How Nature Makes Earth Aroma

If you have ever dug into a flower bed, walked through a plowed field being readied for planting, or ventured outside after a gentle rain shower, then you have smelled geosmin, a chemical responsible for the characteristic odor of fresh, moist earth. Brown University chemists now have completed a series of studies unraveling precisely how nature goes about making this bicyclic alcohol (Nat. Chem. Biol., DOI: 10.1038/nchembio.2007.29).

"One nice thing about geosmin is that essentially everyone has smelled it, even if they did not know what it was or where it comes from," says chemistry professor David E. Cane, who along with graduate students Jiaoyang Jiang and Xiaofei He carried out the research.

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," Cane says.

Despite the longtime interest, scientists haven't been able to fully decipher how geosmin is made, until now. In 2003, Cane's group and others identified the gene in the soil bacterium Streptomyces coelicolor that codes for the magnesium-dependent enzyme that converts farnesyl diphosphate into germacradienol, which is a geosmin precursor. Farnesyl diphosphate is a common intermediate used by organisms in the biosynthesis of hundreds of different terpenes, Cane notes.

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).

In completing the work, Cane, Jiang, and He now provide the details of the enzymatic process. The team altered specific amino acids in the enzyme and characterized the chemical products of the mutant enzymes. In doing so, the researchers found that the enzyme folds in such a way that two halves harbor independent active sites with distinct catalytic functions. The N-terminal half of the enzyme uses farnesyl diphosphate to crank out germacradienol, which subsequently is handed off to the C-terminal half to complete the transformation into geosmin.

The two-part enzyme-mediated process is a "real surprise," Cane says. "This is the first bifunctional enzyme found for this type of terpene."

The researchers believe that microbiologists working in water purification—where removing geosmin has proven difficult—and in the food and beverage industries will be interested in their finding. By understanding precisely how geosmin is made, a method to block its formation could avoid the sometimes musty taste of water, wine, fish, and other foods, Cane says.