Biosynthesis of monoterpenes. Enantioselectivity in the enzymatic cyclization of linalyl pyrophosphate to (-)-endo-fenchol.

摘要: The conversion of geranyl pyrophosphate to (-)-endo-fenchol is considered proceed by the initial isomerization substrate (-)-(3R)-linalyl and subsequent cyclization this bound intermediate. To test stereochemical scheme, phosphatase-free preparations cyclase from fennel (Foeniculum vulgare M.) fruit were repeatedly incubated with a sample (3RS)-[1-3H2]linalyl until approximately 50% precursor was converted bicyclic monoterpenol end product. residual linalyl isolated enzymatically hydrolyzed free alcohol, linalool, which resolved chiral phase capillary gas-liquid chromatography derived threo erythro mixture 1,2-epoxides. predominance (3S)-enantiomer in indicated that (3R)-enantiomer preferred for (-)-(1S)-endo-fenchol. This conclusion subsequently confirmed preparation direct testing (3R)-1Z-[1-3H] pyrophosphate, afforded Km value lower than observed relative velocity nearly three times higher. (3S)-1Z-[1-3H]Linalyl not an effective biosynthesis but did, anomalous cyclization, give rise low levels enantiomeric (+)-(1R)-endo-fenchol as well other products. These results support proposed model also suggest step rate limiting coupled isomerization-cyclization (-)-endo-fenchol.