Amino acids are the building blocks of proteins. They contain an amino group, a carboxyl group, and a side chain. There are 22 protein amino acids that are polymerized to form proteins, which carry out important structural and functional roles in the body. Amino acids can also be classified based on their chemical properties and metabolic fates. The peptide bond forms when amino acids condense, linking them together into polypeptides and proteins.

1. Chemistry of amino acids and
proteins
Bile M(md)

2. Amino Acids
• Chemically, amino acids are organic acids that
contain one or more amino (–NH2) group(s).
• They are the structural units of proteins and
are obtained from them by hydrolysis.
• But they also attain important functions in
their free form.

3. Amino Acids
• Each amino acid has a carboxylic group (–
COOH), an amine group (–NH2) and a
characteristic side chain, radical or R-group (–
R).

4. Functions of amino acids
• They are polymerized to form proteins.
• Presence of specific amino acids at the active
site of enzymes is vital for their catalytic
activity
• Some amino acids (glucogenic) can be
converted to carbohydrates.

5. Functions of amino acids
• The carbon skeleton and nitrogen of amino
acids is used for the synthesis of purine and
pyrimidine bases for nucleotides and nucleic
acids.
• Glycine and methionine help in the
detoxification mechianisms.

6. Functions of amino acids
• Certain amino acids give rise to biologically
improtant derivatives:
1. Gycine is a precursor for ‘
 heme’ of hemoglobin and
 creatine that acts as the mediator of energy in muscles
2. Tyrosine is the precursor for a number of hormones
 Thyroxine,
 triiodothyronine,
 epinephrine and nor-epinephrine and
 skin pigment melanin.

7. Functions of amino acids
3. Tryptophan can give rise to
vitamin niacin and
neurotransmitter, Serotonin.
4. Histidine can be converted to the mediator
of allergic reactions i.e. histamine.

8. Cont’’
• Although about 300 amino acids exist in cells,
only 22 of them are polymerizable into protein
structure and are known as protein amino
acids, whereas, those that do not occur in
proteins are called as non-protein amino
acids.

9. Classification of Amino Acids
• The amino acids can be classified on the basis
of three criteria viz
• A) Chemical,
• B) Biological or nutritional, and
• C) Metabolic.

10. Classification of Amino Acids
• Chemical classification
• is based upon the number of amino groups to
carboxyl groups in the amino acid and is the most
widely used classification.
• The three groups of amino acids under this
classification are:
• a) Neutral amino acids
• b) Acidic amino acids
• c) Basic amino acids

11. Classification of Amino Acids
• a) Neutral amino acids that are mono-amino
mono-carboxylic; Glycine tyrosine tryptophan
• b) Acidic amino acids that are mono-amino
dicarboxylic; glutamic acid and asparatic acid
• c) Basic amino acids that are diamino (or
complex amino) mono-carboxylic amino
acids,lysine, histidine and arginine

12. Classification of Amino Acids
• Biological classification
– is based upon whether the amino acids can be
synthesized in human body or not into
• Essential amino acids
– that are not synthesizable in the body in adequate
amounts and must be supplied in the diet; and
• non-essential amino acids
– that are synthesizable in the body and there is no
diet dependency for them

13. Classification of Amino Acids
• Biologically and nutritionally, the dietary
deficiency or imbalance of any of the essential
indispensable amino acids leads to nutrition
deficiency disorders that affect both growth
and health

14. Ten amino acids are essential
• Ten amino acids are essential
– Valine, Isoleucine, Threonine, Tryptophan,
Arginine, Leucine, Lysine, Methionine, Phenyl
alanine and Histidine.
• For simplicity and memorization, the group of
essential amino acids abbreviated as ‘VITTAL
LyMPH’, where each letter stands for one
amino acid.

15. Arginine and Histidine are semi-essential:
• Arginine and Histidine are semi-essential:
– The healthy adult human body synthesizes just
enough arginine and histidine but in the childhood
growth period, sickness, convalescence and during
pregnancy such amount is not enough and requires
dietary supplementation.
– Therefore, these two amino acids are semi-essential.

16. Dietary source of essential amino
acids
• The main dietary source of essential amino
acids is proteins of high biological value, i.e.,
digestible proteins that contain all of them in
well-balanced proportions.
• These proteins include
– all animal proteins (e.g., milk, egg, meat, liver, fish
and chicken) and
– a few plant proteins (e.g., beans and lentils).

17. Dietary source of essential amino
acids
• The remaining non-essential amino acids are
diet independent because they are
synthesized in the body, but they are
metabolically as important as the essential
ones

18. Metabolic classification
• Metabolic classification
– based upon the catabolic fate of the carbon
skeleton of the amino acids inside the body into:
– Ketogenic amino acids
– Glucogenic amino acids
– The mixed amino acids

19. Metabolic classification
• Ketogenic amino acids:
– Only two amino acids are purely ketogenic viz. lysine and
leucine that catabolically give intermediates convertible
into acetyl-CoA or acetoacetyl-CoA.
• Glucogenic amino acids:
– Fourteen of the twenty protein amino acids give rise to
intermediates of glycolysis or Krebs' cycle (including;
pyruvate, oxaloacetate, -ketoglutarate and propionate)
convertible by gluconeogenesis into glucose.
• The mixed amino acids:
– These are amino acids, the carbon skeleton of which is
catabolized to produce the glycolytic intermediates as well
as the acetyl CoA derivatives:

20. The Peptide Bond
• Condensation of the primary –COOH group of
an amino acid and primary –NH2 group of
another amino acid forms the peptide (amide)
bond.
• Two amino acids form dipeptide (notice, a
miss name), 3 amino acids form tripeptide, etc
and >20 amino acids form a polypeptide and
less number is an oligopeptide

21. Peptides of biological importance:
• Bradykinin the local antiinflammatory, vasodilator and
hypotensive hormone that is a potent smooth muscle
relaxant is formed of 9 amino acids
• Oxytocin the posterior pituitary hormone that induces
uterine contraction is also nonapeptides, i.e., formed
of 9 amino acids.
• Thyrotropin releasing hormone (TRH), the
hypothalamic hormone responsible for the stimulation
of pituitary to secrete thyrotropin (TSH), is a tripeptide
• Insulin hormone is formed from 2 polypeptide chains
connected together by 2 disulfide linkages

22. Proteins
• Proteins are the fundamental building blocks
of the cell life,
• Chemically, proteins are organic compounds
with a high molecular weight formed of
carbon, oxygen, hydrogen and nitrogen and
may also contain sulfur, phosphorus, coloring
non-protein organic groups and metal ions

23. Proteins
• A protein molecule may be composed of a
single polypeptide, i.e., monomeric or several
ones, i.e., polymeric.
• Simple proteins are formed of amino acids
only, whereas, some proteins have a non-
proteinaceous moiety attached covalently or
non-covalently to the polypeptide chain(s),
these proteins are called as complex or
conjugated proteins.

24. Functions of proteins
• Structural: Proteins are the main structural
component in bone, muscles, cytoskeleton
and cell membrane.
• Nutrition: Provide the body with essential
amino acids, nitrogen and sulfur.
• Catalytic: All metabolic enzymes are proteins
in nature.
• Endocrine: Most of hormones and all
receptors are protein in nature.

25. Functions of proteins
• Defense:
– The antibodies (immunoglobulins) and
complement system that play an important role in
the body’s defensive mechanisms are proteins in
nature.
• Osmotic Potential:
– Plasma proteins are responsible for most effective
osmotic pressure of the blood

26. Functions of proteins
• Blood clotting factors are proteins.
• Transport role: Proteins carry lipids in the blood
forming lipoprotein complexes. Proteins also carry,
hormones, e.g., thyroid hormones and minerals, e.g.,
calcium, iron and copper. Hemoglobin (a chromo-
protein) carries O2 from the lung to tissues is a protein.
• Membrane transport: The plasma membrane is
lipoprotein and is semipermeable in nature. The
proteins in the membranes act as channels or
transporters to allow selective molecules/ions to cross
into or out of the cells.

27. Functions of proteins
• Gene Regulation: Control cellular activities
through control of gene expression: Most
factors required for DNA replication,
transcription and mRNA translation are
protein in nature.
• Signal Transduction: Cell-environment,
intercellular and intracellular signal
transduction is carried out largely by proteins.