Secondary metabolites constitute a group of organic compounds that are not directly involved in an organism's usual growth, development, or reproduction. Unlike primary metabolites, intermediates, and products of basic cellular metabolism, secondary metabolites are considered part of a biochemical pathway heading out of primary metabolism. Often, secondary metabolites play critical roles in the defense mechanism of plants, signaling, and interaction with the environment. The class of secondary metabolites encompasses many subdivisions, with terpenes, phenols, and nitrogenous compounds being most notable for their structural and functional diversity. Notably, this blog will describe the biosynthesis of these three classes of secondary metabolites and what these compounds do.
Introduction to Secondary Metabolites
Secondary metabolites are usually categorized into three major groups:
Terpenes
Phenols
Nitrogenous Compounds
These compounds have been synthesized through different biochemical pathways. Their ecological roles include defense against herbivores, pathogens, and competition with other plants.
Terpenes:
Biosynthesis of Terpenes:
Terpenes are described as the most significant class of secondary metabolites, with structures that are more than 40,000 known. They are classified based on their units:
Monoterpenes: Two isoprene units
Sesquiterpenes (C15): Three isoprene units
Diterpenes (C20): Four is isoprene units
Triterpenes (C30): Six isoprene units
Tetraterpenes (C40): Eight isoprene units
Polyterpenes: Multiple isoprene units
Terpenes are synthesized through the mevalonate pathway (MVA) or the methylerythritol phosphate pathway (MEP):
Mevalonate Pathway (MVA):
This is an eukaryotic cell cytosol-specific pathway that converts acetyl-CoA to isopentenyl pyrophosphate (IPP), the terpene building block. Methylerythritol Phosphate Pathway (MEP): Occurs in the plastids of plants and some bacteria, beginning from pyruvate and glyceraldehyde-3-phosphate and leading to the formation of IPP.
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Role of Terpenes
Defense: Terpenes can act as toxins or deterrents to herbivores and pathogens. For example, pyrethrins from Chrysanthemum spp. are used as natural insecticides.
Attraction: Many terpenes are volatile, and their odors lure pollinators into the flowers.
Protection: Some terpenes, like carotenoids, protect plants from ultraviolet radiation.
Phenols
Synthesis of Phenols Phenolic compounds are considered to be one or more such hydroxide groups attached to an aromatic ring. The most important pathways involved in the biosynthesis of phenols include shikimate and phenylpropanoid pathways:
Shikimic pathway
The pathway leading to the synthesis of aromatic amino acids—phenylalanine, tyrosine, and tryptophan—that are precursors for many phenolic compounds.
The phenylpropanoid pathway
The phenylpropanoid pathway begins with phenylalanine and produces cinnamic acid, as well as the bulk of its derivatives, which act as precursors for a wide range of phenolic compounds.
Roles of Phenols
Antioxidants: Most of these are phenolic compounds, including flavonoids and tannins, that have high antioxidant potential for plant defense against oxidative stress.
Antimicrobial: Many phenolic compounds have antimicrobial activity, giving plants a mechanism for resistance to fungal and bacterial infection.
Structural: Lignin, a three-dimensional aromatic polymer, is a vital plant cell wall constituent and is responsible for structural rigidity in plants.
Nitrogen Compounds
Biosynthesis of Nitrogen Compounds
Nitrogenous secondary metabolites are synthesized from amino acids. Such groups of compounds include alkaloids, cyanogenic glycosides, and glucosinolates, with all having nitrogen in them.
Alkaloids: Those derived from a wide variety of amino acids, but primarily derived from tryptophan, tyrosine, and ornithine. The biosynthetic process involves decarboxylation and other chemical modifications of amino acids to create complicated nitrogen-rich structures.
Cyanogenic glycosides: These are synthesized from amino acid precursors, particularly valine, isoleucine, and phenylalanine. The process involves the conversion of these amino acids to hydroxynitriles, which are glycosylated.
Glucosinolates are derived from some amino acids, including methionine and tryptophan, synthesized by the sequence of reactions from amino acid chain elongation to oxidation and glycosylation. Importance of Nitrogenous Compounds
Defense: Alkaloids often serve as potent poisons to many herbivores and pathogens. One of many examples is nicotine at work as an insect deterrent in tobacco plants.
Signaling: Nitrogenous compounds are also widely used as signalling molecules. Notable in this class are jasmonates, which arise from the fatty acid linolenic acid and play a central role in plant defense signaling.
Detoxification: Cyanogenic glycosides can release hydrogen cyanide upon tissue damage, thus deterring herbivores with toxicity.
Conclusion
Terpenes, phenolic compounds, and nitrogenous compounds are part of secondary metabolites, which the plant cannot do without survival and interaction with the environment. It is this diversity in the biosynthetic pathway of secondary metabolites that underpins their multifunctional performance within plant biology and ecology. Known compounds could be helpful for insight into plant defense modalities and for putative uses in agriculture, medicine, and industry. From this research into the biosynthesis and functions of these secondary metabolites, new strategies for crop protection could be developed, new pharmaceuticals could be discovered, and more exploration could be completed on sustainable uses of compounds derived from plants.