3. The Proteins speak:
“We are the basis of structure
and function of life;
Composed of twenty amino acids,
the building blocks;
Organized into primary, secondary, tertiary
and quaternary structure;
Classified as simple, conjugated
and derived proteins.”
4. AMINO ACIDS
- group of organic compounds
containing two functional groups:
amino group (-NH2) basic
carboxyl group (-COOH) acidic
5. General Structure of Amino Acids
H H
R C COOH R C COOH
NH2 NH3
General Structure Exists as ion
6. ⍺ - amino acids
amino
groups – attached to the
carboxyl same carbon Atom
⍺ - carbon atom binds to a side chain
represented by R (different for
each of the 20 amino acids
found in proteins)
Ionized forms how they exist
7. Classification of Amino Acids
based on polarity
of the R group
4 groups
Polarity reflects the functional
role of AA in
protein structure
8. 1. Non polar AA-
hydrophobic (water hating)
No charge on the ‘R’ group
Examples are:
Alanine Methionine
Leucine Phenylalanine
Isoleucine Tryptophan
Valine Proline
9. 3 Polar AA with R group. (+) ‘ ’
carries (+) charge
Examples:
Histidine Arginine Lysine
4 Polar AA with R group. (-) ‘ ’
• carries (-) charge
• Examples:
Glutamic Acid Aspartic Acid
10. 2 Polar AA with no charge.
on R group‘ ’
no charge on the ‘R’ group
possess groups hydroxyl
sulfhydryl
amide
participate in hydrogen bonding of
protein structure
Examples:
Asparagine Glycine Cysteine
Tyrosine Serine
Threonine Glutamine
11. A.Physical Properties
1. Solubility - soluble in water and insoluble in
organic solvents
2. Melting Points - melt at higher
temperatures often 200°C
3. Taste
sweet (Gly, Ala, Val)
tasteless (Leu)
bitter (Arg, Ile)
Sodium Glutamate
– salt of Glutamic Acid – flavoring agent
12. 4. Optical Properties
- Assymetric a carbon atom is
attached to 4
different groups
exhibiting optical isomerism
4 distinct groups R
H - held by
an
COOH -⍺
carbon
13. All AA except Glycine possess
optical isomers due to
asymmetric -carbon atom⍺
Some AA (Isoleucine, Threonine)
2nd
asymmetric carbon
14. D- and L- forms of AA based on
the structure of glyceraldehyde
CHO CHO
H C OH OH C
H
CH2OH CH2OH
D-Glyceraldehyde L-
Glyceraldehyde
15. R R
H C NH2 H2N C
H
COOH COOH
D-Amino Acid L-
Amino Acid
The proteins are composed of L- amino⍺
acids
16. 5. Amino acids as ampholytes
can donate a proton or accept a
proton
AA contain both acidic (-COOH)
and basic (-NH2) groups
17. Zwitterion or dipolar ion:
Zwitter
from German word – means
“hybrid”
Zwitter ion (or dipolar ion)
a hybrid molecule containing (+)
and (-) ionic groups
18. AA rarely exist in a neutral form with free
carboxylic (-COOH) and free Amino (-NH2)
groups
Strongly acidic pH (low pH) AA (+)
charged (cation)
Strongly alkaline pH (high pH) AA (-)
charged (anion)
Each AA has a characteristic pH (e.g.
Leucine, pH – 6.0), at which it carries both
(+) and (-) charges and exist as zwitterion
19. Existence of an amino acid as Cation, Anion and
Zwitterion
H
H໋ R C COOH
H໋
NH2
H Ami no Aci d
H
R C COOH
R C COO ¯
NH3໋ H NH2
Cat i on H໋ H໋ Ani on
( l ow pH) R C
20. B. Chemical Properties
General Reactions mostly due to the 2
functional groups
Reactions due to - COOH group
1. AA from salts (-COONa) with bases
and esters (-COOR’) with alcohols
2. Decarboxylation
- AA undergo decarboxylation to
produce corresponding
21. 3. Reaction with Ammonia
- the carboxyl group of dicarboxylic
AA reacts with NH3 to form amide
Asparatic Acid + NH3 Asparagine
Glutamic Acid + NH3 Glutamine
22. Reactions due to -NH2 group
4. The Amino groups behave as bases and
combine with acids (e.g. HCl) to form
salts (-NH3 + Cl¯)
5. Reaction with NINHYDRIN
- the ⍺-AMINO ACIDS react with
Ninhydrin to form a purple, blue or
pink colour complex (Ruhemann’s
purple)
23. Amino acid + Ninhydrin Keto acid
+ NH3 + CO2 +
Hydrindantin
Hydrindantin + NH3 + Ninhydrin
Ruhemman’s
purple
Ninhydrin reaction – quantitative
determination of AA
24. 6. Colour reactions of Amino Acids
- AA can be identified by specific colour
reactions
Color Reactions of proteins / AA
Reaction Specific
group or AA
1. Buiret Reaction Two peptide linkages
2. Ninhydrin Reaction ⍺-Amino acids
3. Xanthoproteic Reaction Benzene ring of
aromatic AA (Phe, Tyr,
Trp)
4. Million’s reaction Phenolic Group (Tyr)
25. 6. Sakaguchi Reaction Guanidino Group
(Arg)
7. Nitroprusside Reaction Sulfhydryl groups
(Cys)
8. Paulys’ test Imidazole ring (His)
9. Sulfur test Sulfhydryl groups
(Cys)
10. Folin – Coicalteau’s Phenolic groups
test (Tyr)
26. 7. Transamination
- important reaction in AA
metabolism
- transfer of an amino group from an
amino acid to a keto acid to form a
new AA
8. Oxidative deamination
- AA undergo oxidative deamination
to liberate free ammonia
