2. Amino acids
• Amino acids are building blocks of proteins. Proteins are composed
of 20 different amino acid.
• Amino acids are organic compounds having an amino group
attached to a chain containing an acid group. Amino acid derived
from proteins have the amino group on a-carbon that is the carbon
atom next to the carboxyl group.
• R group may be hydrogen, aliphatic group, or aromatic ring.
3. CLASSIFICATION (ON THE BASIS OF
STRUCTURE)
Amino acids are classified as neutral, acidic, or basic according
to no of amino acids and carboxyl group in the molecule.
1) Neutral amino acids - contain one amino acid and one
carboxyl group
R
C
COOH
NH2
2) Acidic amino acids - contain one amino group and two
carboxyl group
HOOC
CH
NH2
COOH
4. 3) Basic amino acids – contain two amino groups and one
carboxyl group.
H2N
CH
NH2
COOH
5. CLASSIFICATION – ON THE BASIS OF POLARITY
1) Non polar amino acids – the are also referred to as hydrophobic
(water hating) They have no charge on R group.
Glycine (Gly)
Methionine (Met)
Alanine (Ala)
Phenylalanine (Phe)
Valine (Val)
Tryptophan (Trp)
Leucine (leu)
Isoleucine
(Ile)
6. 2) POLAR AMINO ACIDS WITH NO CHARGE ON R GROUP
They carry no charge on R group. They possess groups such
as hydroxyl, sulfhydril and amide.
Serine (Ser)
Threonine (Thr)
Tyrosine (Tyr)
Cysteine (cys)
Asparagine (Asn)
Glutamine (Gln)
7. 3) POLAR AMINO ACID WITH POSITIVE R GROUP
Lysine (Lys)
Arginine (Arg)
Histidine (His)
4) Polar amino acid with negative R group
Aspartic acid (Asp)
Glutamic acid (Glu)
8. CLASSIFICATION ON THE BASIS OF NUTRTION
Eesential amino acids – cannot be synthesized by
the body. Therefore they must be present in our
diet.
Arginine*
Histidine*
Isoleucine
Leucine
Valine
Lysine
Methionine
Threonine
Phenylalanine
Tryptophan
* Can be synthesized by adults but not by growing
children. They are also called as semi essential
amino acids.
9.
Non essential amino acids –
They are synthesized in our body. Hence they need
not to be consumed in the diet.
Alanine
Asparagine
Aspartate
Glutamate
Glutamine
Glycine
Proline
Serine
Cysteine
Tyrosine
10. CLASSIFICATION ON THE BASIS OF METABOLIC
FATE
1) Glycogenic amino acid – these amino acids
serve as a precursor for the formation of glucose or
glycogen.
eg. alanine, aspartate, glycine, methionine etc.
2) Ketogenic amino acid – fat can be synthesized
from these amino acids.
eg. Lucine and lysine
11. PROPERTIES OF AMINO ACIDS
Solubility: most of the amino acids are soluble in water and
insoluble in organic solvents.
Melting point: melt at higher temperature
(above 200 c)
Taste: sweet - glycine, alanine, valine
tasteless – leucine
bitter – arginine, isoleucine
Optical activity: all the amino acids except glycine possess
optical isomers due to presence of asymmetric carbon atom.
glycine
12.
Alanine and all other amino acids have an
asymmetric carbon at position 2 (the a-carbon
atom). For this reason they all are optically active and exist
in D and L forms. Which are non super-imposable mirror
images.
13. Amino acids as ampholytes : amino acids contain both acidic
(-COOH) and basic (-NH2) groups. They can donate a proton and
accept a proton. Hence they are also called as ampholytes.
Zwitter ions : Amino acids also exist in zwitter ion form. zwitter
ion is a hybrid molecule that contain both positive as well as
negative ionic groups. eg. leucine
- at isoelectric ph - carries no net charge
14. CHEMICAL PROPERTIES
A) Reactions due to carboxylic group
1)amino acids form salts (-COONa) with bases and
estres (-COOR) with alcohols.
2) Deacarboxylation : amino acids undergo
decarboxylation to produce amines.
H2N-CH2-COOH + Ba(OH)2
glycine
CH3-NH2 + BaCO3 + H2O
methylamine
3) Reaction with ammonia : form amides
aspartic acid + NH3
glutamic acid + NH3
aspargamine
glutamine
15. REACTIONS DUE TO NH2 GROUP
1 ) Amino groups behave as bases and combine with acids
(eg.HCl) to form salts.
2) Reaction with ninhydrin
the a-amino acid react
with ninhydrin to form a
purple, blue or pink colour
complex.
Ninhydrin reaction is used
for the quantitative
determination of amino
acids and proteins.
16.
Oxidative deamination :The amino acids undergo
oxidative deamination to liberate free ammonia.
Transmethylation: transfer of amino group from an
amino acid to a keto group is called
transmethylation
