This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
|Superclasses:||Biosynthesis → Secondary Metabolites Biosynthesis → Terpenoids Biosynthesis → Sesquiterpenoids Biosynthesis|
Geosmin is the substrate that gives fresh moist soil its typical "earthy" smell. When present in water, wine or fish, it conveyes an undesirable muddy flavor. The human taste buds are very sensitive to geosmin, and can detect a concentration as small as 0.7 ppb (parts per billion). Geosmin was first isolated from actinomycetes [Gerber65].
Geosmin is produced by a number of microorganisms, including most Streptomyces strains [Bentley81] and several species of cyanobacteria [Tsuchiya81, Wu91, Saadoun01, Blaha04], myxobacteria [Dickschat05], fungi [Pisarnitskii, La05] and liverworts [Spörle91]. It has been suggested that the distinctive smell of geosmin enables camels to find water in the desert. In return, the animals aid the bacteria by spreading their spores [Simons03].
As is the case of many other secondary metabolites, geosmin is synthesized from (2E,6E)-farnesyl diphosphate (FPP). In Streptomyces coelicolor a single enzyme, geosmin synthase / octalin synthase / germacradienol synthase, catalyzes all steps required. It is a bifunctional enzyme, with its N-terminal domain responsible for converting (2E,6E)-farnesyl diphosphate to (1E,4S,5E,7R)-germacra-1(10),5-dien-11-ol, and the C-terminal domain catalyzing the transformation of (1E,4S,5E,7R)-germacra-1(10),5-dien-11-ol to (-)-geosmin, in a process that generates acetone [Jiang06, Jiang07, Nawrath08]. Both steps require Mg2+.
Bentley81: Bentley R, Meganathan R (1981). "Geosmin and methylisoborneol biosynthesis in streptomycetes. Evidence for an isoprenoid pathway and its absence in non-differentiating isolates." FEBS Lett 125(2);220-2. PMID: 7227551
Blaha04: Blaha L, Sabater S, Babica P, Vilalta E, Marsalek B (2004). "Geosmin occurrence in riverine cyanobacterial mats: is it causing a significant health hazard?." Water Sci Technol 49(9);307-12. PMID: 15237639
Jiang06: Jiang J, He X, Cane DE (2006). "Geosmin biosynthesis. Streptomyces coelicolor germacradienol/germacrene D synthase converts farnesyl diphosphate to geosmin." J Am Chem Soc 128(25);8128-9. PMID: 16787064
La05: La Guerche S, Chamont S, Blancard D, Dubourdieu D, Darriet P (2005). "Origin of (-)-geosmin on grapes: on the complementary action of two fungi, botrytis cinerea and penicillium expansum." Antonie Van Leeuwenhoek 88(2);131-9. PMID: 16096689
Nawrath08: Nawrath T, Dickschat JS, Muller R, Jiang J, Cane DE, Schulz S (2008). "Identification of (8S,9S,10S)-8,10-dimethyl-1-octalin, a key intermediate in the biosynthesis of geosmin in bacteria." J Am Chem Soc 130(2);430-1. PMID: 18095691
Spörle91: Spörle, J., Becker, H., Allen, N.S., Gupta, M.P. (1991). "Occurrence of (-)-geosmin and other terpenoids in an axenic culture of the liverwort Symphyogyna brongniartii." Z. fur Naturfors. C 46, 183-188.
Tsuchiya81: Tsuchiya Y, Matsumoto A, Okamoto T (1981). "[Identification of volatile metabolites produced by blue-green algae, Oscillatoria splendida, O. amoena, O. geminata and Aphanizomenon sp]." Yakugaku Zasshi 101(9);852-6. PMID: 6820045
Cane03: Cane DE, Watt RM (2003). "Expression and mechanistic analysis of a germacradienol synthase from Streptomyces coelicolor implicated in geosmin biosynthesis." Proc Natl Acad Sci U S A 100(4);1547-51. PMID: 12556563
Cane06: Cane DE, He X, Kobayashi S, Omura S, Ikeda H (2006). "Geosmin biosynthesis in Streptomyces avermitilis. Molecular cloning, expression, and mechanistic study of the germacradienol/geosmin synthase." J Antibiot (Tokyo) 59(8);471-9. PMID: 17080683
Gust03: Gust B, Challis GL, Fowler K, Kieser T, Chater KF (2003). "PCR-targeted Streptomyces gene replacement identifies a protein domain needed for biosynthesis of the sesquiterpene soil odor geosmin." Proc Natl Acad Sci U S A 100(4);1541-6. PMID: 12563033
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