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New discovery: if it weren't for this enzyme, decomposing pesticide would take millennia

October 25, 2005

CHAPEL HILL - An enzyme inside a bacterium that grows in the soil of potato fields can - in a split second - break down residues of a common powerful pesticide used for killing worms on potatoes, researchers have found.

That may be expensive for farmers but lucky for the environment because University of North Carolina at Chapel Hill scientists have now discovered that if that particular enzyme weren't there, it would take 10,000 years for just half of the widely used pesticide to decompose. And the chemical would remain in the soil of the potato fields where it is now used in colossal amounts, contaminating groundwater and posing a threat to human and animal health.

A report on the unusual discovery appears online in the Proceedings of the National Academy of Sciences Monday (Oct. 24). Authors are Christopher M. Horvat, a UNC chemistry major from Spruce Pine, who plans to become a physician, and Dr. Richard V. Wolfenden, Alumni Distinguished professor of biochemistry and biophysics at the UNC School of Medicine.

"The half-life of the pesticide is longer, by several orders of magnitude, than the half-lives of other known environmental pollutants in water," Wolfenden said. "The half lives of atrazine, aziridine, paraoxon and 1, 2-dichloroethane, for example, are five months, 52 hours, 13 months and 72 years, respectively."

In contrast, the half-life of the potato pesticide residue chloroacrylate - 10,000 years - matches the half-life of plutonium-239, the hazardous isotope produced in nuclear power plants, he said.

The bacteria Pseudomonas pavonaceae have evolved in the soil in which the potato pesticide 1, 3-dichloropropene is used and can grow on it as their only source of carbon and energy, the scientist said. The enzyme responsible for degrading the pesticide may have evolved since the chemical's first use on potato fields in 1946. Common names for the agricultural product are Shell D-D and Telone II.

"There is also a possibility, which I consider strong, that this surprising enzyme may have already existed in the bacteria and that it catalyzes another, so-far unidentified reaction that these bacteria require for normal metabolism," Wolfenden said. "The apparently novel catalytic activity of the enzyme may be a lucky side reaction of an enzyme that evolved to act on some natural substance yet to be identified."

Horvat carried out the work in Wolfenden's laboratory by analyzing what happened to the pesticide's residue at various temperatures and then extrapolating the results to room temperature to see how long the pesticide would last if the bacteria weren't busy digesting it in the blink of an eye.

"It was just amazing that this enzyme can degrade something so quickly when otherwise it would take thousands of years," Horvat said. "It's really a neat picture of what evolution and natural selection can do."

Although it's hard to predict, the work may have implications for people designing enzymes later on, he said. Using enzymes in reactions can greatly reduce the cost of a lot of chemical processes.

Finding the enzyme in bacteria in fields never before exposed to the pesticide Shell D-D would demonstrate that that it had not evolved in the past 50 years, Wolfenden said.

"What is also remarkable, and unexpected, is that in the bacteria that contain this 'new' enzyme, CaaD, there is another enzyme, tautomerase, that has a structure similar to that of CaaD and catalyzes a reaction that's involved in conventional metabolism," he said. "So it's thought that tautomerase and CaaD may have a common evolutionary origin. The surprise is that the 'new ' enzyme is better at catalyzing this new reaction than the 'old' enzyme is at catalyzing that conventional reaction."

If the enzyme did appear in just the past 50 years, that would be extraordinary example of the "majestic hand of evolution at work," Wolfenden said.

For an undergraduate to publish a paper in such a prestigious scientific journal as the Proceedings of the National Academy of Sciences also is quite unusual, he said. That success in part reflects UNC's continuing efforts to involve undergraduates in cutting-edge research. The National Institutes of Health supported the study.

University of North Carolina at Chapel Hill