The document classifies and describes the 20 standard amino acids. It divides them into categories based on their R-groups: non-polar aliphatic, polar uncharged, aromatic, positively charged, and negatively charged. Within each category it provides examples and describes key properties of amino acids like glycine, alanine, valine, leucine, isoleucine, methionine, phenylalanine, tyrosine, tryptophan, lysine, arginine, histidine, aspartic acid, and glutamic acid. It also lists the essential and nonessential amino acids for human nutrition.

1. Classification of Amino Acids
By
Mrs Sanchita Choubey
(M.Sc., PGDCR, Pursuing Ph. D)
Assistant Professor of Microbiology
Dr. D Y Patil Arts Commerce and Science College Pimpri, Pune

2. Structure of Amino acid

3. Classification of Amino Acids
• Nutritional Based on R group
- Essential - Non polar aliphatic R group
- Non-essential - Polar uncharged R group
- Aromatic R group
- Positively charged R group
- Negatively charged R group

4. The hydrocarbon R group in this
class of amino acids is nonpolar and
hydrophobic. Glycine has the
simplest amino acid structure. The
bulky side chain of valine, isoleucine
and leucine are important in
promoting hydrophobic interactions
within protein structures.

5. Gly amino acid
Glycine was the first amino acid to be isolated from a protein, in this
case gelatin, and is the only one that is not optically active (no d- or l-
stereoisomers). Structurally the simplest of the α-amino acids, it is
very unreactive when incorporated into proteins. Even so, glycine is
important in the biosynthesis of the amino acid serine, the coenzyme
glutathione, purines and heme, a vital part of hemoglobin.
Ala amino acid
Discovered in protein in 1875, alanine makes up 30 % of the residues
in silk. Its low reactivity contributes to the simple, elongated structure
of silk with few cross-links which gives the fibers strength, stretch
resistance and flexibility. Only the l-stereoisomer participates in the
biosynthesis of proteins.

6. Val amino acid
The structure of valine was established in 1906, after first being
isolated from albumin in 1879. Only the l-stereoisomer appears in
mammalian protein. Valine can be degraded into simpler
compounds in the body, but in people with a rare genetic condition
called maple syrup urine disease, a faulty enzyme interrupts this
process and can prove fatal if untreated.
Leu amino acid
Leucine was isolated from cheese in 1819 and from muscle and
wool in its crystalline state in 1820. In 1891, it was synthesized in
the laboratory.
Only the l-stereoisomer appears in mammalian protein and can be
degraded into simpler compounds by the enzymes of the body.
Some DNA binding proteins contain regions in which leucines are
arranged in configurations called leucine zippers.

7. Ile amino acid
Isoleucine was isolated from beet sugar molasses in 1904. The hydrophobic nature of
isoleucine’s side chain is important in determining the tertiary structure of proteins in
which it is included.
Those suffering from a rare inherited disorder called maple syrup urine disease, have a
faulty enzyme in the degradation pathway common to isoleucine, leucine, and valine.
Without treatment, metabolites build up in patient’s urine contributing the distinctive odor
that gives the condition its name.
Met amino acid
Methionine was isolated from the milk protein casein in 1922, and its structure solved by
laboratory synthesis in 1928.Methionine is an important source of sulfur for numerous
compounds in the body, including cysteine and taurine. Linked to its sulfur content,
methionine helps to prevent fat accumulation in the liver, and helps to detoxify metabolic
wastes and toxins.
Methionine is the only essential amino acid that is not present in significant amounts of
soybeans and is therefore produced commercially and added to many soy meal
products.

8. Their aromatic side chains are relatively
nonpolar. All can participate in hydrophobic
interactions. The OH group of tyrosine can
form hydrogen bond and can act as an
important functional group in the activity of
some enzymes.

9. Phe amino acid
Phenylalanine was first isolated from a natural source (lupine sprouts) in 1879 and
subsequently synthesized chemically in 1882. The human body is ordinarily able to break
down phenylalanine into tyrosine, however in individuals with the inherited
condition phenylketonuria (PKU), the enzyme that performs this conversion lacks activity. If
left untreated, phenylalanine builds in the blood causing retarded mental development in
children. On in 10,000 children are born with the condition, adopting a diet low in
phenylalanine early in life can ease the effects.
Tyr amino acid
In 1846 tyrosine was isolated from the degradation of the casein (a protein from cheese),
following which it was synthesized in the laboratory and its structure determined in 1883.
Only present in the l-stereoisomer in mammalian proteins, humans can synthesize tyrosine
from phenylalanine. Tyrosine is an important precursor to the adrenal hormones
epinephrine and norepinephrine, thyroid hormones including thyroxine and the hair and
skin pigment melanin. In enzymes, tyrosine residues are often associated with active sites,
alteration of which can change enzyme specificity or wipe out activity entirely.
Suffers of the serious genetic condition phenylketonuria (PKU) are unable to convert
phenylalanine to tyrosine, whilst patients with alkaptonuria have a defective tyrosine
metabolism, producing distinctive urine which darkens when exposed to the air.

10. Tryptophan (symbol Trp or W) is an α-amino acid that is used
in the biosynthesis of proteins. Tryptophan contains an α-
amino group, an α-carboxylic acid group, and a side chain
indole, making it a non-polar aromatic amino acid. It is
essential in humans, meaning that the body cannot synthesize
it and it must be obtained from the diet. Tryptophan is also a
precursor to the neurotransmitter serotonin, the hormone
melatonin, and vitamin B3. It is encoded by the codon UGG.

11. The R group of these
amino acids is more
soluble in water, or
hydrophilic than those of
non polar amino acids,
because they contain
functional groups that
form hydrogen bond with
water

12. The amino acids in which the R group have a
net positive charge at pH 7.0

13. Lysine
Lysine (symbol Lys or K)[2] is an α-amino acid that is used in the
biosynthesis of proteins. It contains an α-amino group (which is
in the protonated −NH3+ form under biological conditions), an α-
carboxylic acid group (which is in the deprotonated −COO− form
under biological conditions), and a side chain lysyl
((CH2)4NH2), classifying it as a basic, charged (at physiological
pH), aliphatic amino acid. It is encoded by the codons AAA and
AAG. Like almost all other amino acids, the α-carbon is chiral
and lysine may refer to either enantiomer or a racemic mixture of
both. For the purpose of this article, lysine will refer to the
biologically active enantiomer L-lysine, where the α-carbon is in
the S configuration.

14. Arginine
Arginine, also known as l-arginine (symbol Arg or R), is an α-
amino acid that is used in the biosynthesis of proteins. It
contains an α-amino group, an α-carboxylic acid group, and
a side chain consisting of a 3-carbon aliphatic straight chain
ending in a guanidino group. At physiological pH, the
carboxylic acid is deprotonated (−COO−), the amino group is
protonated (−NH3+), and the guanidino group is also
protonated to give the guanidinium form (-C-(NH2)2+),
making arginine a charged, aliphatic amino acid. It is the
precursor for the biosynthesis of nitric oxide. It is encoded
by the codons CGU, CGC, CGA, CGG, AGA, and AGG.

15. Histidine
Histidine (symbol His or H) is an α-amino acid that is used in the
biosynthesis of proteins. It contains an α-amino group (which is in
the protonated –NH3+ form under biological conditions), a
carboxylic acid group (which is in the deprotonated –COO− form
under biological conditions), and an imidazole side chain (which is
partially protonated), classifying it as a positively charged amino
acid at physiological pH. Initially thought essential only for infants,
it has now been shown in longer-term studies to be essential for
adults also. It is encoded by the codons CAU and CAC.

16. Amino acids having R
group with a net negative
charge at pH 7.0, with a
second carboxyl group

17. Aspartic Acid
Aspartic acid symbol Asp the ionic form is known as aspartate), is
an α-amino acid that is used in the biosynthesis of proteins. Like all
other amino acids, it contains an amino group and a carboxylic acid.
Its α-amino group is in the protonated –NH+3 form under
physiological conditions, while its α-carboxylic acid group is
deprotonated −COO− under physiological conditions. Aspartic acid
has an acidic side chain (CH2COOH) which reacts with other amino
acids, enzymes and proteins in the body. Under physiological
conditions (pH 7.4) in proteins the side chain usually occurs as the
negatively charged aspartate form, −COO−.

18. Glutamic acid (symbol Glu or E;[4] the ionic form is known
as glutamate) is an α-amino acid that is used by almost all
living beings in the biosynthesis of proteins. It is non-
essential in humans, meaning the body can synthesize it. It is
also an excitatory neurotransmitter, in fact the most
abundant one, in the vertebrate nervous system. It serves as
the precursor for the synthesis of the inhibitory gamma-
aminobutyric acid (GABA) in GABA-ergic neurons.
Glutamic acid

19. Structures and abbreviations of the standard
amino acids

20. Nutritional Classification of Amino acids
Essential Amino
Acids:
Need to supplied in daily diet
1. Lysine
2. Leucine
3. Isoleucine
4. Metionine
5. Tryptophan
6. Phenylalanine
7. Threonine
8. Valine
Nonessential Amino Acids:
Need not be supplied in daily diet
• Alanine
• Asparagine
• Glycine
• Tyrosine
• Serine
• Proline
• Cysteine
• Histidine(essential for children)
• Glutamine(conditionally
essential)
• Arginine(conditionally essential)
• Glutamate