The basic structure of antibodies (immunoglobins) is show by the diagram to the right. It represents specifically an antibody of the IgG class. Other classes we will study are IgM, IgA and IgE. All of these are built from the basic Y-shaped structure shown to the right. Observe that there are four polypeptide chains: two identical light chains and two identical heavy chains. Notice especially the two identical binding sites for a specific antigen.
To the left, one bar represents the polypeptide chain of a heavy chain and another the polypeptide chain of a light chain. The divisions in each bar represent the domains that comprise each chain. The heavy chain has four domains, while the light chain has two. The variable domains differ in antibodies of the same class that bind different antigens. Particularly important in this figure are the hypervariable regions of the variable domains. These are the portions of the light and heavy chains that allow a specific antibody molecule to bind a specific antigen.
T cell receptors are likewise formed from constant and variable domains. But in this case there is only one binding site. Moreover, T cell receptors are always attached to a membrane by transmembrane domains.
Now we are ready to examine the figure to the left, which shows structure of a variable domain and a constant domain. In this case, we are looking at domains from a T cell receptor. But the principle is identical for domains of antibodies.
As you can see, both are based on a beta-sheet secondary structure. All molecules of the immunoglobin family are constructed from similar domains.
In the variable domain, the hypervariable regions are found in the loops that extend between the portions that from the beta sheet structure. These loops are the portions that specifically bind the antigen. By necessity, the portions of the polypeptide chain that create the beta sheet secondary structure are conserved.
Finally, observe the actual three dimensional structure of the two variable domains of an antibody binding an antigen. Notice the precise "lock and key" relationship between the antigen and antibody. (The antigen in this case is a specific portion of an envelope protein of the HIV virus binding to a corresponding anti-HIV antibody.)
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