Dr Nazia Khan
Assistant professor
College of medicine
Amino Acids
Amino acids are a group of organic compounds containing two functional
groups – amino (basic) and carboxylic group (acidic).
Amino acids are the building blocks of proteins.
The general structure of amino acids includes an α-amino group , an αcarboxylate group and a variable side chain (R) .
Properties of AA
 Physical properties-
 Colorless
 Crystalline
 May be sweet(Glycine, Alanine, Valine),
tasteless(Leucine) or bitter(Arginine,Isoleucine).
Aspartame- An artificialsweetener contains Aspartic
acid and Phenylalanine
 Soluble in water, acids, alkalis but insoluble in organic
solvents
 High melting point(More than 200°c)
Optical Properties of AA
 The α carbon of each amino acid is attached to four
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different groups and is thus optically active carbon atom.
Glycine is exceptional because there are two hydrogen
substituents at the α carbon, thus it is optically inactive.
Amino acids with asymmetric centre at the αcarbon can
exist in two forms, D and L forms that are mirror images of
each other and are called Enantiomers.
All amino acids found in proteins are of L-configuration
D- amino acids are found in some antibiotics and in
bacterial cell walls.
Isoelectric Point
 Amino acids can exist as ampholytes or zwitterions in
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solution, depending upon pH of the medium
The pH at which the amino acids exist as zwitterions,
with no net charge on them is called Isoelectric pH or
Isoelectric point.
In acidic medium, the amino acids exist as cations
In alkaline medium , they exist as anions.
Due to no net charge, there is no electrophoretic
mobility at Isoelectric pH.
Solubility and buffering capacity are also minimum at
Isoelectric pH
Classification of AA
Amino acids can be classified in 4 ways:
 1. Based on structure
 2. Based on the side chain characters
 3. Based on nutritional requirements
 4. Based on metabolic fate
Classification based on Structure
Simple amino acids:
 - Glycine , Alanine
Branched chain amino acids:
 - Valine,Leucine and Isoleucine
OH group-containing amino acids:
 - Serine and Threonine
Sulfur-containing amino acids:
 - Cysteine, Cystine and Methionine.
Amide group-containing amino acids:
 - Glutamine and Asparagine
Acidic Amino Acids:
 Aspartic acid and Glutamic acid
Basic amino acids:
 Arginine, Lysine and Histidine
Aromatic amino acids
 Phenyl alanine, tyrosine and tryptophan
Imino acid
 Proline
Classification based on side chain character
 A. Amino acids with a non-polar side-chain:
e.g.: Alanine, Valine, Leucine, Isoleucine,
Phenylalanine, Tryptophan, Proline
 Each of these amino acids has a side chain that does not
participates in hydrogen or ionic bonds.
 Side chains of these amino acids can be thought of as
“Oily” or lipid like, a property that promotes hydrophobic
interactions.
 B) Amino acids with a polar but uncharged side-chain:
e.g. Serine, Threonine, Tyrosine, Cysteine, Asparagine and
Glutamine.
 These amino acids are uncharged at neutral
 They can also participate in hydrogen bond formation.
 C) Amino acids with a charged side-chain
 a) Amino acids with a positively charged side- chain: The basic
amino acids
Ex: Lysine, Arginine and Histidine
 b) Amino acids with a negatively charged side- chain: The
acidic amino acids
Ex: Glutamic acid and Aspartic acid
 They are hydrophilic in nature. Can participate in Hydrogen
bond formation
Classification based on Nutritional requirement
 I. Essential amino acids: These amino acids cannot be
synthesized in the body and have to be present essentially
in the diet.
 Examples-Valine, Isoleucine, Leucine, Lysine, Methionine,
Threonine, Tryptophan and Phenylalanine.
 II. Semi-essential amino acids: These amino acids can be
synthesized in the body but the rate of synthesis is lesser
than the requirement(e.g. during growth, repair or
pregnancy)
 Examples-Arginine and Histidine.
 III. Non-essential amino acids: These amino acids are
synthesized in the body, thus their absence in the diet does
not adversely affect the growth.
 Examples- Glycine, Alanine, and the other remaining amino acids.
Classification based on metabolic fate
 The carbon skeleton of amino acids can be used either
for glucose production or for the production of ketone
bodies, Based on that
 I. Both glucogenic and ketogenic amino acids:
 Isoleucine, Tyrosine, Phenylalanine and Tryptophan
 II. Purely Ketogenic amino acids:
 Leucine and Lysine
 III. Purely Glucogenic amino acids:
 The remaining 14 amino acids are glucogenic. Alanine, valine
,serine, threonine, glycine, methionine, asparagine, glutamine,
cysteine, cystine, aspartic acid, glutamic acid, histidine and
arginine.
AMINO ACIDS NOT FOUND IN PROTEINS
1.
2.
3.
4.
5.
CARNITINE
CITRULLINE
GLUTATHIONE
TAURINE
ORNITHINE
Carnitine
Citrulline
Functions of AA
 Incorporated into tissue proteins
 Act as precursors for haem, creatine, glutathione,
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Porphyrins, purines and pyrimidines synthesis
Niacin, Serotonin and melatonin are synthesized from
Tryptophan
Melanin, thyroid hormone, catecholamines are
synthesized from Tyrosine
GABA (neurotransmitter) is synthesized from
Glutamic acid
Nitric oxide, a smooth muscle relaxant is synthesized
from Arginine
Reactions of AA
 1) Reactions due to amino group:
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Oxidative deamination:
Transamination:
Formation of carbamino compound
 2) Reactions due to carboxyl group
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Decarboxylation
Formation of amide linkage
 3) Reactions due to side chain
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Ester formation
Formation of disulphide bonds
Transmethylation
 4) Reaction due to both amino and carboxyl groups
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Formation of peptide bond
 Thank u.
 Extra points and detailed information about reactions
of Amino acids.
Derived Amino Acids:
 Non-α-amino acids e.g.: β-alanine, γ-amino butyric acid
(GABA), δ-amino Levulinic acid
 Derived and Incorporated in tissue proteins: e.g.:
Hydroxy-proline, hydroxy-lysine
 Derived but not incorporated in tissue proteins: e.g.:
Ornithine, Citrulline, Homocysteine, Argino succinic
acid.
 Seleno cysteine is the 21st Amino Acid
 Pyrolysine is 22nd aminoacid.
Special Groups in AA
 Arginine- Guanidinium group
 Phenyl Alanine- Benzene group
 Tyrosine- Phenol group
 Tryptophan- Indole group
 Histidine- Imidazole group
 Proline- Pyrrolidine group.
Reactions due to amino group
 Oxidative deamination:
 α amino group is removed and corresponding α-keto
acid is formed. α-keto acid produced is either converted
to glucose or ketone bodies or is completely oxidized.
 Transamination:
 Transfer of an α amino group from an amino acid to an α
keto acid to form a new amino acid and a corresponding
keto acid.
 Formation of carbamino compound
 CO2 binds to α amino acid on the globin chain of
hemoglobin to form carbamino hemoglobin
 The reaction takes place at alkaline pH and serves as a
mechanism for the transfer of Carbon dioxide from the
tissues to the lungs by hemoglobin.
Reactions due to carboxyl group
 1) Decarboxylation- Amino acids undergo alpha
decarboxylation to form corresponding amines.
 Examples Glutamic acid  GABA
 Histidine Histamine
 Tyrosine  Tyramine
 2) Formation of amide linkage
• Non α carboxyl group of an acidic amino acid reacts with
ammonia by condensation reaction to form corresponding
amides .
Aspartic acid Asparagine
Glutamic acid  Glutamine
Reactions due to side chains
1) Ester formation
 OH containing amino acids e.g. serine, threonine can
form esters with phosphoric acid in the formation of
phosphoproteins.
 OH group containing amino acid can also form:
Glycosides – by forming
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O- glycosidic bond with carbohydrate residues.
2)Formation of disulphide bonds
 Cysteine has a sulfhydryl group( SH) group and can
form a disulphide (S-S) bond with another cysteine
residue.
 The dimer is called Cystine
 Two cysteine residues can connect two polypeptide
chains by the formation of interchain disulphide chains
3)Transmethylation
 The methyl group of Methionine can be transferred after
activation to an acceptor for the formation of important
biological compounds
D)Reactions due to both amino & carboxyl groups:
-Formation of peptide bond
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