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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