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

Abstract

Geosmin (1) is responsible for the characteristic odor of moist soil, as well as off-flavors in drinking water and foodstuffs1,2. Geosmin is generated from farnesyl diphosphate (FPP, 2) by an enzyme that is encoded by the SCO6073 gene in the soil organism Streptomyces coelicolor A3(2) (ref. 3). We have now shown that the recombinant N-terminal half of this protein catalyzes the Mg2+-dependent cyclization of FPP to germacradienol (3) and germacrene D (4), 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 (6), which was previously suggested to be an enzyme-bound intermediate in the cyclization of FPP to germacradienol.

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Figure 1: Organization of protein domains and conserved Mg2+-binding motifs in S. coelicolor germacradienol-geosmin synthase.

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Acknowledgements

We thank S. Schulz of the Technische Universität Braunschweig for a gift of synthetic octalin and T.-L. Shen for assistance with the mass spectrometric analysis. This research was supported by US National Institutes of Health Grant GM30301 to D.E.C.

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X.H. prepared the D86E, L90D and S233A mutants of SCO6073 and carried out the initial steady state kinetic characterization and product identification of these variant proteins. J.J. carried out all other experiments described, including the GC-MS identification of 6. The experiments were conceived by J.J., X.H. and D.E.C., and the manuscript was prepared by J.J. and D.E.C.

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Correspondence to David E Cane.

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Supplementary Figs. 1–4, Supplementary Scheme 1, Supplementary Tables 1 and 2, and Supplementary Methods (PDF 259 kb)

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Jiang, J., He, X. & Cane, D. Biosynthesis of the earthy odorant geosmin by a bifunctional Streptomyces coelicolor enzyme. Nat Chem Biol 3, 711–715 (2007). https://doi.org/10.1038/nchembio.2007.29

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