All proteins are macromolecules because of their very high molecular weights. These are the polymers, i.e., chain-like molecules produced by joining a number of small units of amino acids called monomers. The amino acids are, therefore, regarded as ‘building blocks of proteins.
Each amino acid is a nitrogenous compound having both an acidic carboxyl (— COOH) and a basic amino (— NH2) group. R stands for the side chains that are different for each amino acid. R can be as simple as a hydrogen atom (H) or a methyl group (— CH3) or a more complex structure. The first carbon is the part of the carboxyl group. The second carbon, to which is attached the amino group, is called the α-carbon. The α-carbon of most amino acids is joined by covalent bonds to 4 different groups. Thus, the α-carbon in all the amino acids is asymmetric except in glycine where the α-carbon is symmetric.
Structure of 20 standard amino acids
1. Alanine – ala – A
2. Arginine – arg – R
3. Asparagine – asn – N
4. Aspartic acid – asp – D
5. Cysteine – cys – C
6. Glutamine – gln – Q
7. Glutamic acid – glu – E
8. Glycine – gly – G
9. Histidine – his – H
10. Isoleucine – ile – I
11. Leucine – leu – L
12. Lysine – lys – K
13. Methionine – met – M
14. Phenylalanine – phe – F
15. Proline – pro – P
16. Serine – ser – S
17. Threonine – thr – T
18. Tryptophan – trp – W
19. Tyrosine – tyr – Y
20. Valine – val – V
Twenty Amino acids can be grouped according to the characteristics of the side chains as follows:
Aliphatic – alanine, glycine, isoleucine, leucine, proline, valine.
Aromatic – phenylalanine, tryptophan, tyrosine.
Acidic – aspartic acid, glutamic acid.
Basic – arginine, histidine, lysine.
Hydroxylic – serine, threonine.
Sulphur-containing – cysteine, methionine.
Amidic (containing amide group) – asparagine, glutamine.
Properties of 20 standard amino acids
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