The pathway for producing ethylene
New research lays out the chemical steps used by a naturally occurring enzyme to convert a common chemical compound into ethylene—a plant hormone important for fruit ripening and an industrial chemical used in the production of plastics and textiles. A paper describing the research by scientists at Penn State appears online Aug. 12 in the journal Science.
"Because ethylene is so important in the manufacturing industry for making plastics, solvents and textiles, it is one of the most abundantly produced compounds on Earth," said Rachelle Copeland, a recent doctoral graduate from Penn State and first and co-corresponding author of the paper. "Currently, petroleum is our main source of ethylene for these uses. However, plants and some microbes produce ethylene naturally. Understanding the step-by-step chemical process used by these plants and microbes could help us move away from petroleum-based ethylene production."
The aptly named "ethylene-forming enzyme (EFE)" is able to transform a common chemical compound—2-oxoglutarate, which is found in almost all organisms where it plays a role in metabolism—into ethylene, but researchers had been unable to precisely characterize the mechanism employed by the enzyme. The reaction required for this transformation is fundamentally different from reactions driven by enzymes closely related to EFE.
Enzymes are proteins that initiate or speed up the chemical reactions necessary to sustain life, most of which require atoms, clusters of atoms, or small molecules—collectively known as cofactors—to make these reactions happen. EFE belongs to a class of enzymes that promote reactions of various types of molecules with oxygen, enabled by an iron cofactor and 2-oxoglutarate co-substrate.
"Our lab group has been studying enzymes related to EFE for close to 20 years," said Carsten Krebs, professor of chemistry and of biochemistry and molecular biology at Penn State and an author of the paper. "EFE is unique amongst this family of enzymes because it breaks down 2-oxoglutarate in two different ways. The first is well characterized, but the second, the one that produces ethylene, has been a mystery until now."
The research team dissected the chemical pathway for ethylene formation by EFE by inserting isotopes—atoms that differ in atomic weight and can be traced as the reaction is in progress—into the various products. In this way the team could track individual atoms to see where they go over the course of the reaction. Separately, they also made chemical modifications to both the enzyme and the 2-oxoglutarate to see how the reaction and products were altered.
"Using these techniques, we could see that EFE initiates the reaction between 2-oxoglutarate and oxygen in a very different way from other related enzymes," said Copeland. "It inserts the oxygen between two carbon atoms of 2-oxoglutarate, which produces a unique intermediate compound that the enzyme then breaks down into ethylene."
The location of the inserted oxygen atom had been computationally predicted but had not been shown experimentally until now.
"There have been several mechanisms proposed over the years to explain how EFE converts 2-oxoglutarate into ethylene, but there have been no experimental data to distinguish among them," said J. Martin Bollinger Jr., professor of chemistry and of biochemistry and molecular biology at Penn State and an author of the paper. "Rachelle designed these experiments to look at the most fundamental aspects of the reaction. Where do the individual atoms go? And it maps out an unmistakably clear mechanism."