The Basic Structure Of Immunoglobulins

Introduction:

The history of the chemical structure of the IgG molecule is quite fascinating and is the model after which the study of other immunoglobulin has been patterned. In 1972, a joint Nobel Prize was awarded to Edelman and Porter for their contributions towards the immunoglobulin properties.

The basic structure of an immunoglobulin can be written as H₂ L₂. It consists of two heavy chains and two light chains. There are certain regions called as Variable region (V) and Constant region(C) on both the chains. A variable region is so named because the sequence of amino acids varies from one immunoglobulin to another immunoglobulin of the same class. In contrast, a constant region consists of same amino acid sequence in a particular class of Immunoglobulin. Light chains are of two types which exist in all classes of immunoglobulin. They are K -Kappa and L- Lamda light chains. The heavy chains of Ig A, Ig G, Ig D, Ig M and Ig E are designated as α, γ, δ , µ and ε chains respectively.

Heavy chains of Ig G: There are four regions on heavy chains of Ig G molecule-one variable and 3 constant regions. Each of these regions is called as domain. They are designated as V H1, CH1, CH2 and CH3.each constant region consists of about 110 amino acids and variable region consists of 121 amino acids. Thus the heavy chain of Ig G constitutes about 450 amino acids. Amino terminal is located towards antigen binding site and carboxyl terminal towards the end of Fc fragment.

Light chains: Each Ig G light chain consists of a Variable region and a Constant region. The variable region is present at antigen binding site towards amino terminal and the constant region towards carboxyl terminal .An immunoglobulin never contain two light chains of different type(Kappa and Lamda) and always consists of two light chains of the same type(either two Lamda or two Kappa).

Immunoglobulins are proteins. However the heavy chains consist of oligosaccharide (carbohydrate molecules) hence an immunoglobulin may be considered as glycoproteins.