Biosynthesis of the earthy odorant geosmin by a bifunctional Streptomyces coelicolor enzyme

Abstract

Geosmin is responsible for the characteristic odor of moist soil, as well as off-flavors in drinking water and foodstuffs. Geosmin is generated from farnesyl diphosphate (FPP, 2) by an enzyme that is encoded by the SCO6073 gene in the soil organism Streptomyces coelicolor A32 (ref. 3). We have now shown that the recombinant N-terminal half of this protein catalyzes the Mg2+-dependent cyclization of FPP to germacradienol and germacrene D, while the highly homologous C-terminal domain, previously thought to be catalytically silent, catalyzes the Mg2+-dependent conversion of germacradienol to geosmin. Site-directed mutagenesis confirmed that the N- and C-terminal domains each harbor a distinct, independently functioning active site. A mutation in the N-terminal domain of germacradienol-geosmin synthase of a catalytically essential serine to alanine results in the conversion of FPP to a mixture of sesquiterpenes that includes an aberrant product identified as isolepidozene, which was previously suggested to be an enzyme-bound intermediate in the cyclization of FPP to germacradienol.

An overview of diversity, occurrence, genetics and toxin production of bloom-forming Dolichospermum (Anabaena) species

“…Taste and odor (T & O) problems, mainly caused by secondary metabolites of microorganisms in aquatic ecosystems, have been frequently reported worldwide (Jiang et al, 2007;Giglio et al, 2008;Wang et al, 2011;Kutovaya and Watson, 2014). Geosmin and 2-methylisoborneol (2-MIB) are sesquiterpene and monoterpene molecules, respectively, that are responsible for the most unpleasant and widely distributed earthy-musty off-flavors in freshwater bodies.…”

Changes in Volatile Compounds of Chinese Rice Wine Wheat Qu During Fermentation and Storage

“…For this reason, the increase of some terpenes observed in wheat Qu may be attributed to the β-glucosidase activity originating from microbiota present in the medium. Geosmin is produced by a number of microorganisms, including most Streptomyces and several species of myxobacteria, cyanobacteria and fungi 14 . Terpenes, which were present in small amounts in wheat Qu, were not detected in the Chinese rice wine 18 .…”

2-Methylisoborneol production characteristics of Pseudanabaena sp. FACHB 1277 isolated from Xionghe Reservoir, China

“…Recently, the biosynthesis pathways of 2-MIB and geosmin have been analyzed in actinomycetes and cyanobacteria. Geosmin is catalyzed by the bifunctional single sesquiterpene cyclase (geosmin synthase) from farnesyl diphosphate (FPP) (Jiang et al 2007). 2-MIB is generated from geranyl diphosphate (GPP) via a two-step process: GPP is methylated by methyltransferase, followed by the cyclization of 2-methylgeranyl diphosphate by monoterpene cyclase (Kakimoto et al 2014).…”