Bisabolene diterpenes

Bisabolene diterpenes Although this class of diterpenes was the last one to be discovered in Eremophila, its existence was predicted because of the occurrence of bicyclic and tricyclic diterpenes in this genus which, formally at least, could be derived from further cyclization of a putative intermediate containing this ring system (100). 

Examples of monoterpenes include the linear aldehyde citral, which is found in many essential oils, and the (cis) alcohol geraniol, a major component of oil of geranium. Cyclic monoterpenes include limonene, menthol, pinene, camphor, and carvone, major components of lemon oil, mint oil, turpentine, camphor oil, and caraway oil, respectively. Sesquiterpenes include farnesol, a component of rose oil, and bisabolene, a component of Bisabol myrrh. The diterpenes include phytol, a component of chlorophyll, and vita-... 

To date over 30 plant terpenoid synthases have been cloned as cDNAs, and many of these were found to encode enzymes of secondary metabolism (43). Isolation and analysis of six genomic clones encoding monoterpene ((—)-pinene and (—)-limonene), sesquiterpene ((E)-a-bisabolene and S-selinene) and diterpene (abietadiene) synthases from Abies grandis, and a diterpene (taxadiene) synthase from Taxus brevifolia have been reported (44). Overexpression of a cotton farnesyl diphosphate synthase (EPPS) in transgenic Artemesia annua has resulted in 3- to 4-fold increase in the yield of the sesquiterpenoid anti-malarial drug, artemisinin, in hairy roots (45). 

This a-domain is common to all class I terpene synthases and is the only domain in bacterial and fungal class I enzymes such as the pentalenene synthase from Streptomyces ey oliatus or the trichodiene synthase from Fusarium sporotri-chioides (Fig. 87.14a) [191,194]. Plant hemi-, mono-, and sesquiterpene synthases exhibit a second helical p-domain that resembles a barrel structure. An example of this class is given by the (+)-5-cadinene synthase from G. arboreum (Fig. 87.14b) [195]. In plant diterpene cyclases and in the exceptional case of the ( )-a-bisabolene synthase from A. grandis (Fig. 87.14c), a third helical y-domain with a barrel-like structure is present [197, 200, 202]. The p- and y-domains in the plant enzymes are nonfunctional but are required for correct enzyme folding. 

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