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Abstract

The C₁₂ ‘earthy’ odorant geosmin is derived from the C₁₅ metabolite farnesyl diphosphate. Metabolic transformation now seems to be catalyzed by a bifunctional protein having two operatively independent sesquiterpene synthase domains. The domains are catalytically linked through the passive diffusion of a C₁₅ alcohol product of the N-terminal catalytic domain to the C-terminal catalytic domain for the final steps of geosmin formation.

Figure 1

Schematic depiction of the bifunctional germacradienol-geosmin synthase. Each half of the protein (N-terminal domain, dark blue; C-terminal domain, light blue) adopts the α-helical fold commonly observed in terpene synthases. The functionally independent domains are joined by a ~50-amino-acid-residue linker. The dark blue N-terminal domain converts FPP into germacradienol, which then diffuses into the active site of the light blue C-terminal domain, whereupon germacradienol undergoes transformation to geosmin. Various postulated and identified mechanistic intermediates are shown.