Biosynthesis and function of aromatic amino acids in plants

Most aromatic amino acids in plants are formed by three main types of aromatic acids;

•    Phenylalanine
•     Tyrosine
•     Tryptophan

These three important aromatic amino acids are exclusively synthesized by Shikimic acid pathway that is unique to plants and microbes. This pathway got its name by an important intermediate forms,called Shikimic acid.

Shikimic acid pathway starts from the condensation of Erythrose-4-phosphate with Phosphoenolpyruvate (PEP). PEP is provided by the glycolysis while Erythrose-4-P comes from either oxidative pentose phosphate pathway or Calvin cycle. Therefore Shikimic acid pathway is combined with other important metabolic pathways of the cell. 

The condensation produces 3-deoxy-D-arabinoheptulosonic acid-7-phosphate (DAHP). DAHP undergoes another series of reactions including condensation with another molecule of PEP to give out Chorismic acid. Shikimic acid forms as an intermediate in this reaction and regarded as the key intermediate.

Chorismate is a central intermediate giving rise to two products; Prephenate and Anthranillic acid. Shikimic acid pathway is shown simply as below.

The synthesis of aromatic amino acids is important as these amino acids are the precursors for the synthesis of defense and repair compounds.

Phenylalanine

·         Flavonoids: in plant pigments (eg: Anthocyanine), act against pathogens. Antioxidants.

·         Coumarins: Has appetite-suppressing properties.

·         Liginin: In lignicolous fungi

Tyrosine

·         Tocopherol: Antioxidant in cornifers.

·         Plastoquinone: Important in photosynthesis.

·         Cyanogenic glucosides: Phytoanticipants. Important in plant defense against herbivores due to bitter taste and release of toxic hydrogen cyanide upon tissue disruption.

Tryptophan

·         Alkaloides: provides protection as it prevents insects and herbivores eating the plant.

·         Plant growth regulators