What is it about?
Ethylene is important in industry and biological signaling. In plants, ethylene is produced by oxidation of 1-aminocyclopropane-1-carboxylic acid, as catalyzed by 1-aminocyclopropane-1-carboxylic acid oxidase. Bacteria catalyze ethylene production, but via the four-electron oxidation of 2-oxoglutarate to give ethylene in an arginine-dependent reaction. Crystallographic and biochemical studies on the Pseudomonas syringae ethylene-forming enzyme reveal a branched mechanism. In one branch, an apparently typical 2-oxoglutarate oxygenase reaction to give succinate, carbon dioxide, and sometimes pyrroline-5-carboxylate occurs. Alternatively, Grob-type oxidative fragmentation of a 2-oxoglutarate–derived intermediate occurs to give ethylene and carbon dioxide. Crystallographic and quantum chemical studies reveal that fragmentation to give ethylene is promoted by binding of l-arginine in a nonoxidized conformation and of 2-oxoglutarate in an unprecedented high-energy conformation that favors ethylene, relative to succinate formation.
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Why is it important?
The plant-signaling molecule ethylene is biosynthesized from 1-aminocyclopropane-1-carboxylic acid (ACC), as catalyzed by ACC oxidase, which is homologous to the 2-oxoglutarate (2OG) oxygenases, but which does not use a 2OG cosubstrate. Bacteria produce ethylene in a highly unusual reaction that involves oxidative 2OG fragmentation. Biophysical studies on a Pseudomonas ethylene-forming enzyme (EFE) reveal how structural and stereoelectronic factors enable the EFE to bias reaction away from normal 2OG oxygenase catalysis involving two-electron substrate oxidation concomitant with succinate formation, toward the arginine-dependent four-electron oxidation of 2OG to give ethylene. The results imply that negative catalysis, with respect to ethylene formation, has operated during the evolution of 2OG oxygenases and will be useful in protein engineering aimed at optimizing ethylene production.
Perspectives
Ethylene is of industrial importance and is a vital signaling molecule in plants, where it has roles in germination, senescence, and stress responses. Bacteria engineered to produce ethylene using the Pseudomonas syringae pv. phaseolicola ethylene-forming enzyme (PsEFE) have been developed to ripen fruit as an alternative to the use of synthetic ethylene. We describe biochemical, structural, and modeling studies supporting a branched mechanistic pathway for PsEFE that can lead either to ethylene via oxidative fragmentation of 2OG or to succinate via a more typical 2OG oxygenase reaction, which sometimes results in P5C formation.
Martine I Abboud
University of Oxford
Read the Original
This page is a summary of: Structural and stereoelectronic insights into oxygenase-catalyzed formation of ethylene from 2-oxoglutarate, Proceedings of the National Academy of Sciences, April 2017, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.1617760114.
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