In this article we will discuss about the structure of IgG, explained with the help of suitable diagrams.
Electron microscopic studies have revealed that IgG is a Y-shaped tetrapeptide protein (Fig. 8.20) formed of two identical light chains and two identical heavy chains. So an antibody is represented as H2L2.
Light Chains:
Each light chain is formed of about 214 ammo acids. Light chains are of two types in all the classes of immunoglobulin: Kappa (k) and lambda (λ) chains. A given antibody has either both Kappa or both lambda type light chains.
The amino acid sequence analysis of light chains has shown that a light chain is formed of 2 parts:
Variable (V) of 1-108 amino acids from the N-terminal end and varies from one antibody to another; and Constant (C) of 109- 214 amino acids and is characteristic of the antibody. Each light chain also has two intiachain disulphide (S-S) loops, one in variable part and other in constant part.
Heavy Chains:
These are two in number. Each heavy chin is formed of 440 amino acids. The analysis of primary structure of heavy chains has revealed that each heavy chain is formed of two parts: Variable region (formed of 1-118 amino acids from the N-terminal end) and Constant region (formed of 119-440 amino acids and about three times in length to that of light chain). The constant region of each heavy chain has three interchain -S-S-loops. Similarly, the variable region also has a -S-S-loop. So each heavy chain of IgG has four -S-S-loops.
The part of the hypervariable regions on the antibody which binds the antigen is called the paratrope, while the corresponding part of the antigen which is in contact with the paratope is called the epitope.
Two light chains and two heavy chains of IgG are interlinked through -S-S- (disulphide) bonds. There are three disulphide bonds in IgG.