**4. Metabolic pathways for the formation of phenolic secondary metabolites**

As discussed above both primary and secondary metabolites share some of the precursor compounds and there is delicate balance in the production of these two metabolites. Accordingly carbon fluxes are diverted between the pathways.

Phenolics are formed by three different biosynthetic pathways:


The addition of hydroxyl groups to the phenyl ring plays the major role, which involved in the biosynthesis of phenolic acids [19].

I.Shkimate/Phenylpropanoids Pathway

The precursor compound for phenyl propanoid pathway is phenylalanine which can form various types of phenolics which range in the number of aromatic rings from 1 to 6 which also differ in the substitution pattern (**Figure 2**). Hydroxycinnamic acids (HCAs) (C6–C3) which include caffeic, ferulic, *p*-coumaric and sinapic acids varies in the degrees of hydroxylation and methylation at C6 position. The cleavage of aliphatic side chain of P-coumaric acid can lead to the formation of hydroxybenzoic acids like salicylic, vanillic, gallic and syringic acids.

The synthesis of chorismic acid from the precursors phosphoenol pyruvate and erythrose-4-phosphate acts as a precursor for the synthesis of cinnamic acid derivatives. Various derivatives of benzoic acids are formed from chorismic acid via oxidative and non oxidative pathways and the precursor for protocatechuic acid is isochorismic acid.

The condensation of 3 C2 residues with an activated hydroxycinnamic acid products are two classes of metabolites with a second aromatic ring linked to the phenylpropanoid moiety, the flavonoids (C6–C3–C6) and the stilbenes (C6–C2–C6).

II.Acetate/Malonate or Polyketide Pathway

The acetate-malonate pathway is the fatty acids both those 1o metabolites which arise universally and the more infrequent compounds with a limited distribution. This pathway also makes an important contribution to plant aliphatic and aromatic compounds; these are biosynthesised through the formation of polyketides.

Acetyl coenzyme-A is the precursor of the acetate-malonate pathway. This is a metabolite of very importance in both 1o and 2o metabolism. The metabolic pool of acetyl CoA is incessantly replenished by glycolysis and the catabolism of fatty acids and amino acids, which depletes for the synthesis of fatty acids, steroids, polyketides, terpenoids, aromatic compounds and acetyl esters and amides (**Figure 3**).

III.Acetate - Mevalonate Pathway

Mevalonic acid is the 1o precursor of steroids with phenolic ring biosynthesised by plants. It is consequent from acetyl CoA through the transitional formation of acetoacetyl CoA and 3-hydroxy-3-methylglutaryl CoA (HMG CoA) these reactions being catalyzed by acetyl CoA acetyltransferase and HMG CoA synthase respectively (**Figure 4**).

#### **Figure 2.**

*Synthesis of plant phenolic compounds by shikimate pathway.*

**Figure 3.**

*The acetyl coenzyme a metabolie process.*

*Phenolic Compounds - Chemistry, Synthesis, Diversity, Non-Conventional Industrial...*

#### **Figure 4.**

*The acetate/mevalonate pathway for the formation of IPP, the basic five-carbon unit of terpenoid biosynthesis. Synthesis of each IPP unit needs three molecules of acetyl-CoA.*
