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Amino Acids, Peptides and Proteins
The Proteins speak:
“We are the basis of structureand 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.”
AMINO ACIDS
- group of organic compounds containing two functional groups:
amino group (-NH2) basiccarboxyl group (-COOH) acidic
General Structure of Amino Acids
H H
R C COOH R C COOH
NH2 NH3
General Structure Exists as ion
⍺ - amino acids
aminogroups – attached to
thecarboxyl 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
Classification of Amino Acids based on polarity
of the R group
4 groups
Polarity reflects the functional role of AA in protein structure
1. Non-polar AA
hydrophobic (water hating)No charge on the ‘R’ groupExamples are:
Alanine MethionineLeucine
PhenylalanineIsoleucine
TryptophanValine Proline
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
2. Polar AA with no charge on ‘R’ group
no charge on the ‘R’ group possess groups hydroxyl
sulfhydrylamide
participate in hydrogen bonding of protein structure
Examples:Asparagine Glycine
Cysteine Tyrosine Serine Threonine Glutamine
A.Physical Properties
1. Solubility - soluble in water and insoluble in organic solvents
2. Melting Points - melt at higher temperatures often 200°C
3. Tastesweet (Gly, Ala, Val)tasteless (Leu)bitter (Arg, Ile)Sodium Glutamate – salt of Glutamic Acid – flavoring
agent
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 NH3໋
All AA except Glycine possess optical isomers due to asymmetric ⍺-carbon atom
Some AA (Isoleucine, Threonine) 2nd asymmetric carbon
D- and L- forms of AA based onthe structure of glyceraldehyde
CHO CHO
H C OH OH C H
CH2OH CH2OH
D-Glyceraldehyde L-Glyceraldehyde
R R
H C NH2 H2N C H
COOH COOH
D-Amino Acid L-Amino Acid
The proteins are composed of L-⍺ amino acids
5. Amino acids as ampholytes
can donate a proton or accept a
proton
AA contain both acidic (-COOH) and basic (-NH2) groups
Zwitterion or dipolar ion:
Zwitter from German word – means
“hybrid”
Zwitter ion (or dipolar ion) a hybrid molecule containing
(+) and (-) ionic groups
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
Existence of an amino acid as Cation, Anion and Zwitterion
H
H໋ R C COOH H໋
NH2
H Amino Acid H
R C COOH R C COO ¯
NH3 ໋ H NH2
Cation H໋ H໋ Anion (low pH) R C COO ¯ (high pH)
NH3 ໋Zwitterion
(Isoelectric pH)
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 amines
3. Reaction with Ammonia
- the carboxyl group of dicarboxylic AA reacts with NH3 to form amide
Asparatic Acid + NH3 AsparagineGlutamic Acid + NH3 Glutamine
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)
Amino acid + Ninhydrin Keto acid + NH3 + CO2 + Hydrindantin
Hydrindantin + NH3 + Ninhydrin Ruhemman’s purple
Ninhydrin reaction – quantitative determination of AA and proteins
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 linkages2. Ninhydrin Reaction ⍺-Amino acids3. Xanthoproteic Reaction Benzene ring of
aromatic AA (Phe, Tyr, Trp)
4. Million’s reaction Phenolic Group (Tyr)5. Hopkins – Cole Reaction Indole Ring (Trp)
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)
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
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