Structure of Proteins
Each protein has specific properties, which are determined by the number and the specific sequence of amino acids in a molecule and upon the shape, which the molecule assumes as the chain folds into its final, compact form.
There are four levels of organization, which are described below.
(i) Primary structure
Primary structure comprises the number and sequence of amino acids in a protein molecule.
F. Sanger was the first scientist who determined the sequence of amino acids in a protein molecule.
Example
- F. Sanger concluded that insulin is composed of 51amino acids in two chains. One of the chains has 21amino acids and the other has 30 amino acids and they are held together by disulphide bridges.
- Haemoglobin is composed of four chains, two alpha and two beta chains. Each alpha chain contains 141amino acids, while each beta chain contains 146amino acids.
(ii) Secondary structure
Secondary structure tells us about the helix structure or other regular configuration of polypeptide chains.
Polypeptide chains do not lie flat. They usually coil in a helix or into some other regular configuration.
Example
- One of the common secondary structure is the Ξ± -helix. It involves a spiral formation of the basic polypeptide chain. The Ξ±-helix is a very uniform geometric structure with 3.6 amino acids in each turn of the helix.
- The helical structure is kept by the formation of hydrogen bonds among amino acid molecules in successive turns of the spiral
-pleated sheet is formed by fold backs of the polypeptide.
(iii) Tertiary structure
Usually a polypeptide chain bends and folds upon itself forming a globular shape. Tertiary structure tells us about shape of protein after bending and folding.
Tertiary structure is maintained by three types of bonds, namely ionic, hydrogen and disulphide (-S-S-).
Example
In aqueous environment, the most stable tertiary structure (conformation) is that in which hydrophobic amino acids are buried inside while hydrophilic amino acids are on the surface of the molecule.
Polypeptide chains in keratin (fibrous protein) and in hemoglobin
(globular protein) are held together to form respective functional proteins.
(iv) Quaternary structure
In many highly complex proteins, polypeptide tertiary chains are aggregated and held together by hydrophobic interactions, hydrogen and ionic bonds. This specific arrangement is the quaternary structure.
Example
Hemoglobin, the oxygen carrying protein of red blood cells, which exhibits such a structure.
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