What is the Fundamental Structure of Immunoglobulin?

Experiments conducted by Rodney Porter and Gerold Edleman between 1950 to 1960 revealed the structure of immunoglobulin and both the scientists shared Nobel Prize in 1972 for their efforts.

Their experiment revealed that the immunoglobulins are made up of two identical pairs of polypeptides chains as shown in the figure, one pair is short and the other pair is long.

Based on the molecular weights, the polypeptide chains of short pair are referred as “light (L) chains” and the polypeptide chains of long pair are referred as “Heavy (H) chains” Light chains are composed of 220 amino acid residues, while heavy chains are composed of 440-550 amino acids.

Both the chains contain several homologous units the “domines” with 110 amino acid residues. An intra chain disulphide bond in each domine forms a loop of about 60 amino acids.

The amino acid sequences in domines towards the carboxyl end are constant and are known as “constant domines”. But the amino acid sequences are not constant in the domine present near to the amino (NH₂) terminal of polypeptide chain and are known as “variable domines”.

The four polypeptide chains are held together by covalent disulfide (-S-S-) bonds and the immunoglobulin molecule gets Y or T shape.

At the junction of the Y or T shaped structure a flexible region known as “hinge region” is present to facilitate the function of immunoglobulin. The two arms from the hinge region with variable domains are capable of binding with antigens hence they are referred as “Fraction of antigen binding” regions or “Fab” regions in short.

The other region that is joined to the two arms of Y or T structure does not possess antigen binding capacity. The immunoglobulin, on brief treatment with lytic enzymes, such as papain and pepsin generates two or three pieces, with or without Fab regions.

The single piece without Fab region under goes crystallization during cold storage. Hence this part is referred as “Fc fragment” or fraction of “crystallization”. Being glycoproteins, significant amount of carbohydrate is present in them.