Terpenes and their Classification

What are terpenes ?
Terpenes are secondary metabolites which are produced by Plants for their defense mechanism or required as per their metabolism. They are unsaturated and conjugated compounds may be aromatic or alliphatic

1. Head Tail Head Isoprene The terpenoids form a large and structurally diverse family of natural products derived from C5 isoprene units joined in a head to- tail fashion .

2. Tail to tail Tail to tail

3. IPP is isomerized to the other isoprene unit, DMAPP, by an isomerase enzyme which stereospecifically removes the pro-R proton (HR) from C-2, and incorporates a proton from water on to C-4. While the isomerization is reversible, the equilibrium lies heavily on the side of DMAPP.

4. MONOTERPENES (C10) Combination of DMAPP and IPP via the enzyme prenyl transferase yields geranyl diphosphate ( GPP). This is believed to involve ionization of DMAPP to the allylic cation , addition to the double bond of IPP, followed by loss of a proton. Stereochemically , the proton lost (H R) is analogous to that lost on the isomerization of IPP to DMAPP. This produces a monoterpene diphosphate , geranyl PP, in which the new double bond is trans (E).

5. Linalyl PP and neryl PP are isomers of geranyl PP, and are likely to be formed from geranyl PP by ionization to the allylic cation,which can thus allow a change in attachment of the diphosphate group (to the tertiary carbon in linalyl PP) or a change in stereochemistry at the double bond (to Z in neryl PP)

6. SESQUITERPENES (C15) Addition of a further C5 IPP unit to geranyl diphosphate in an extension of the prenyl transferase reaction leads to the fundamental sesquiterpene precursor, farnesyl diphosphate (FPP) . Again, an initial ionization of GPP seems likely, and the proton lost from C-2 of IPP is stereochemically analogous to that lost in the previous isoprenylation step. FPP can then give rise to linear and cyclic sesquiterpenes . Because of the increased chain length and additional double bond, the number of possible cyclization modes is also increased, and a huge range of mono-, bi-, and tri-cyclic structures can result.

7. The stereochemistry of the double bond nearest the diphosphate can adopt an E configuration (as in FPP), or a Z configuration via ionization, as found with geranyl / neryl PP.

8. DITERPENES (C20) The diterpenes arise from geranylgeranyl diphosphate ( GGPP), which is formed by addition of a further IPP molecule to farnesyl diphosphate in the same manner as described for the lower terpenoids . One of the simplest and most important of the diterpenes is phytol , a reduced form of geranylgeraniol,which forms the lipophilic side-chain of the chlorophylls, e.g. chlorophyll a. Related haem molecules, porphyrin components of haemoglobin , lack such lipophilic side-chains. Available evidence suggests that geranylgeranyl diphosphate is involved in forming the ester linkage, and the three reduction steps necessary to form the phytol ester occur after attachment to the chlorophyll molecule .

9. TRITERPENES (C30) Triterpenes are not formed by an extension of the now familiar process of adding IPP to the growing chain. Instead, two molecules of farnesyl PP are joined tail to tail to yield the hydrocarbon squalene , originally isolated from the liver oil of shark ( Squalus sp.). Squalene was subsequently found in rat liver and yeast, and these systems were used to study its biosynthetic role. As a precursor of triterpenes and steroids; several seed oils are now recognized as quite rich sources of squalene , e.g. Amaranthus cruentus ( Amaranthaceae ).

10. TETRATERPENES (C40) The tetraterpenes are represented by only one group of compounds, the carotenoids , though several hundred natural structural variants are known. These compounds play a role in photosynthesis, but they are also found in non-photosynthetic plant tissues, in fungi and bacteria. Formation of the tetraterpene skeleton, e.g. phytoene , involves tail-to-tail coupling of two molecules of geranylgeranyl diphosphate (GGPP) in a sequence essentially analogous to that seen for squalene and triterpenes .

11. A cyclopropyl compound, prephytoene diphosphate (compare presqualene diphosphate ,)is an intermediate in the sequence, and the main difference between the tetraterpene and triterpene pathways is how the resultant allylic cation is discharged. For squalene formation, the allylic cation accepts a hydride ion from NADPH, but for phytoene biosynthesis, a proton is lost, generating a double bond in the centre of the molecule, and thus a short conjugated chain is developed .

12. In plants and fungi, this new double bond has the Z ( cis ) configuration, whilst in bacteria, it is E (trans). This triene system prevents the type of cyclization seen with squalene . Conjugation is extended then by a sequence of desaturation reactions, removing pairs of hydrogens alternately from each side of the triene system, giving eventually lycopene , which, in common with the majority of carotenoids , has the all- trans configuration.

13. The extended π-electron system confers colour to the carotenoids , and accordingly they contribute yellow, orange, and red pigmentations to plant tissues. Lycopene is the characteristic carotenoid pigment in ripe tomato fruit ( Lycopersicon esculente; Solanaceae ). The orange colour of carrots ( Daucus carota ; Umbelliferae / Apiaceae ) is caused by β-carotene , though this compound is widespread in higher plants.