Notes on Amino acids - An Introduction - Structure and Classification of Amino acids -20 Amino acids

Amino acids are a set of 20 different molecules used to build proteins. Proteins consist of one or more chains of amino acids called polypeptides. The sequence of the amino acid chain causes the polypeptide to fold into a shape that is biologically active. The amino acid sequences of proteins are encoded in the genes.

Amino acids

General Structure of Amino acids

General Structure

Central carbon (C^α) attached to:

Hydrogen (H)
Amino group (-NH₂)
Carboxyl group (-COOH)
Side chain (R)

1. Enantiomers: The two non-super imposable mirror images. Molecules are classified as D form (Dextrorotatory ) and L (Laevorotatory) form depending on whether they rotate the plane of plane –polarized light clock wise or anticlockwise.

2. Naturally occurring amino acids are L (Laevorotatory) form.

3. D-form aminoacids are rarely found in bacterial cell walls and certain antibiotics (Gramicidin-S, Polymyxin, Actinomycin-D and Valiomycin).

4.N-acetylmuramic acid (NAM) is a D-aminoacid containing tetrapeptide.

5. Peptide bond: covalent bond formed between the α -amino group of one amino acid and α -carboxylic group of other forming a -CO-NH- linkage.

Peptide bond

Example of condensation reaction
Exergonic
Rigid plannar
Tripeptide means: 3 aminoacids +2 peptide bond

6. All aminoacids have high melting point more than 200⁰C.

7. Soluble in water and alcohol, insoluble in non-polar solvents.

8. Aminoacids can exist as ampholytes/Zwitter ions.

Amino acids Classification

Classification of amino acids

20 Different Amino Acids

9. Essential Amino acids:

Arginine(Arg/R), Histidine(His/H), Isoleucine (Ile/I), Leucine (Leu/L), Lysine (Lys/K), Methionine (Met/M), Phenylalanine (Phe/F), Threonine(Thr/T), Tryptophan (Trp/W), Valine (Val/V).

Memory tip: Any Help In Learning These Little Molecule Proves Truly Valuable.

This stands for Arginine, Histidine, Isoleucine, Leucine, Threonine, Lysine, Methionine, Phenylalanine, Tryptophan, Valine.

10. Non Essential Amino acids:

Alanine(Ala/A),Asparagine(Asn/N),Aspartate(Asp/D),Cysteine(Cys/C),Glutamate(Glu/E),Glutamine(Gln/Q),Glycine(Gly/G),Proline(Pro/P),Serine(Ser/S),Tyrosine(Tyr/Y).

11. Histidine and Arginine are essential amino acids for infants but not healthy adults.

12. Cysteine and Tyrosine are considered to be semi-essential amino acids. They are required by premature infants and adults who are ill.

13.Simplest amino acid: Glycine- An aminoacid without asymmetrical carbon atom.

14.Acidic aminoacids: Glutamic acid and Aspartic acid

Glutamic acid -Most common amino acid undergo oxidative deamination.

15.Basic aminoacids: Lysine and Arginine

Arginine contain guanidium group
Histones are rich in basic aminoacids

16. Neutral aminoacids: Glycine, Valine and Phenylalanine.

17. Alcoholic aminoacid: Serine and Threonine.

18. Aromatic aminoacids: Tryptophan, Tryrosine, Phenylalanine

Aromatic aminoacids absorb UV –light .This account for the characterization strong absorbance of light by most proteins at wavelength of 280 nm
Tryrosine forms skin pigment melanin and two hormones Adrenaline and Thyroxine

19.Heterocyclic aminoacids: Tryptophan and Histidine

Tryptophan (Bulkiest amino acid) has indole ring
Tryptophan: precursor of IAA (Indole 3-acetic acid- plant hormone) and nicotinamide (a β vitamin )
Amino acids with buffering activity-Histidine
Histidine :only aminoacid having an ionisable side chain with a pka near neutrality (pk value=6.1)
Buffering capacity of plasma proteins and haemoglobin is mainly due to Histidine residues
Histidine :more present in active site of an enzymes
Histidine contain imidazole group
Histidine found in intracellular and extracellular fluids of most animals and plants.

20. Sulphur containing aminoacid –Cysteine, Methionine

21. Aminoacids with amide group: Asparagine, Glutamine

22. Proline (Secondary imino amino acid) is rarely found within alpha helix segments.

23 Selenocysteine: 21^st aminoacid present in human protein.

24. Un common aminoacids:

· Collagen (A fibrous protein of connective tissue): 4 hydroxyproline, 5-hydroxyglycine.

· Myosin (A contractile protein of muscle):6-N Methyllysine

· Prothrombin (Blood clotting protein):γ-carboxyglutamate

25.Non-PROTEIN AMIINO ACIDS:

Ornithine and Citrulline are involved in the biosynthesis of Arginine and in the Urea cycle.

26. Glutathione is a co enzyme of oxidation –reduction reactions which consist of glutamic acid, glycine and cysteine. This chemical is important in preventing oxidative damage to erythrocytes.

27.Aspartame(Nutrasweet) is artificial sweetner, synthetic dipeptide, chemically it is -L-aspartyl-L-phenylalanine

28. Neurotransmitters:

GABA (Gamma Amino Butryic Acid)-a derivative of glutamic acid
Dopamine-derivative of tyrosine

29. Histamine- histidine derivative, allergic reaction mediator

30. Amino acids as helix breakers- Proline-Glycine-Tyrosine-Asparagine

31.Bend producing amino acid :Glycine-Proline -serine -Threonine

32.Sickle cell haemoglobin differs from normal haemoglobin by a single amino acid. In the β chain of sickle cell haemoglobin ,a valine (a hydrophobic acid) has replaced a glutamic acid( a negatively charged amino acid)

33. pI denotes isoelectric point/pH

pI=1/2(pk1+pk2)