In this article we will discuss about the structure of immunoglobulins, explained with the help of a suitable diagram.
Identification of antibodies in the serum protein fraction was demonstrated in the classic experiments of A. Tiselius and E.A. Kabat in 1939. However, the basic structure of —globulin molecule was revealed by Rodney R. Porter and Gerald Edehman in 1950s and 1960s for which they received Nobel Prize in 1972. Porter and Edelman using ultracentrifugation differentiated γ -globulin (gamma globulin) fraction into high molecular weight fraction bearing sedimentation constant of 19 S and low molecular weight fraction with a sedimentation constant of 7 S.
The 7 S fraction bearing a 1, 50,000 MW gamma (γ) globulin was designated as immunoglobulin G or IgG by them. On subjecting brief digestion of IgG with the enzyme papain two similar fragments each with MW of 45,000 were obtained by Porter which were called Fab fragments since they retained the “antigen binding” activity and the one fragment with MW 50,000 was called Fc fragment for it crystallized during cold storage.
Similar experiments were performed by Alfred Nisonoff using the enzyme pepsin and on brief digestion a single 1,00,000 MW fragment composed of two Fab-like fragments to which he designated F(ab’)2 Like Fab fragments the F(ab’)2 fragments precipitated antigens.
The Fc fragment, however, was not recovered on digestion with pepsin, which got digested into multiple fragments. The experiments of Porter revealed that IgG has two 50,000 MW polypeptide chains designated as heavy (H) chains and two 25,000 MW chains designated as light (L) chains.
By using antisera from goats it was concluded that Fab consists of portions of a heavy and a light chain and that Fc contains only heavy chain components. Hence, Porter and Edelman proposed the prototype structure for IgG according to which the IgG molecule consists of two identical H chains and two identical L chains which are linked by disulphide bridges (Fig. 18.1).
