Cheiro de terra molhada

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Issue 35, June 2003 The earth’s perfume
Vivienne Baillie Gerritsen


ave you ever brought a glass of wine – or drinking water – to your lips and discovered a musty taste? 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 to geosmin; 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 in the 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 smell that 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 involved in 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 germacradienol synthase. 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 actually necessary 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 biochemical pathway 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 farnesyl pyrophosphate. 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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