1. Amino Acids
By,
Dr. Kayeen Vadakkan
Assistant Professor,
Department of Biotechnology
St. Mary's College Thrissur,
Kerala , India
2. Amino Acids
• Amino acids are a group of organic compounds containing two
functional groups— amino and carboxyl.
• The amino group (—NH2) is basic while the carboxyl group (—
COOH) is acidic in nature.
• Amino acids bind to each other by peptide bond and form
proteins.
4. Classification of amino acids
The classification of amino acids are done in various criteria such
as
1. Based on Structure
2. Based on Polarity
3. Based on Nutritional requirements
5. Based of structure amino acids are classified into 7.
1. Amino acids with aliphatic side chains.
( Example: glycine, alanine, valine, leucine)
2. Hydroxyl group containing amino acids.
( Example :Serine, threonine)
3. Sulfur containing amino acids
(Example : Cysteine)
4. Acidic amino acids
(Example : Aspartic acid and glutamic acid)
Classification of amino acids based on
Structure
6. 5. Basic amino acids.
( Example: lysine, arginine)
6. Aromatic amino acids
( Example :Phenylalanine, tyrosine and tryptophan
7. Imino acids (It has an imino group (=NH), instead of an amino group (–NH2)
(Example : proline)
7. Classification of amino acids based on Polarity
Based on
Polarity
Amino Acids
1. Non-polar amino
acids.
(Example : alanine,
leucine, isoleucine)
2. Polar amino
acids with no
charge on ‘R’
group.
(Example: glycine,
serine, threonine)
3. Polar amino
acids with positive
charge on ‘R’
group.
(Example: lysine,
arginine and
histidine)
4. Polar amino
acids with negative
charge on ‘R’
group.
(Example: glutamic
acid )
8. Classification of amino acids based on nutritional
requirements
Based on nutritional
requirements
Amino Acids
1. Essential amino
acids.
(Example
:Arginine, Valine,
Histidine, )
2. Non essential
amino acids
(Example: glycine,
alanine, serine)
9. Peptide bond Formation
• The amino acids are held together in a protein
by covalent peptide bonds or linkages. These
bonds are rather strong and serve as the
cementing material between the individual
amino acids.
• The peptide bond is rigid and planar with
partial double bond in character.
• It generally exists in trans configuration. Both
–C=O and –NH groups of peptide bonds are
polar and are involved in hydrogen bond
formation.
• When the amino group of an amino acid
combines with the carboxyl group of another
amino acid, a peptide bond is formed
10. General Chemical reactions of Amino Acids
• The general reactions of amino acids are mostly due to the presence of two functional
groups namely carboxyl (–COOH) group and amino (–NH2) group.
1. Decarboxylation
Amino acids undergo decarboxylation to produce corresponding amines.
11. 2. Formation of Amide
The carboxyl group of dicarboxylic amino acids reacts with NH3 to form amide
3. Transamination
Transamination is the process by which amino groups are removed from amino acids and
transferred to acceptor keto-acids to generate the amino acid version of the keto-acid and the
keto-acid version of the original amino acid
12. 4. Oxidative deamination
Deamination is the removal of an amino group from a molecule. Enzymes that catalyse
this reaction are called deaminases.
The amino group is removed from the amino acid and converted to ammonia.
13. Amphoteric property of Amino acids
Amino acids contain both acidic (–COOH) and basic (–NH2) groups. They can donate a proton
or accept a proton, hence amino acids are regarded as ampholytes.
Zwitterion or dipolar ion
Zwitter ion (or dipolar ion) is a hybrid molecule containing positive and
negative ionic groups.
• The amino acids rarely exist in a neutral form with free carboxylic (–COOH)
and free amino (–NH2) groups.
• In strongly acidic pH (low pH), the amino acid is positively charged (cation)
while in strongly alkaline pH (high pH), it is negatively charged (anion).
• Each amino acid has a characteristic pH (e.g. leucine, pH 6.0) at which it
carries both positive and negative charges and exists as zwitterion
14. • Isoelectric pH (symbol pI) is defined as
the pH at which a molecule exists as a
zwitterion or dipolar ion and carries no
net charge.
• Thus, the molecule is electrically neutral.
The pI value can be calculated by taking
the average pKa values corresponding to
the ionizable groups.
15. Titration curve of alanine
• The existence of different ionic forms of amino acids can be more easily
understood by the titration curves.
• At low pH, alanine exists in a fully protonated form as cation. As the
titration proceeds with NaOH, alanine loses its protons and at isoelectric
pH (pI), it becomes a zwitterion. Further titration results in the formation
of anionic form of alanine.
16. Titration curve of alanine
9.7
6.0
2.34
• Alanine exists as cation at pH
2.34 and anion at pH 9.7. At
the isoelectric pH (pI=6.0),
alanine is found as zwitterion.
• Thus the pH of the medium
determines the ionic nature of
amino acids.
