7.1.3 Antibody structure and function
Antibodies as they are found in nature are all based on a Y-shaped molecule consisting
of four polypeptide chains held together by disulphide bonds (see Fig. 7.2). There are two
pairs of chains, known as heavy (H) and light (L); each member of the pair is identical.
Functionally the base of the Y is known as theconstant regionand the tips of the arms
are the variable region. The amino acid structure in the constant region is fairly fixed in
an individual but varies between animal species. The amino acid structure in the variable
region is composed of between 110 and 130 amino acids and it is variations in these that
forms the different binding sites of the antibodies. The ends of both the heavy and light
chains are variable and the antigen-binding site is formed by a combination of the two.
The variable part of the antibody contains two further areas, theframeworkand
hypervariableregions. There are three hypervariable and four framework regions per
binding site. The hypervariable regions are structurally supported by the framework
regions and form the area of direct contact with the antigen.
Antibodies can be fragmented by enzymatic degradation and the subunits produced
are sometimes used to describe portions of the antibody molecule. Treatment with the
enzyme papain gives rise to three fragments: two antigen-binding fragments (Fab)
and one constant fragment (Fc). The enzyme digests the molecule at thehinge region
and the resulting Fab fragments retain their antigen-binding capability. The Fc
fragment has no binding region and has no practical use. Fab fragments are some-
times prepared and used for some immunochemical applications. Their smaller size
may mean that they can bind to antigens in certain situations where the larger native
molecule would have difficulty binding.Antigen
binding siteFabLHHLHingeCH^2CH^3FcCLCH^1VHVL
FvConstant
regionsVariable
regionsFig. 7.2Immunoglobulin G.270 Immunochemical techniques