Structure of immunoglobulin (Ig)

 

       Structure of immunoglobulin (IgG) 

Antibody is a specific substance produced in the body in response to antigen and bind specifically with antigen and try to destroy or eliminate from the body.   The immunoglobulin are large group of closely related heterogeneous glycoprotein. The larger chain are called heavy chain and smaller chain are called light chain. Both the light chain and both the heavy chain are identical. The light chain are only  half the length of the heavy chain. Two heavy chains are joined together by disulfide bond. Heavy and light chain are also joined by disulfide bond. The tail of Y is Fc region is attached to B cell and the arms called Fab region which bind with antigen. Antigen binding sites are formed by the groove created between light and heavy chain at their N terminus.

     Introduction:-

•      Ig are glycoprotein, comprise 20-25% of total serum protein.

•      Chemically, antibody are globulin, hence they are called immunoglobulin.

•      Ig denotes chemical structure of protein.

•      Antibody refers to biological activity and function of protein.

Basic Structure of IgG:-

1.HEAVY CHAINS

Ø Two identical heavy chain held together by disulphide bond.

Ø Has constant region & variable region.

Ø Consists total 450 aminoacid,110 a.a. in variable region.

Ø Amino acid sequence of N-terminus/variable region is variable. 

2.LIGHT CHAIN

Ø Two L chain attach with H chain by di-sulphide bond.

Ø Either kappa or lambda, never both in one immunoglobulin.

Ø Consists 110 amino acid in each constant & variable region.

Ø Aminoacid sequence of N-terminus is variable,              C-terminus is constant.

3.Hinge region

Ø Area of H chain in C region between CH1 and CH2 domain.

Ø More flexible and more exposed to enzymes and chemical.

Chemically digested structure of IgG

1.Papin digestion:-

= Acts on Hinge region and produce one Fc and two separate Fab fragments.

2.Pepsin digestion:-

= Acts on Hinge region to produce single (Fab)₂ fragment and act on Fc segment to produce multiple Fc fragments.

Classification of immunoglobulin :

1.      IgG

2.      IgM

3.      IgA

4.      IgE

5.      IgD

         Types of IgG

Ø IgG in Human serum, Dog & Cat:IgG₁,IgG₂,IgG₃,IgG₄

Ø In cattle & Sheep: IgG₁,IgG₂,IgG₃

Ø In horse: IgG₁,IgG₂,IgG₃,IgG₄,IgG₅,IgG₆

Ø Interchain disulphide bond less in IgG₁ & more in IgG₂,IgG₃,IgG₄

Ø Mainly IgG₂ produce immunological response against bacterial polysaccharides.

Properties and function:-

Ø IgG is 7S Immunoglobulin(molecular weight.=150kDa)  *S=svedberg unit

Ø Contain 3% carbohydrate, half life 23 days.

Ø Smallest immunoglobulin, can cross blood vessel & placental barrier.

Ø Passive immunity to new born animal.

Ø Acts as complement fixing antibody, provide long lasting immunity.