Structures of Protein

 Proteins are made up of building blocks called amino acids. There are about 20 different amino acids that link together in different combinations. The building blocks of proteins are amino acids, which are small organic molecules that consist of an alpha (central) carbon atom linked to an amino group, a carboxyl group, a hydrogen atom, and a variable component called a side chain .

Primary structure of protein:

By primary structure of protein we mean the linkage of amino acids through polypeptide bonds.

Secondary Structure Of Protein:

Secondary structure of protein refers to regular, short ranged recurring arrangements in space of adjacent amino acid residues in a polypeptide chain.

This structure is stabilized by the hydrogen bonds between the carbonyl oxygen and amide nitrogens.

This structure includes the α-helix, β-sheets and β- turns.

α-helix:

The backbone adopts a cylindrical spiral structure and there are 3.6 AAs residues per helical turn.

The α helix is stabilized by hydrogen bonds between an amide hydrogen of one amino acid and a carbonyl oxygen four amino acids away.

The R groups always point outside in order to maintain connection with other molecules.  

 β-sheets:

In β-sheets structure the β- strands come in pairs or multiple copies and they connect with each other through hydrogen bonds.

Carbonyl oxygens and amide nitrogens in the strands form the Hydrogen bonds.

The terminal with carboxylic acid is called Carboxyl Terminus and the amide ones is called Amino Terminus. 

The strands of a β pleated sheet may be parallel, pointing in the same direction (meaning that their N- and C-termini match up), or antiparallel, pointing in opposite directions (meaning that the N-terminus of one strand is positioned next to the C-terminus of the other).

The β- strands in protein are 5 to 8 AAs long.  

  β- turns:

    This structure is the most common form of protein structure. This is a type of non-regular secondary structure in proteins that causes a change in direction of the polypeptide chain.

   β- turns consist of 3 to 4 amino acids.

  Glycine and Proline are common in turns 

Tertiary Structure:

3D structure of a polypeptide is called its tertiary structure.

R groups of the amino acids interact among themselves to form the tertiary structure.

The R groups interact with each other through Hydrogen bonding,Ionic bonding ,London dispersion force, dipole-dipole interactions.

The only covalent bond in the tertiary structure is the disulphide bond.

Covalent linkage between the -SH (thiol) groups of two cysteine monomers is the example of the Disulphide bond.