PLANT BIOCHEMISTRY 



CH3 GH3 



I I 



C=CH— GH2— CH2C=CH— CH.— CH2C=CH— CH, 



GH 



3 



squalene 



farnesol. The diterpenes occur infrequently in nature. One triter- 

 pene, squalene, found in liigher plant oils, is presumably an interme- 

 diate in sterol synthesis. 



Although the terpenes are traditionally considered to be derivatives 

 of isoprene, mevalonic acid has been implicated as an active interme- 

 diate in squalene synthesis. In vitro studies using purified enzyme 

 fractions from rat liver have shown a rapid incorporation of mevalonic 

 acid into squalene when incubated with manganese or magnesium 

 ions, ATP, TPN + , and glucose-6-phosphate. The cof actors for syn- 

 thesis using plant enzymes have not been examined in this detail. 



GH3 



I 

 CH2GH2G— CH2GOOH 



I 1 



OH OH 



mevalonic acid 



Probably the same metabolic intermediate fiuictions in syntheses of 

 mono-, di-, and polyterpenes. The relationship to the basic isoprene 

 unit is evident, requiring only the removal of two molecules of water 

 and decarboxylation. 



H H CH3 H 



I I I • I 



HC C — C C — [C00]H 



The carotenoids (tetraterpenes) and other photochemical pigments 

 are discussed in Chapter 9. These high-molecular-weight terpenes are 

 non-volatile and therefore are not classified as essential oils. 



Plants contain a number of volatile or essential oils which are not 

 members of the terpene class. Oil of wintergreen (methyl salicylcate) 

 from the mint family, alloisothiocyanate of the Cruciferae, and an- 

 thranilic acid esters (components of the odor of orange blossoms) are 

 examples of this varied group. 



The essential oils have no known metabolic function in the plant. 

 They may be of service to the plant through their attraction of insects, 

 thereby aiding fertilization. Other members seem to repel insects as 

 a result of their pungent odor. 



