Sept 17 : Scientists at Brown University have found out the origins of the scent of freshly turned soil.

In their study in Nature Chemical Biology, the chemists describe how an unusual bifunctional enzyme produces geosmin, the organic compound responsible for the scent.

Geosmin, which literally translates to “earth smell,” was scientifically identified more than 100 years ago.

In soil, bacteria produce the chemical compound. In water, blue-green algae make it. Along with the pleasant scent of warm, moist soil, geosmin is also responsible for the muddy “off” taste in some drinking water.

But, until recently, scientists knew little about how geosmin was made.

Only a few years ago, David Cane, professor of chemistry at Brown who oversaw the research, found the gene responsible for geosmin formation in Streptomyces coelicolor, a strain of plant-munching bacteria found in soil.

Last year, the team discovered that a single protein converted farnesyl diphosphate to geosmin.

“Everyone is familiar with the wonderful smell of warm earth. Now we know precisely how it is made,” said Prof. Cane.

In their new work, Prof. Cane and his team found that this protein, called germacradienol/geosmin synthase, folded into two distinct but connected parts, similar to a dumbbell.

One piece was responsible for the first half of the reaction, cranking out a chemical that wafted over to the companion bit of protein, which then produced geosmin.

“We found that geosmin is created by this bifunctional enzyme. The two steps of the process that forms geosmin are metabolically related. This finding was a real surprise. This is the first bifunctional enzyme found for this type of terpene, the class of chemicals geosmin belongs to,” he said.

Jiaoyang Jiang, a graduate student in the Department of Chemistry and lead author of the journal article, said microbiologists working in water purification plants would be most interested in knowing the origins of geosmin.

“By understanding precisely how the substance is synthesized, these experts may find a way to block it – avoiding the foul taste that keeps people away from the tap. Geosmin may smell good in the garden, but not in the glass,” she said. (ANI)