Let us make an in-depth study of the terpenes. After reading this article you will learn about 1. Classification of Terpenes and 2. Biosynthesis of Terpenes.
Introduction to Terpenes:
These substances constitute the largest group of secondary plant products and show some of the properties of lipids. They are insoluble in water and are derived from the union of a common 5-carbon unit called as isoprene which has a branched carbon skeleton. Isoprene in turn is derived from basic 5-C unit called as isopentane.
Except for isoprene itself, the isoprenoids or terpenoids are dimers, trimers, tetramers or polymers in which isoprene units are usually joined in a head to tail manner:
However, sometimes due to extensive metabolic modifications it is not easily possible to mark out the original 5-C units in isoprenoids.
Classification of Terpenes:
Terpenes are classified into many categories based on the no. of carbon atoms and isoprene residues present in their structure:
(i) Monoterpenes. They consist of 10-C atoms or two isoprene residues.
(ii) Sesquiterpenes. These contain 15-C atoms or three isoprene residues.
(iii) Diterpenes. These contain 20-C atoms or four isoprene residues.
(iv) Triterpenes. These consist of 30-C atoms or six isoprene units.
(v) Tetraterpenes. These consist of 40-C atoms or eight isoprene residues.
(vi) Polyterpenes. These consist of large number of isoprene residues.
Biosynthesis of Terpenes:
Biosynthesis of terpenes may be studied in two parts:
(A) Synthesis of Activated 5-C units:
Isoprene unit is almost entirely synthesized from acetyl-CoA through mevalonic acid pathway as shown in Fig. 24.2 Three acetyl-CoA molecules are joined together in stepwise manner to from a six-carbon intermediate, mevalonic acid. Mevalonic acid is then pyrophosphorylated utilizing 2 ATP molecules to form mevalonic acid pyrophosphate (MVA- PP).
Decarboxylation and dehydration of MVA-PP result in the formation of activated 5-C unit called as isopentenyl pyrophosphate (IPP). The latter can be isomerized to another activated 5- C unit called as dimethylallyl pyrophosphate (DPP). Both these activated 5-C units are building blocks of terpenes in plants.
(B) Condensation of activated 5-C units IPP and DPP to form terpenes:
Terpenes are ultimately formed by condensation of activated 5-C units IPP and DPP as shown in Fig. 24.3. and briefly described below:
(i) IPP and DPP unite to form 10-C geranyl phyrophosphate (GPP) which is precursor of monoterpenes.
(ii) GPP unites with another molecule of IPP to give rise to 15-C farnesyl pyrophosphate (FPP) which is precursor of sesquiterpenes.
(iii) FPP unites with IPP to form 20-C compound geranylgeranyl pyrophosphate (GGPP) which is precursor of diterpenes.
(iv) FPP dimerises to form 30-C compound which after elimination of two pyrophosphate groups (2PP) gives rise to squalene. The latter is precursor of triterpenes and steroids.
(v) GGPP can dimerise to form 40-C compound which after elimination of two pyrophosphate groups (2PP) gives rise to phytoene. The latter is precursor of tetraterpenes.
(vi) Polyterpenes are polymers containing large number of isopentenyl units.
