Biochemistry

Anyone for a Cuppa?

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Science  07 Mar 2008:
Vol. 319, Issue 5868, pp. 1309
DOI: 10.1126/science.319.5868.1309c

Inhaling the soothing aroma from a mug of peppermint tea renews the sense of wonder at the delightful mix of monoterpenes produced by the hybrid plant Mentha X piperita. Its oil, which accumulates within a structure called a peltate glandular trichome, contains high levels of (-)-menthol and low levels of the precursor (+)-pulegone and the side product (+)-menthofuran. The critical balance between the activities of pulegone reductase (which produces menthones that are converted into menthols) and menthofuran synthase (which diverts pulegone into menthofuran) can be disturbed in less-than-ideal environments, such as low light.

Rios-Estepa et al. have collected the measured biochemical parameters of kinetic constants and metabolite concentrations, validated them where feasible against independently derived quantities (gland volume, oil density, and terpene molecular weight), and built a biosynthetic model that simulates the developmental time courses under optimal and low-light conditions of menthol, pulegone, and menthofuran levels. An earlier genetic engineering study had suggested that menthofuran turned down the transcription of pulegone reductase, but plugging this constraint into the model failed to reproduce the observed changes in terpene levels. Instead, the observations could be replicated by assuming that menthofuran acted as a competitive inhibitor of the reductase, a supposition confirmed by Lineweaver-Burk analysis of in vitro enzyme assays. — GJC

Proc. Nat. Acad. Sci. U.S.A. 105, 2818 (2008).

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