Constant fragment , antibody molecules

Figure 32.3. The structure of the antibody molecule. All immunoglobulins consist of a basic subunit made up of four polypeptide chains (two light and two heavy) bound together by disulfide bonds. The variable region of the molecule contains the antigen binding site. Papain digestion yields a constant fragment (Fc) and variable fragments (Fab).

The cell receptor binding site is probably located in the 25 A deep canyon, which circulates around each of 12 pentamer vertices. An antibody molecule, whose Fab fragment (figure 3.16) has a diameter of the order of 35 A, would have difficulty in reaching the canyon floor, its entrance being blocked by the canyon rim. Thus, the residues in the deeper recesses of the canyon would not be under immune selection and could remain constant, permitting the virus to retain its ability to seek out the same cell receptor. It was expected that the canyon would be present for all picornaviruses (e.g. poliomyelitis, foot and mouth disease and hepatitis viruses but see section 10.5.1.2). 

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