Mechanisms of Limonene Toxicity and Tolerance Elucidated
Limonene, a major component of citrus peel oil, has a number of applications related to microbiology. Limonene has antimicrobial properties, but also has potential as a biofuel component, making it the target of renewable production efforts through microbial metabolic engineering. For both applications, an understanding of microbial sensitivity or tolerance to limonene is crucial, but the mechanism of limonene toxicity was unknown. Researchers at the Department of Energy’s Joint BioEnergy Institute have characterized a limonene-tolerant strain of Escherichia coli and found a mutation in a gene encoding alkyl hydroperoxidase, which alleviates limonene toxicity. They found that the acute toxicity previously attributed to limonene was largely due to the common oxidation product limonene hydroperoxide, which forms spontaneously in aerobic environments. The mutant AhpC protein was able to alleviate this toxicity by reducing the hydroperoxide to a more benign compound. The researchers found that the degree of limonene toxicity is a function of its oxidation level and that nonoxidized limonene has relatively little toxicity to wild-type E. coli cells. These results have implications for both the renewable production of limonene and limonene’s applications as an antimicrobial.
Chubukov, V., F. Mingardon, W. Schackwitz, E. E. Baidoo, J. Alonso-Gutierrez, Q. Hu, T. S. Lee, J. D. Keasling, and A. Mukhopadhyay. 2015. “Acute Limonene Toxicity in Escherichia coli Is Caused by Limonene Hydroperoxide and Alleviated by a Point Mutation in Alkyl Hydroperoxidase AhpC,” Applied and Environmental Microbiology 81, 4690-6. DOI: 10.1128/AEM.01102-15.