Linalool biosynthesis

Raguso, R. A. and Pichersky, E. (1999). A day in the life of a linalool molecule chemical communication in a plant-pollinator system. Part 1 Linalool biosynthesis in flowering plants. Plant Species Biology 14 95-120. 

Acyclic monoterpene alcohols are present in many essential oils and can be isolated by distillation, although nowadays, they are often synthesized from other sources. Geraniol and linalool (Fig. 7) are the most important monoterpene alcohols and together with nerol are the primary products in terpene biosynthesis [44]. a-Terpineol is a frequently used cyclic monoterpene alcohol in the fragrancing industry because of its typical lilac odor and is also partly responsible for the typical... 

Among the H-NMR data of isoprenoids practically exclusively those of monoterpe-nes are of interest in the present context. As already outlined before, these compounds are normally synthesised in plants via the deoxyxylulose-phosphate pathway and show therefore H-pattems in agreement with their common biosynthesis and its modulations [111], e.g. in the case of R(-)-linalool, distinct deuterium depletions at position C-2 and C-9, corresponding to (D/H)j and (D/H) in the NMR nomenclature but individual shifts from a common mean value, depending on the botanical origin [230] (see Fig. 6.22 and Tables 6.12 and 6.13). 

Two elements of the cyclization have yet to be addressed the isomerization of geranyl pyrophosphate to linalyl pyrophosphate (or the equivalent ion-pair) and the construction of bicyclic skeleta. Studies on the biosynthesis of linalool (61), and on the analogous nerolidyl system in the sesquiterpene series (52), have shown this allylic transposition to occur by a net suprafacial process, as expected. On the other hand, the chemical conversion of acyclic or monocyclic precursors to bicyclic monoterpenes, under relevant cationic cyclization conditions, has been rarely observed (47,62-65) and, thermodynamic considerations notwithstanding (66), bicyclizations remain poorly modeled. 

The acyclic terpene alcohols geraniol, linalool, and citronellol are the most important terpene alcohols used as fragrance and flavor substances. Geraniol and linalool are, in addition to nerol and lavandulol, primary products in terpene biosynthesis. The fully saturated alcohols tetrahydrogeraniol and tetrahydrolinalool are also... 

Biosynthesis. The starting compound is geranyl pyrophosphate (see Terpenes). Acyclic D. are formed by hydrolysis of the pyrophosphate residne (e.g. phytol). Geranylgeranylpyrophosphate is probably converted easily to geranyl linalool, which is then converted to bi- and tricyclic compoimds (Rg.). In a few of the cyclic D-, e. g. abietic acid, there is a migration of the substituents. The gibberellins are derived from the labda-diene type of D. 

The tremendous diversity of volatile monot-erpenes, sesquiterpenes and diterpenes arises from enzymatic modification of the non-volatile prenyl diphosphate intermediates GPP, FPP and GGPP through the action of terpene synthases (TPS) [57, 58], many of which have the distinctive ability to catalyze the formation of multiple products from a single prenyl diphosphate snb-strate [59-61]. A recently isolated monoterpene synthase from Nicotiana suaveolens flowers was found to produce a blend of 5 cyclic and 2 acyclic monoterpenes, all of which are components of N. suaveolens floral scent [62]. In Arabidopsis, two terpene synthases account for the biosynthesis of nearly all sesquiterpenes found in the floral volatile blend [63]. In addition to sesquiterpenes, Arabidopsis flowers also emit monoterpenes dominated by P-myrcene and (5)-linalool. While P-myrcene could likely be synthesized by multi-product monoterpene synthases [64, 65], one single-product monoterpene synthase may be solely responsible for the emission of (5)-linalool [65]. In snapdragon, two specialized single-product monoterpene synthases are responsible for the biosynthesis of the two floral monoterpenes, myrcene and ( )-P-ocimene [66]. The reaction mechanism of TPSs involves the formation of carbocationic intermediates which can then be differentially metabolized to form multiple products [57, 58]. 

Benzaldehyde is considered to be the character-impact compoimd for cherry aroma (77), and is the key component in imitation cherry flavor compositions (18), The biosynthesis of this character-impact compound has been attributed to the acid or enzymic hydrolysis of the glucoside amygdalin in cherry pips. The importance of benzaldehyde in the aroma of cherry is emphasised by Schmid and Grosch (19), On sniffing stepwise diluted flavor extracts from cherries, seven compounds were revealed to have the highest aroma values - these compounds were benzaldehyde, linalool, hexanal, /ran5-2-hexenal, phenylacetaldehyde, (Z, )-2,6-nonadienal and eugenol. 

The key prenyltransferase in the biosynthesis of monoterpenes is GPP synthase. To date, only a few GPP synthases have been fully characterized (Burke et al. 1999 Bouvier et al. 2000). Enzyme-catalysed isomerization of GPP via loss of pyrophosphate as a leaving group produces the allylic linalyl cation (16). Attack by an extraneous nucleophile can take place at either end of the allylic system and ultimately gives rise to linear monoterpenes such as geraniol (17), linalool (18) and linalyl acetate (19), all common components of a number of essential oils (Figure 3.9). 

Martin, D.M., Chiang, A., Lund, S.T., Bohlmann, J., 2011. Biosynthesis of wine aroma transcript profiles of hydroxymethylbutenyl diphosphate reductase, geranyl diphosphate synthase, and linalool/nerolidol synthase parallel monoterpenol glycoside accumulation in Gewurztraminer grapes. Planta 236, 919-929. 

---