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Amino Acids
Amino acids are the building blocks of proteins. Proteins are polymers of amino
acids, with each amino acid residue joined to its neighbor by a specific type of
covalent bond called peptide bond. Twenty different amino acids are commonly
found in proteins. All 20 of the common amino acids are α-amino acids.
They are defined as the organic acids which have a carboxyl group and an
amino group bonded to the same carbon atom (the α-carbon) (Fig). They differ
from each other in their side chains, or R groups, which vary in structure, size, and
electric charge, and which influence the solubility of the amino acids in water.
General structure of an amino acid. This structure is common to all but one of
the α-amino acids. (Proline, a cyclic amino acid, is the exception.) The R group or
side chain (red) attached to the α-carbon (blue) is different in each amino acid.
Classification of Amino Acids: There are different ways of classifying the amino
acids based on the structure and chemical nature, nutritional requirement,
metabolic fate etc.
1. Amino acid classification based on the structure
2. Nutritional classification of amino acids
3. Amino acid classification based on their metabolic fate
1. Amino acid classification based on the structure: Depending upon their
reaction in the solution amino acids are classified into three main classes as,
Neutral, Acidic and Basic.
I. Neutral Amino Acids: These amino acids have a carboxylic acid group
(-COOH) and an amino group (-NH3). This is the largest group of amino
acids and can further divided into following sub classes.
1. Aliphatic Amino Acids: these amino acids contains aliphatic side chain.
These include Glycine, Alanine, Valine, Leucine, and Isoleucine.
2. With side chain containing hydroxyl group (-OH): These include
Serine, Threonine, and Tryptophan.
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3. With side chain containing sulfur: These include Cysteine and
Methionine.
4. With side chain containing Aromatic Ring: These include
Phenylalanine, Tyrosine and Tryptophan.
5. Imino acid: e.g. Proline
II. Acidic amino Acids: These amino acids have two carboxylic acid group (-
COOH) and one amino group (NH3). These include Glutamic Acid,
Glutamine, Aspartic Acid, and Asparagine.
III. Basic Amino Acids: These include Histidine, Lysine and Arginine.
2. Nutritional classification of amino acids: The amino acids which
cannot be synthesized in the body but are essential for proper growth and
maintenance of good health, therefore need to be supplied through the diet are
called essential amino acids. The essential amino acids are listed below.
Arginine, Valine, Histidine, Isoleucine, Leucine, Lysine, Methionine,
Phenylalanine, Threonine, and Tryptophan. (A.V.,HILL, M.P., TT)
Of the ten amino acids listed above, two amino acids, namely, arginine and
histidine can be partely synthesized in adults. Therefore, they are considered as the
semi essential amino acids.
Those amino acids which can be synthesized in the body, so, may not be the
essential components of the diet are called non-essential amino acids. These
include- Glycine, Alanine, Serine, Tyrosine, Cysteine, Proline, Aspartate,
Asparagine, Glutamate and Glutamine.
3. Amino acid classification based on their metabolic fate: The carbon skeleton
of amino acids can serve as a precursor for the synthesis of glucose ( glycogenic)
or fat (ketogenic) or both. From metabolic view point, amino acids are divided into
three groups:
1. Glycogenic amino acids: These amino acids can serve as precursors for the
formation of glucose or glycogen. e.g. alanine, aspartate, glycine, methionine etc.
2. Ketogenic amino acids: Fat can be synthesized from these amino acids. Two
amino acids leucine and lysine are exclusively ketogenic.
3. Glycogenic and ketogenic amino acids: The four amino acids isoleucine,
phenylalanine, tryptophan, tyrosine are precursors for synthesis of glucose as well
as fat.
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Importance of Amino Acids
1. Amino acids are the building blocks of proteins. Proteins are polymers of
amino acids, with each amino acid residue joined to its neighbor by a
specific type of covalent bond called peptide bond. Twenty different amino
acids are commonly found in proteins.
2. Some amino acids are converted to carbohydrates and are known as the
glucogenic amino acids.
3. Specific amino acids give rise to specialized products.
a. Tyrosine forms precursor for the synthesis of Thyroid hormones (T3 and
T4), Catecholamines epinephrine and nor-epinephrine and black pigment
Melanin.
b. Glycine, Arginine, and Methionine are required for the synthesis of
creatine.
c. Tryptophan is utilized for the synthesis of vitamin Niacin.
d. Glycine and cysteine are required for the synthesis of bile salts.
e. Glutamate, cysteine and Glycine are required for the synthesis of
Glutathion.
f. Histidine is converted to histamine on Decarboxylation.
g. Serotonine, a neurotransmitter, is synthesized from tryptophan.
h. Glycine is required for the synthesis of Heme.
i. Some amino acids such as Glycine, Cysteine, are used in detoxification.
j. Methionine acts as active methyl group donor in the form of S-Adenosyl
methionine.
k. Cysteine and Methionine are the sources of sulfur in the body.
Physical Properties
• They are colorless, crystalline substances.
• They are more soluble in water then in the polar solvents.
• Tyrosine is soluble in hot water.
• They have high melting point usually more than 200° C.
• They have high dielectric constant.
• They possess a large dipole moment.
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Only L- α Amino Acids Occur in Proteins (Optical property):
All amino acids except Glycine exist in D and L isomer (Enentiomer). Only L-
amino acids occur in plant and animal proteins.
Amphoteric nature and Isoelectric pH:
• When an amino acid is dissolved in water, it exists in solution as the dipolar
ion, or zwitterion. The pH at which the amino acids are present in the
form of zwitterion is called Isoelectric pH (pI). A zwitterion can act as
either an acid (proton donor):
Amino acids are ampholytes acting both as acid and base. At specific pH, called
isoelectric pH (PI), amino acids are present in the form of dipolar ion, called
Zwitterion, that carry equal amount of positive and negative charges. On acidic
side of pI amino acids exist as a cation by accepting a H and on alkaline side, it
exist in the form of anion by donating a proton.
