All living organisms are dependent on energy and supply of building blocks to construct/maintain their own tissues for their existence and survival and need to transform and interconvert a vast number of organic compounds to enable them to live , grow and reproduce. These requirements are accomplished in living organisms by utilizing compounds which are synthesized (anabolism) and/or degraded (catabolism) by means of enzyme-mediated and carefully regulated chemical reactions, collectively referred to as metabolism. The chemical reaction pathways involved are termed as metabolic pathways. The compounds which are produced and/or degraded during the metabolism are collectively known as metabolites.
Some of the crucially important molecules for life include carbohydrates, proteins, fats, and nucleic acids. Apart from minor variations, the metabolic pathways for synthesizing and modifying carbohydrates, proteins, fats, and nucleic acids are found to be essentially the same in all organisms. These metabolic processes demonstrate the fundamental unity of all living matter and are collectively known as primary metabolism. The compounds involved in the primary metabolic pathways are termed as primary metabolites.
In contrast to primary metabolic pathways, which synthesize, degrade, and transform compounds commonly encountered in all organisms, there also exists another type of metabolism, the secondary metabolism, which is concerned with compounds which have a much more limited occurrence in nature. These compounds are called secondary metabolites, and are synthesized and modified by only specific organisms, or groups of organisms, and are an expression of the individuality of species. Secondary metabolites are not necessarily produced under all conditions. In the vast majority of cases the function of secondary metabolites and their benefit to the organism are yet unknown. Some of the secondary metabolites are undoubtedly synthesized for easily appreciated reasons, e.g. as toxic agents for defense against predators, as colouring materials to attract or warn other species or as volatile attractants towards the same or other species, but it is logical to assume that all do play some vital role for the well-being of the producer organism.
The secondary metabolic pathways provide most of the pharmacologically active natural products. Thus it is fairly obvious that the human diet could be both unpalatable and remarkably dangerous if all plants, animals, and fungi produced the same range of organic compounds. Some common secondary metabolites are alkaloids, flavonoids, saponins, steroids and terpinoids etc.
The basic building blocks required for the synthesis of secondary metabolites are derived from primary metabolism and the number of available basic building blocks is surprisingly limited but a vast array of secondary metabolites can be synthesized from a limited number of basic building blocks. The most important building blocks utilized in the biosynthesis of secondary metabolites are derived from the intermediates acetyl coenzyme A (acetyl-CoA), shikimic acid, mevalonic acid, and methylerythritol phosphate. These intermediates are employed respectively in the acetate, shikimate, mevalonate, and methylerythritol phosphate metabolic pathways,
Many of the secondary metabolites present in plants, animals and micro-organisms have been found to possess pharmacological properties and many of these are used to treat a variety of mild to serious human ailments.
Flavonoids constitute a large group of secondary metabolites which occur in a variety of structural forms. These are widespread in higher plants and are also found in some lower plants, including algae. All flavonoids contain fifteen carbon atoms in their basic nucleus and these are arranged in a C6 - C3 - C6 configuration i.e., two aromatic rings linked by a three carbon unit which may or may not form a third ring. Majority of flavonoid compounds have yellow colour. The yellow colour of the flowers and fruits is due to the presence of flavonoids in the form of glycosides.