Definition of Phenylpropanoids:
Phenylpropanoids are a large class of plant secondary metabolites derived from the amino acid phenylalanine through the phenylpropanoid pathway. These compounds are characterized by a basic structure of a C6-C3 skeleton, which consists of a benzene ring (C6) linked to a three-carbon propene chain (C3). Phenylpropanoids are diverse in structure and function, including simple phenolics, lignans, lignins, coumarins, stilbenes, and flavonoids. They serve multiple roles in plants, including structural components, UV protection, defense against pathogens, and attraction of pollinators.
Biosynthesis:
The phenylpropanoid biosynthetic pathway begins with the deamination of phenylalanine by the enzyme phenylalanine ammonia-lyase (PAL), which converts phenylalanine into cinnamic acid. Cinnamic acid undergoes hydroxylation, methylation, and glycosylation to form various phenylpropanoids, which are subsequently modified to produce a wide range of compounds such as:
– Lignins: Provide structural integrity to plant cell walls.
– Lignans: Act as phytoestrogens and have antioxidant properties.
– Coumarins: Serve as defense compounds and UV protectants.
– Stilbenes: Possess antifungal and antibacterial properties.
– Hydroxycinnamic acids: Such as caffeic acid and ferulic acid, are involved in plant growth and defense.
 Functions:
– Structural Roles: Lignins and suberin provide mechanical strength and rigidity to cell walls.
– Defense Mechanisms: Act as phytoalexins, antimicrobial agents, and insect repellents.
– Attractants and Signals: Participate in pollinator attraction and allelopathy.
– Regulation: Involved in plant growth, development, and adaptation to environmental stress.
Definition of Flavonoids:
Flavonoids are a diverse group of naturally occurring polyphenolic compounds derived from the phenylpropanoid pathway. They are characterized by a 15-carbon skeleton structure, which consists of two aromatic rings (A and B) connected by a three-carbon bridge forming an oxygen-containing heterocyclic ring (C), leading to a C6-C3-C6 structure. Flavonoids are widely distributed in the plant kingdom and are responsible for the pigmentation of flowers, fruits, and leaves, as well as for a range of physiological functions.
 Classification:
Flavonoids are classified into several subclasses based on the oxidation state of the central pyran ring and the degree of hydroxylation, methylation, and glycosylation:
– Flavones: (e.g., apigenin, luteolin) found in leaves and have antioxidant properties.
– Flavonols: (e.g., quercetin, kaempferol) present in a wide variety of fruits and vegetables.
– Flavanones: (e.g., naringenin, hesperidin) primarily found in citrus fruits.
– Flavanols (Catechins): (e.g., catechin, epicatechin) abundant in tea and cocoa.
– Anthocyanins: (e.g., cyanidin, delphinidin) responsible for red, blue, and purple colors in flowers and fruits.
– Isoflavones: (e.g., genistein, daidzein) primarily found in legumes and act as phytoestrogens.
 Biosynthesis:
Flavonoid biosynthesis involves the phenylpropanoid pathway, where phenylalanine is converted to cinnamic acid by phenylalanine ammonia-lyase (PAL), followed by several enzymatic steps leading to the formation of chalcones. Chalcones are the key intermediates that undergo cyclization, hydroxylation, methylation, and glycosylation to produce different flavonoid subclasses.
 Functions:
– UV Protection: Flavonoids absorb harmful UV-B radiation, protecting plant tissues.
– Antioxidant Activity: Scavenge free radicals and reduce oxidative stress.
– Defense against Pathogens: Act as antimicrobial, antifungal, and antiviral agents.
– Signal Molecules: Involved in signaling between plants and symbiotic organisms, such as in the nodulation process in legumes.
– Pigmentation: Responsible for the coloration of flowers and fruits, aiding in pollination and seed dispersal.
Conclusion:
Phenylpropanoids and flavonoids are critical components of plant metabolism with a wide range of structural diversity and functions, from protecting against environmental stress to contributing to plant development and reproduction. Both groups play vital roles in human health as well, offering antioxidant, anti-inflammatory, anticancer, and other bioactive properties.
