Living plants are solar-powered biochemical and biosynthetic laboratory which

manufactures both primary and secondary metabolites from air, water, minerals

and sunlight.

The primary metabolites like sugars, amino acids and fatty acids that are needed

for general growth and physiological development of plant are widely distributed

in nature and are also utilized as food by man.

The secondary metabolites such as alkaloids, glycosides, flavonoids, volatile oils

etc are biosynthetically derived from primary metabolites. They represent

chemical adaptations to environmental stresses, or serve as defensive, protective or

offensive chemicals against microorganisms, insects and higher herbivorous

predators. They are some times considered as waste or secretory products of plant

metabolism and are of pharmaceutical importance.

A primary metabolite is directly involved in the normal growth, development, and

reproduction.

A secondary metabolite is not directly involved in those processes, but usually has

important ecological function.

MEVALONIC ACID PATHWAY

Biosynthesis of Mevalonic Acid

A molecule of acetyl-CoA undergoes a Claisen condensation with a molecule of

malonyl-CoA (as in fatty acid biosynthesis; see Biochemistry of Yeast

Fermentation – Fatty Acids) to form acetoacetyl-CoA which reacts with a third

molecule of manonyl-CoA in an Aldol Condensation to give 3-hydroxy-3-

methylglutaryl-CoA (HMG-CoA). Reduction with HMG-CoA reductase yields

mevaldic acid which is reduced again to mevalonic acid.

Conversion of Mevalonic acid to Isopentenyl pyrophosphate (3-methylbut-3-

enylpyrophosphate)

Mevalonate is converted into 3-phospho-5-pyrophosphomevalonate by three

consecutive phosphorylations. This labile intermediate loses CO2 and phosphate

and yields 3-isopentenyl pyrophosphate (IPP).

Isomerization Of Isopentenyl Pyrophosphate And Prenyl Transferase

Reactions

IPP is isomerized to dimethylallyl pyrophosphate (DMAPP) with IPP isomerase.

The isomerization involves addition of a proton to the sp2 carbon of isopentenyl

pyrophosphate and generation of the more stable tertiary carbocation. Elimination

of a proton yields the most stable alkene (trisubstituted, Zaitev’s Rule).

Next IPP reacts with DMAPP to give geranyl pyrophosphate (GPP). In the first

step of the reaction, IPP acts as a nucleophile and displaces a pyrophosphate group

from DMAPP. Pyrophosphate is an excellent leaving group. Its four OH groups

have pKa values of 0.9, 2.0, 6.6, and 9.4. Therefore three of the four groups will

be primarily in their basic forms at physiological pH (pH 7.3). A proton is

removed in the next step, again yielding the most stable alkene. GPP rearranges to

give the monoterpenes. GPP reacts with a second molecule of IPP to produce

farnesyl pyrophosphate (FPP). FPP is the precursor to the sesquiterpenoids, and

the steroids. FPP reacts with a third molecule of IPP to produce geranylgeranyl

pyrophosphate (GGPP). GGPP is the precursor of the carotenoids.

Significance of Mevalonic acid pathway:

Isoprenoid Compounds:

The isoprenoids are built up of C5 isoprene units and the nomenclature of the main

classes reflects the number of isoprene units present (Table 1).

Monoterpenes:

Acyclic Monoterpenes:

In monoterpene diphosphate, geranyl PP, the double bond is trans (E). 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) (Figure 5.9). These three

compounds can give rise to a range of linear monoterpenes found as components

of volatile oils used in flavouring and perfumery (Figure 5.10). E.g.: Nerol,

Geraniol, citronellol, linalool, hotrienol

MonoCyclic Monoterpenes:

Cyclic monoterpenes can not be expected to occur with the precursor geranyl

diphosphate, because the E stereochemistry of the double bond being unfavourable

for ring formation (Figure 5.9). Neryl PP or linalyl PP, however, do have

favourable stereochemistry, and either or both of these would seem more

immediate precursors of the monocyclic menthane system, formation of which

could be represented as shown in Figure 5.12, generating a carbocation (termed

menthyl or α-terpinyl) having the menthane skeleton. It has been found that

monoterpene cyclase enzymes are able to accept all three diphosphates, with

linalyl PP being the best substrate, and it appears they have the ability to isomerize

the substrates initially as well as to cyclize them. It is convenient therefore to

consider the species involved in the cyclization as the delocalized allylic cation

tightly bound to the diphosphate anion, and bond formation follows due to the

proximity of the π-electrons of the double bond (Figure 5.12).

The menthyl cation could be quenched by attack of water, in which case the

alcohol α-terpineol would be formed, or it could lose a proton to give limonene

(Figure 5.13). The menthyl cation, although it is a tertiary, may be converted by a

1,3-hydride shift into a favourable resonance stabilized allylic cation (Figure 5.13).

This allows the formation of α- and β-phellandrenes by loss of a proton from the

phellandryl carbocation. E.g. of monocyclic terpenoids are p-cymene, thymol,

carvacrol, menthol, isomenthol, neomenthol, menthone, isomenthone,

neoisomenthone, pulegone, isopulegone, carveol, carvone, piperitone,

piperitenone, isopiperitenone, isopiperitenol,

Dicyclic Monoterpenes:

Alternatively, folding the cationic side-chain towards the double bond (via the

surface characteristics of the enzyme) would allow a repeat of the cyclization

mechanism, and produce bicyclic bornyl and pinyl cations, according to which end

of the double bond was involved in forming the new bonds (Figure 5.14). Borneol

would result from quenching of the bornyl cation with water, and then oxidation of

the secondary alcohol could generate the ketone camphor. As an alternative to

discharging the positive charge by adding a nucleophile, loss of a proton would

generate an alkene. Thus α-pinene and β-pinene arise by loss of different protons

from the pinyl cation, producing the double bonds as cyclic or exocyclic

respectively. A less common termination step involving loss of a proton is the

formation of a cyclopropane ring as exemplified by 3-carene and generation of the

carane skeleton.

The bicyclic pinyl cation, with a strained four membered ring, rearranges to the

less strained five-membered fenchyl cation (Figure 5.14), a change which

presumably more than makes up for the unfavourable tertiary to secondary

carbocation transformation. This produces the fenchane skeleton, exemplified by

fenchol and fenchone. The isocamphyl tertiary carbocation is formed from the

bornyl secondary carbocation by a Wagner–Meerwein rearrangement, and so leads

to camphene. A hydride shift converting the menthyl cation into the terpinen-4-yl

cation only changes one tertiary carbocation system for another, but allows

formation of α-terpinene, γ-terpinene, and the α-terpineol isomer, terpinen-4-ol.

A further cyclization reaction on the terpinen-4-yl cation generates the thujane

skeleton, e.g. sabinene and thujone.

Sesquiterpenes:

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. 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 (Figure 5.26). In some systems, the

tertiary diphosphate nerolidyl PP (compare linalyl PP, page 172) has been

implicated as a more immediate precursor than farnesyl PP (Figure 5.26). This

allows different possibilities for folding the carbon chain, dictated of course by the

enzyme involved, and cyclization by electrophilic attack on to an appropriate

double bond.

E,E-Farnasyl cation, E,Z-Farnasyl cation and nerolidyl cation give

rise to matricin, -santonin, humulene, thapsigargin, bisabolene,

-bisabolol, caryophyllene, artemisinic acid, -cardinene,

zingiberene, sesqui-phellandrene, thapsigargin, parthenolide,

chamazulene.

Diterpenoids:

The diterpenes arise from geranylgeranyl diphosphate (GGPP). 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. E.g. of diterpenoids are

Triterpenoids:

Two molecules of farnesyl PP are joined tail to tail to yield the

hydrocarbon squalene which act as precursor of triterpenes and

steroids.

Cyclization of squalene is via the intermediate squalene-2,3-

oxide (Figure 5.55), produced in a reaction catalysed by a

flavoprotein requiring O2 and NADPH cofactors.

If squalene oxide is suitably positioned and folded on the enzyme

surface, the polycyclic triterpene structures formed can be

rationalized in terms of a series of cyclizations, followed by a

sequence of concerted Wagner–Meerwein migrations of methyls

and hydrides (Figure 5.55).

The cyclizations are carbocation mediated and proceed in a step-

wise sequence (Figure 5.56).

Examples of triterpenoids prepared from squalene are euphol, lupeol,

dammarenediol, taraxasterol, -amyrin, -amyrin. E.g. of triterpenoid saponin are

hopan-22-ol, tetrahymanol, quillaic acid, barringtogenol, glycyrrhizic acid,

liquiritin, isoliquiritin, carbenoxolone sodium, panaxadiol, panaxatriol, oleanolic

acid, syringin.

Tetraterpenes:

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

(Figure 5.67).

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. 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 (Figure 5.67). The tetraterpenes are represented by

only one group of compounds, the carotenoids. E.g.

Lycopene, -carotene, -carotene, -carotene,

zeathanthin, lutein, violaxanthin, capsanthin, fucoxanthin,

astaxanthin.

Steroids:

The steroids are modified triterpenoids containing the tetracyclic ring system of

lanosterol but lacking the three methyl groups at C-4 and C-14.