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. That is
why the substance is of interest to water purification experts and
even vintners, who want to keep the benign but pungent substance out
of their wine.
Until recently, scientists knew little about
how geosmin is made. Then, a few years ago, Cane 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 converts farnesyl diphosphate to
geosmin.
In their new work, Cane and his lab team found that this protein,
called germacradienol/geosmin synthase, folds into two distinct but
connected parts, similar to a dumbbell. One piece is responsible for
the first half of the reaction, cranking out a chemical that wafts
over to the companion bit of protein, which then produces geosmin.
�We found that geosmin is created by this bifunctional enzyme,� Cane
said. �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.�
Jiaoyang Jiang, a Brown graduate student in the Department of
Chemistry and lead author of the journal article, said microbiologists
working in water purification plants will be most interested in
knowing the origins of geosmin. By understanding precisely how the
substance is synthesized, Jiang said, 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.
Xiaofei He, a former Brown graduate student, contributed to the
research. The work was funded by the National Institute of General
Medical Sciences.
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