Phenylpropanoid pathway

The Phenylpropanoid pathway is a key metabolic route within plants that produces a variety of secondary metabolites derived from the amino acid phenylalanine. These metabolites play crucial roles in plant structure, pigmentation, and defense mechanisms. The pathway is responsible for the biosynthesis of important compounds such as flavonoids, lignin, and tannins, which are essential for plant development and survival.

Overview

The phenylpropanoid pathway begins with the deamination of phenylalanine to form cinnamic acid, a reaction catalyzed by the enzyme phenylalanine ammonia-lyase (PAL). This step is considered the gateway to the phenylpropanoid pathway and is critical for the subsequent biosynthesis of various phenylpropanoid end products.

Key Enzymes and Reactions

Following the formation of cinnamic acid, a series of enzymatic reactions transform this compound into different intermediates and eventually into various phenylpropanoid derivatives:

• Cinnamate 4-hydroxylase (C4H) hydroxylates cinnamic acid to p-coumaric acid.
• 4-Coumarate:CoA ligase (4CL) activates p-coumaric acid to p-coumaroyl-CoA, which is a central intermediate for the branching pathways leading to different classes of phenylpropanoids.

1. 1. 1. Flavonoid Biosynthesis

From p-coumaroyl-CoA, the pathway diverges to produce flavonoids through the action of complex enzyme sequences including chalcone synthase (CHS). Flavonoids are significant for their roles in UV protection, flower coloring, and plant reproduction.

1. 1. 1. Lignin Biosynthesis

Lignin biosynthesis involves the polymerization of monolignols, which are derived from p-coumaroyl-CoA through the actions of enzymes such as cinnamoyl-CoA reductase (CCR) and cinnamyl alcohol dehydrogenase (CAD). Lignin is a major component of wood and bark, contributing to plant rigidity and water transport.

1. 1. 1. Tannin Biosynthesis

Tannins, which are polyphenolic compounds, are also synthesized from intermediates of the phenylpropanoid pathway. They are important for plant defense against herbivores and pathogens.

Regulation

The phenylpropanoid pathway is tightly regulated at both the transcriptional and post-translational levels. Environmental factors such as light, stress, and pathogen attack can induce the expression of genes encoding enzymes of this pathway, thereby modulating the production of its metabolites.

Ecological and Economic Importance

Phenylpropanoids have significant ecological roles in plant interactions with their environment, including protection against UV radiation and pests. Economically, they are valuable in various industries; for example, flavonoids are used in supplements and cosmetics, while lignin is a key material in the production of biofuels and other bioproducts.

See Also

• Secondary metabolism
• Metabolic pathway
• Plant defense against herbivory