Immunoglobulin G is an antibody class whose conventional four-chain architecture supplies the example above.
An ordinary IgG has two matching immunoglobulin heavy chains and two matching immunoglobulin light chains, abbreviated . Its gamma-type immunoglobulin heavy chains contribute both to recognition and to interactions with immune partners.[ref]
Antibody Structure and Molecular Immunology by Gerald M. Edelman. Gerald M. Edelman, Antibody Structure and Molecular Immunology, printed pp. 35–37, Table 1 and Figure 3: gamma chains, four-chain architecture and separate functional regions
IgG chain continuity and protein disulfide bond connectivity
. Original chain-connectivity schematic. Gold crossbars denote interchain protein disulfide bonds; intrachain crosslinks are omitted. The drawn crossbar count and geometry illustrate connectivity, rather than atomic coordinates or a universal IgG subclass inventory. Details.
A useful way to read this picture is to follow a single polypeptide chain continuously from its N-terminus to its C-terminus, then examine the crosslinks separately. The same immunoglobulin heavy chain contributes to a Fab fragment and to the Fc fragment; these named fragments are not pre-existing separate chains. Conversely, one Fab fragment contains material from two different chains. The protein primary structure and the fragment inventory therefore answer different questions about the same protein.
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