Cheiro de terra molhada

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Issue 35, June 2003
www.proteinspotlight.org

The earth’s perfume
Vivienne Baillie Gerritsen

H

ave you ever brought a glass of wine – or drinking water – to your lips and discovered a mustytaste? Geosmin is what produces it. Geosmin is a germacranoid sesquiterpene or a trans-1,10dimethyl-trans-9-decalol for the more chemically minded. Human taste buds are extremely sensitive
togeosmin; the average person can detect 0.7 parts per billion! The chemical is produced by a
number of microorganisms amongst which the mycelial soil bacteria Streptomyces, which have
become invaluable inthe medical field since they are an important source for naturally occurring
antibacterial and antifungal agents as well as anticancer drugs and immunosuppressants.

Geosmin is the distinct smellthat soil gives off
when it is disturbed or on which it has just
rained. A pleasant smell for most. So pleasant, it
is used to confer an earthy scent to perfumes. A
number of enzymes are involvedin the making
of geosmin, one of which is a key enzyme:
germacradienol synthase.

The genome of Streptomyces coelicolor was
recently sequenced and on it was found the
sequence for germacradienolsynthase. It is a
protein of just over 700 amino acids, of which
little is known save that it is made of two
homologous domains. Out of the two, only one
– the N-terminal one – is actuallynecessary for
geosmin synthesis.
The detail of geosmin biosynthesis is still a bit
of a mystery, though the discovery of the
germacradienol synthase is certainly helping to
unveil the biochemicalpathway which leads to
it. It has been suggested for some time now that
the formation of geosmin in all likelihood
involves the action of a germacranoid
sequiterpene
synthase
on
farnesylpyrophosphate. And this has indeed turned out
to be the case. Germacradienol synthase
probably catalyses the cyclization of the
farnesyl pyrophophate, which is an early step in
geosmin biosynthesis. The...
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