General Biosynthetic Pathways of Secondary Metabolites

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Chapter: Pharmacognosy and Phytochemistry : General Biosynthetic Pathways of Secondary Metabolites

All organisms need to transform and interconvert a vast number of organic compounds to enable them to live, grow, and reproduce. They need to provide themselves with energy in the form of ATP, and a supply of building blocks to construct their own tissues.


General Biosynthetic Pathways of Secondary Metabolites

 

INTRODUCTION

 

All organisms need to transform and interconvert a vast number of organic compounds to enable them to live, grow, and reproduce. They need to provide themselves with energy in the form of ATP, and a supply of building blocks to construct their own tissues. An integrated network of enzyme-mediated and carefully regulated chemical reac-tions is used for this purpose, collectively referred to as ‘intermediary metabolism’, and the pathways involved are termed ‘metabolic pathways’. Some of the crucially important molecules of life are carbohydrates, proteins, fats, and nucleic acids.

 

Despite the extremely varied characteristics of living organisms, the pathways for generally modifying and syn-thesizing carbohydrates, proteins, fats, and nucleic acids are found to be essentially the same in all organisms, apart from minor variations. These processes demonstrate the fundamental unity of all living matter, and are collectively described as ‘primary metabolism’, with the compounds involved in the pathways being termed ‘primary metabo-lites’. Thus degradation of carbohydrates and sugars gener-ally proceeds via the well-characterized pathways known as glycolysis and the Krebs/citric acid/tricarboxylic acid cycle, which release energy from the organic compounds by oxidative reactions. Oxidation of fatty acids from fats by the sequence called β-oxidation also provides energy.

 

In contrast to these primary metabolic pathways, which synthesize, degrade, and generally interconvert compounds commonly encountered in all organisms, there also exists an area of metabolism concerned with compounds which have a much more limited distribution in nature. Such compounds, called ‘secondary metabolites’, are found in 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, and in the vast majority of cases the function of these compounds and their benefit to the organism is not yet known. Some are undoubtedly produced for easily appreciated reasons, for example, as toxic materials providing defence against predators, as volatile attractants towards the same or other species, or as colouring agents to attract or warn other species, but it is logical to assume that all do play some vital role for the well-being of the producer. It is this area of ‘secondary metabolism’ that provides most of the pharmacologically active natural products. It is thus fairly obvious that the human diet could be both unpalatable and remarkably dangerous if all plants, animals, and fungi produced the same range of compounds.

 

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