Proteins are polypeptides. i.e., linear chains of amino acids linked by peptide bonds.

A Peptide bond is formed when –COOH group of one amino acid reacts with –NH2 group of next amino acid by releasing a molecule of water (dehydration).

Proteins are Heteropolymer of amino acids. i.e, different types of amino acids are linked through peptide bonds to form protein.

There are 20 types of amino acids involved in protein synthesis. They are
  1. Alanine (Ala)  
  2. Arginine (Arg)  
  3. Asparagine (Asn)  
  4. Aspartic acid (Asp) ) 
  5. Cystein (Cys)  
  6. Glutamine (Gln)  
  7. Glutamic acid (Glu)  
  8. Glycine (Gly)  
  9. Histidine (His)  
  10. Isoleucine (Ile) 
  11. Leucine (Leu)
  12. Lysine (Lys)
  13. Methionine (Met)
  14. Phenyl alanine (Phe
  15. Proline (Pro)
  16. Serine (Ser)
  17. Threonine (Thr)
  18. Tryptophan (Trp)
  19. Tyrosine (Tyr)
  20. Valine (Val)
Amino acids are 2 types:
  • Essential amino acids: They cannot be synthesized by the body and should be supplied through diet. E.g. Lysine, leucine, isoleucine, tryptophan etc.
  • Non-essential amino acids: They can be synthesized by the body. E.g. Glycine, alanine, serine, arginine etc.
Functions of protein:
  • For growth and tissue repair.
  • Many proteins transport nutrients across cell membranes. E.g. GLUT-4 enables glucose transport into cell.
  • Many proteins act as intercellular ground substance. E.g. collagen.
  • Some proteins function as antibodies to fight infectious organisms (pathogens) such as bacteria.
  • Some proteins act as receptors. E.g. receptors of smell, taste, hormones etc.
  • Many proteins are hormones. E.g. Insulin, glucagon etc. 
  • Many proteins act as enzymes. E.g. trypsin, pepsin, lactase, sucrase etc. 
  • Some proteins act as pigments. E.g. hemoglobin, myoglobin, chlorophyll etc.
Most abundant protein in animal world: Collagen 
Most abundant protein in the biosphere: Ribulose biphosphate carboxylase - oxygenase (RuBisCO)
Structural Levels of protein

Proteins have 4 structural levels. They are
  1. Primary structure
  2. Secondary structure
  3. Tertiary structure
  4. Quaternary structure
  • Primary structure: It describes the sequence of amino acids, i.e. the positional information in a protein. Left end of the chain has first amino acid (N-terminal amino acid). Right end has last amino acid (C-terminal amino acid).
  • Secondary structure: A protein thread is folded in the form of a helix. It has only right handed helices.
  • Tertiary structure: Long protein chain is folded upon itself like a hollow woolen ball. It gives 3-D view of protein. Tertiary structure is necessary for many biological activities of proteins.
  • Quaternary structure: Some proteins are an assembly of more than one polypeptide or subunits. E.g. Hb has 4 subunits (2 α subunits and 2 β subunits).


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