Question

Describe the structure of IgG in detail by referring to the different levels of protein structure

Answer 1

IgG is a type of antibody which is the most common and abundant in blood circulation and approximately 80% of total serum antibodies in human body.

This antibody is secreted by plasna B cells and have 2 antigen binding sites.

Structure:

Immunoglobulin G is a globular protein which is very large monomeric molecule having about 150kDa molecular weight. This antibody is made up of four polypeptide chains, i.e. 2 identical heavy chains and 2 identical or 2 identical light chains and thus possess monomeric "H2L2" structure. These 2 heavy chains are linked with each other and also to the light chains by disulphide bonds. Heavy chains are about 50kDa and light chains are about 25kDa.

Fab domain cintains 2 variable and 2 constant domains and 2 variable domains make variable fragments (Fv). This Fv provides the antigen specificity and have 3 hypervariable loops called as complimentary determining regions (CDR). constant domains give structural framework.

Heavy chains have components like CH1, CH3, CH3, hinge and variable regions of heavy region( VH) while light chains have constant regions of light chains(CL). Thus IgG antibodies shows tetrameric quaternary structure and form Y- like (or fork) shape. At the each end of the fork 2 antigen binding sites are present.

The Fc regions of IgG have highly conserved N-glycosylation site at asparagine 297 at constant region of heavy chains. To this site, N -glycans are attached which is predominantly core fucosylated biantennary structure. Besides this, small structure of these N- glycans are alo bisected by GlcNAc and -2,6 -linked residues of sialic acid.

Fig: different regions of IgG .

Four IgG subclasses( IgG1, IgG2, IgG3 and IgG4 )are present in humans. Out of these four,  IgG1 is most abundant.