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secondary metabolites (Pharmacognosy:- 2) Notes

Description

Definition, classification, properties and test for identification of Alkaloids,
Glycosides, Flavonoids, Tannins, Volatile oil and Resins

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 condi-
tions, 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 appre-
ciated 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 unpalat-
able and remarkably dangerous if all plants, animals, and
fungi produced the same range of compounds.
13.2. THE BUILDING BLOCKS
The building blocks for secondary metabolites are derived
from primary metabolism as indicated in Figure 13.1. This
scheme outlines how metabolites from the fundamental
processes of photosynthesis, glycolysis, and the Krebs cycle
are tapped off from energy-generating processes to provide
biosynthetic intermediates. The number of building blocks
needed is surprisingly few, and as with any child’s con-
struction set a vast array of objects can be built up from a
limited number of basic building blocks. By far the most
important building blocks employed in the biosynthesis of
secondary metabolites are derived from the intermediates
acetyl coenzyme A (acetyl-CoA), shikimic acid, mevalonic
acid, and 1-deoxyxylulose 5-phosphate. These are utilized
respectively in the acetate, shikimate, mevalonate, and
deoxyxylulose phosphate pathways.
In addition to acetyl-CoA, shikimic acid, mevalonic
acid, and deoxyxylulose phosphate, other building blocks
based on amino acids are frequently employed in natural
product synthesis.
Peptides, proteins, alkaloids, and many antibiotics are
derived from amino acids, and the origins of the most
important amino acid components of these are briefly
indicated in Figure 13.1. Intermediates from the glycolytic
pathway and the Krebs cycle are used in constructing many

Subject:- Pharmacognosy 1

Semester:- Sem 3

Course:- Bachelor of pharmacy

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