversion to ethylene by plant tissues proceeds in a stereospecific fashion, the four stereoisomers of 1-amino-2-ethylcyclopropanecarboxylic acid (AEC) were administered to postclimacteric apple (Malus sylvestris Mill., var. Golden Delicious), excised preclimacteric cantaloupe (Cucumis melo L., var. reticulatis Naud cv. PMR45), and etiolated mung bean ( Vigna radiata L., Wilczek, var. Berken) hypocotyls. In each case (lR,2S)-AEC was the preferred substrate yielding 1-butene. In contrast, all AEC isomers were converted equally well to butene by chemical oxidation using NaOCI. Both ACC and AEC appear to be substrates for the same enzyme since both reactions are inhibited in parallel by N2 or Co2+, both reactions are induced in parallel by excision, and when both substrates are present simultaneously each will act as an inhibitor with respect to the other. The aforementioned observations indicate that ACC is stereospecifically converted to ethylene. For AEC to be the most active precursor of 1-butene, the ethyl substituent should be trans to the carboxyl group and the pro-(S) methylene group should be unsubstituted. This observation leads to the suggestion that the enzyme interacts with amino, carboxyl, and pro-(S) methylene groups, a configuration corresponding to a L-amino acid. This view is consistent with the observation that the L-forms of alanine and methionine inhibit the conversion of ACC to ethylene more than the corresponding 1-amino acids in the mung bean hypocotyl system. recently reported an enzymic conversion of ACC to ethylene by a pea seedling extract, some characteristics of their system appear to us not to resemble the natural system. Based on in vivo observations, it is shown that EFE is present widely in plant tissues (1,3,6,7,9,14,17,24), requires 02 in the conversion (1), and is inhibited by high temperature (23), uncouplers of oxidative phosphorylation (23), Co2, (24), and osmotic shock (3). ACC is also known to be readily converted to ethylene nonenzymically by various chemicals (4, 13). This leads to the question of whether the conversion of ACC to ethylene by plant tissues is mediated by a nonenzymic chemical reaction.The ACC molecule contains two methylene groups and possesses reflective symmetry but lacks rotational symmetry. Consequently, the two chemically alike methylene groups can be distinguished by a stereospecific enzyme. Ethyl substitution of each of the four methylene hydrogens of ACC results in four stereoisomers of AEC whose absolute configurations are: (1 R, 2R), (1S, 2S), (IR, 2S), and (1S, 2R) (10, 11). (IS, 2S)-AEC has recently been identified as a hydrolysis product of coronatine, which is a toxin produced by a bacteria, Pseudomonas coronafaciens var. atropurpurea, and induces chlorosis in Italian ryegrass (15). Using the...

Stimulation of Ethylene Production in Bean Leaf Discs by the Pseudomonad Phytotoxin Coronatine

“…This is further supported by the results reported below, and the results ofHoffman et al (8), who showed that an isomer of one hydrolysis product of coronatine, AEC (coronamic acid), was converted to 1-butene through the same enzyme system as used for ACC conversion. AEC did not itself (16,19).…”

“…However, we found no evidence of ethylene stimulation in bean leaf discs when they were treated with either hydrolysis product (Table V). Furthermore, the configuration of our AEC as obtained from the hydrolysis of coronatine (10) would not be the same as that ofthe stereoisomer which Hoffman et al (8) found to act as substrate for the ethylene-forming enzyme. There is further evidence that the functional structure involves the intact molecule.…”

“…1) has a structural region (AEC) which is similar to ACC, an intermediate in the biosynthetic pathway of ethylene in plants. Recent work by Hoffman et al (8) shows that a diastereoisomer of AEC (coronamic acid) is converted enzymically to 1-butene, probably by the same enzyme system that converts ACC to ethylene. These structural similarities and analogies indicated the possibility that coronatine in vivo may interfere with the regulation of ethylene production.…”

Age-Dependent Discrimination between Stereoisomers of 1-Amino-2-Ethylcyclopropane-1-Carboxylic Acid in Carnation Petals

“…Stereodiscrimination between isomers of AEC has also been reported in pea vacuoles (2), pea epicotyls, senescing carnation petals and olive leaves (6). The results ofinhibition studies (3) and competition studies (2) led to the conclusion that both the conversion of ACC to ethylene and of (1 R,2S)-AEC to 1-butene are mediated by the same enzyme. Thus, the ability to discriminate between stereoisomers of AEC became an essential criterion for physiological relevance in studies evaluating enzyme systems capable of converting ACC to ethylene (8).…”

“…Hoffman et al (3) were the first to demonstrate the discriminatory behavior of EFE2 with regard to the oxidation of the four stereoisomers of AEC. In postclimacteric apples, preclimacteric cantaloupe and etiolated mung bean hypocotyls, only (1R,2S)-AEC was converted to 1-butene.…”