General Index
Immunoglobulins
Introduction
The Immunoglobulins constitute an extense superfamily of proteins that include all the Antibodies. Their
structure is interesting enough to get a close look at it.
All the immunmoglobulins have the same structure, consisting in two Heavy chains (H) and two Light chains (L)
linked by disulfide bridges. Both heavy chains and both light chains are identical in each molecule. Heavy chains can be of five different types
(isotypes) called g, a, m, d and e. Light chains can be two types,
k and l.
The isotypes of immunoglobulins are distinguished by their heavy chain. Thus, we have Immunoglobulin G or IgG,
g2k2 or g2l2, Immunoglobulin A or IgA,
a2k2 or a2l2, Immunoglobulin M or IgM,
m2k2 or m2l2, Immunoglobulin D o IgD,
d2k2 or d2l2, and Immunoglobulin E or IgE,
e2k2 or e2l2.
Structure
Immunoglobulin G, the most abundant, is a good example to illustrate the structure of all them.
If we represent in cartoons the molecule, giving a different color to each chain:
The structure of the heavy chain is similar, but with four domains instead of two:
Fab and Fc fragments
Digestion by the proteolytic enzyme Papain destroys the Hinge region and the molecule is cleaved into three fragments, two identical Fab (Antigen Binding) and one Fc (Constant). Going back to the initial representation:
Disulfide bridges
Quaternary structure of the immunoglobulins is maintained by Disulfide bridges established between cysteine residues. Back to the original view: