This lecture was dilivered to MBBS first Professional year. though it can be used to teach any undergraduate courses in Biochemistry.

1. AMINO ACIDS
Dr Kshema Thakur, PhD
Molecular Biology & Biochemistry

2. General structure of amino acids
Monoamino monocarboxylic acids
Or α-amino α-carboxylic acids

3. The 20 Key Amino Acids
• More than 300 amino acids occur
naturally, but 20 of them are especially
important.
• These 20 amino acids are the building
blocks of proteins, the most abundant
biological macromolecules.

4. FUNCTIONS OF PROTEINS
Enzymes
Hormones
Contractile proteins
Antibodies
Transport protein
Tissue proteins
Feather
Spider web
Hair/nails
Mushroom poisons
Lens protein of the eye
Milk proteins
Light producing molecule in firefly
Serve as source of energy during starvation
Molecular instruments for expression of genetic
information
Etc. . .

5. L- and D- Stereoisomers of amino acids
(L-isomers exist mainly in human system)

6. Amino acid Three letter
code
One letter
code
Alanine Ala A
Arginine Arg R
Asparagine Asn N
Aspartic acid Asp D
Cysteine Cys C
Glutamine Gln Q
Glutamic acid Glu E`
Glycine Gly G
Histidine His H
Isoleucine Ile I
Amino acid Three
letter code
One letter
code
Leucine Leu L
Lysine Lys K
Methionine Met M
Phenylalanine Phe F
Proline Pro P
Serine Ser S
Threonine Thr T
Tryptophan Try W
Tyrosine Tyr Y
Valine Val V

7. CLASSIFICATION
1. Based on side chain
Nonpolar
Polar
Aromatic
2. Based on nutritional requirement
Essential
Nonessential
Semiessential
3. Based on metabolic fate
Ketogenic
Glucogenic
Mixed ketogenic and glucogenic

8. Gly is the only amino acid which is achiral
Pro is actually an imino acid (with amino group at secondary
position) and destabilizes α-helices
Met is methyl donor for biological reactions

9. Ser and Thr form sites for phosphorylation at –OH
Cys forms disulphide bridges

10. Formation of Cystine and
disulphide bridges

11. Arg has Guanidinium group
His has Imidazole ring

12. Trp has indole ring
Aromatic amino acids give proteins their characteristic
absorption at 280 nm

13. BASED ON NUTRITIONAL
REQUIREMENT
Essential Semi-essential Non-essential
Isoleucine Arginine Glycine
Leucine Histidine Alanine
Threonine Serine
Lysine Cysteine
Methionine Aspartate
Phenylalanine Asparagine
Tryptophan Glutamate
Valine Glutamine
Tyrosine
Proline

14. BASED ON METABOLIC FATE
1. Glucogenic
Ala, Arg, Asn, Asp, Cys, Gln, Glu,
Gly, His, Meth, Pro, Ser, Thr, Val
2. Ketogenic
Lys, Leu
3. Mixed ketogenic and glucogenic
Ile, Phe, Tyr, Trp

15. MNEMONICS
Polar side chain:
Santa’s Team Crafts New Quilts Yearly
Ser, Thr, Cys, Asn, Gln, Tyr
Non polar side chains:
Grandma Always Visits London In May For Winston’s Party
Gly, Ala, Val, Leu, Ile, Met, Phe, Trp, Pro
Electrically charged:
Dragons Eat Knights Riding Horses
Asp, Glu, Lys, Arg, His
Essential amino acids:
Any Help In Learning These Little Molecules Proves Truly
Valuable
Arg, His, Ile, Leu, Thr, Lys, Met, Phe, Trp, Val

16. Uncommon amino acids

17. NON-α-AMINO ACIDS
β-Alanine
β-Aminoisobutyric acid
γ-Aminobutyric acid (GABA)
δ-Aminolevulinic acid (ALA)
Taurine

18. CHEMICAL & PHYSICAL
PROPERTIES
Of
AMINO ACIDS

19. pI: isoelectric pH (equal amount of postive and negative charge)
Amino acids in solution: Ampholytes
Acid (proton acceptor)
Base (proton donor)
Diprotic in nature

20. TITRATION OF AMINO ACIDS
pI =
pKa1 + pKa2
2
pI = 5.97

21. Properties of amino acids
1.Reactions due to carboxylic group
2.Reactions due to amino group
3.Reactions due to side chain

22. The amino acids will undergo alpha decarboxylation to
form the corresponding “amines”. Thus biologically
important amines are produced from amino acids.
Histidine → Histamine + CO2
Tyrosine →Tyramine + CO2
Tryptophan →Tryptamine + CO2
Lysine →Cadaverine + CO2
Glutamic acid → Gamma Amino Butyric Acid (GABA) +
CO2
DECARBOXYLATION
1. Reactions due to carboxylic group

23. The carboxyl group of amino acids can release a H+ ion with
the formation of Carboxylate (COO–) ions. These may be
neutralized by cations like Na+ and Ca+2 to form Salts. Thus
amino acids react with alkalies to form “Salts”.
1. Reactions due to carboxylic group
Reaction with Alkalies (Salt formation)

24. Amino acid reacts with Amines to
form “Amides”.
1. Reactions due to carboxylic group
REACTION WITH AMINES
Aspartic acid – Asparagine
Glutamate - Glutamine

25. When the amino acids are treated with acids (like
HCl), it forms “Acid Salts”.
2. Reactions with –NH2 group
Reaction with Acids (Salt formation)

26. Ninhydrin reagent reacts with free amino group of amino acid yielding
purple colored product.
Ninhydrin can react with imino acids as proline and hydroxy proline but
gives yellow color.
Reaction with Ninhydrin
2. Reactions with –NH2 group

27. 1- Millon reaction: for phenolic group of tyrosine gives red colored
mass
2- Rosenheim reaction: for trptophan and gives violet ring.
3- Pauly reaction: for imidazole ring of histidine: gives yellow to
reddish product
4- Sakaguchi test: for guanido group of arginine and gives red
color.
5- Lead sulfide test (sulfur test): for sulfur containing amino
acids as cysteine give black color precipitates.
3. Reactions due to side chain (R)

28. Thank you