Amino acids are a set of 20 different molecules used to build proteins. Proteins consist of one or more chains of amino acids called polypeptides. The sequence of the amino acid chain causes the polypeptide to fold into a shape that is biologically active. The amino acid sequences of proteins are encoded in the genes.

1. PROTEINS AND AMINO ACIDS

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Proteins arenitrogenous “macromolecules”
composed of many amino acids
Proteins are macromolecules with a
backbone formed by polymerization of
amino acids in a polyamide structure
They are the most versatile cell
components Composed of C, H, O, N plus
other elements such as S, I, Fe, P etc.
The nitrogen content is around 16 per cent
of the molecular weight of proteins.

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The word protein is derived from Greek word,
proteious meaning primary. So, proteins are the
major components of any living organism.
Proteins are natural substances with high molecular
weights ranging from 5,000 to many millions.
Proteins are most important constituent of cell
membranes and cytoplasm.
Muscle and blood plasma also contain certain
specific proteins.

4. IMPORTANCE OF PROTEINS
• Proteins are the main structural components of the cytoskeleton.
• Biochemical catalysts known as enzymes are proteins.
• Proteins known as immunoglobulins serve as the first line of
defence against bacterial and viral infections.
• Several hormones are protein in nature.
• Structural proteins furnish mechanical support and some of them
like actin and myosin are contractile proteins and help in the
movement of muscle fibre, microvilli, etc.
• Some proteins present in cell membrane, cytoplasm and
nucleus of the cell act as receptors.

5. • The transport proteins carry out the function of
transporting specific substances either across the
membrane or in the body fluids.
• Storage proteins bind with specific substances and store
them, e.g. iron is stored as ferritin.
• Few proteins are constituents of respiratory pigments and
occur in electron transport chain or respiratory chain, e.g.
cytochromes, hemoglobin, myoglobin.
• Under certain conditions proteins can be catabolised to
supply energy.
• Proteins by means of exerting osmotic pressure help in
maintenance of electrolyte and water balance in body.

6. • Protein containing foods are essential for
living organism, because protein is the
most important biological molecules in
building up and maintenances of the
structure of body.

7. AMINO ACIDS
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Proteins when hydrolyzed either by boiling with acids or
through the action of enzymes, are broken down into
their component amino acids.
Proteins are made up from, 20 such standard amino
acids in different sequences and numbers. Cells produce
proteins with strikingly different properties and activities
by joining the same 20 amino acids in many different
combinations and sequences.
This indicates that the properties of proteins are
determined by the physical and chemical properties of
their monomer units, the amino acids.
The names of the amino acids are often abbreviated,
either to three letters or to a single letter. Thus, for
example, proline is abbreviated to Pro or P

8. Functions of Amino Acids
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Apart from being the monomeric constituents of proteins and peptides, amino
acids serve variety of functions.
(a) Some amino acids are converted to carbohydrates and are called as
glucogenic amino acids.
(b) Specific amino acids give rise to specialised products, e.g.
• Tyrosine forms hormones such as thyroid hormones, (T3, T4), epinephrine
and norepinephrine and a pigment called melanin.
• Tryptophan can synthesise a vitamin called niacin.
• Glycine, arginine and methionine synthesise creatine.
• Glycine and cysteine help in synthesis of Bile salts.
.

9. • • Glutamate, cysteine and glycine synthesise
glutathione.
• Histidine changes to histamine on decarboxylation.
• Serotonin is formed from tryptophan.
• Glycine is used for the synthesis of haem.
• Pyrimidines and purines use several amino acids for
their synthesis such as aspartate and glutamine for
pyrimidines and glycine, aspartic acid, Glutamine and
serine for purine synthesis

10. STRUCTURE OF AMINO ACIDS
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All of the 20 standard amino
acids, except for glycine, have
four different groups arranged
tetrahedrally around the C
atom and thus can exist in
either the D or L configuration.
These two enantiomers are
non superimposable mirror
images that can only be
distinguished on the basis of
their different rotation of
plane-polarized light.
Only the L isomer is found in
proteins.
• A basic amino group (-NH2)
• An acidic carboxyl group ( -COOH)
• A hydrogen atom (-H)
• A distinctive side chain (-R)

11. Classification of Amino acids
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They are frequently grouped according to
the chemical nature of their side chains.
Common groupings of amino acids are
aliphatic, hydroxyl/sulfur, cyclic, aromatic,
basic, acidic and acid amides.

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Biological or Physiological Classification
Essential Amino Acids
NON Essential Amino Acids
2. Structural Classification
3. Electrochemical classification
Acidic and
Basic.

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Essential amino acids: These are the ones which are
not synthesised by the body and must be taken in
diet.
Non-essential amino acids: They can be synthesised
by the body and may not be the requisite
components of the diet.
Semi-essential amino acids: These aregrowth
promoting factors since they are not synthesised in
sufficient quantity during growth. They include
arginine and
histidine . They
become essential in
growing children

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Acidic amino acids:
These amino acids
have two
–COOH groups and
one – NH2 group.
They are therefore
monoaminodicarboxy
lic acids.
Aspartic acid (Asp)

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Basic amino acids: This
class of amino acids
consists of those amino
acids which have one –
COOH group and two –
NH2 groups.
Thus they arediamino
monocarboxylic acids.
Arginine, lysine and
hydroxylysine are included
in this group.
Arginine (Arg)

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This is the largest group of amino acids
and can be further subdivided into
Aliphatic
aromatic
heterocyclic
S-containing amino acids.

18. (a) Aliphatic Amino Acids (b) Aromatic Amino Acids
Valine (Val)
Tyrosine (Tyr) or parahydroxy
phenylalanine

19. Heterocyclic Amino Acids
Tryptophan (Trp)