How Nature Makes Earth Aroma —Unusual biosynthesis of geosmin

How Nature Makes Earth Aroma

Unusual biosynthesis of geosmin, a terpene responsible for the pleasant scent of moist soil, is deciphered

Geosmin, ubiquitous in the environment, is a terpene produced by a number of microorganisms, including soil bacteria and cyanobacteria (blue-green algae). Scientists have known about the compound for more than 100 years, but it wasn’t isolated and structurally characterized until 1965.

Besides giving rise to the scent of soil, geosmin and its metabolites can cause undesirable musty smells or off-flavors in water and food. People detect geosmin “at the extraordinarily low threshold of 10 ppt, but no one knows why this should be so or even why geosmin is produced,”says.chemistry professor David E. Cane,

Cane and coworkers suspected that two or more enzymes catalyzing some unknown combination of steps would be required to convert germacradienol to geosmin. But last year, the researchers were surprised to discover that one enzyme alone catalyzes the conversion of farnesyl diphosphate all the way to geosmin by way of germacradienol (J. Am. Chem. Soc. 2006, 128, 8128).

EARTHY ODORANT A bifunctional bacterial enzyme converts farnesyl diphosphate (left) into germacradienol (center) and subsequently into geosmin (right), which is the volatile compound responsible for the characteristic smell of freshly turned soil.

 

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