Molecular Structure of Immunoglobulins
Immunoglobulins are enormous families of related but non-identical
glycoproteins. It has been estimated that every human being is capable of
producing at least 108 different antibody molecules.
The Four-Chain Basic Unit
Immunoglubulin molecule is made up of two different types of
polypeptides. The larger, heavy (H) chains are roughly twice as large as
smaller, light (L) chain. Every immunoglobulin contains equal number of heavy
and light chain poly peptides. The heavy and light polypeptide chains are both
composed of folded globular domains, each of which is 100-110 amino acids long
ad contains a single intra chain disulfide bond.
Immunoglobulin Variable Regions and Constant Regions
All the light chains and all the heavy chains in any single
immunoglobulin protein are identical. However when compared with different
immunoglobulins, there is a wide variation in sequences of the chain. This variation
is mostly occurred in N-terminal domain, where as the sequences of other domain
remain constant. Therefore N-terminal domain in heavy or light chain poly
peptide is referred as variable region.
Hinge Region
A short additional segment of amino acid located between the CH1
and CH2 domains of H chain is called hinge region. It is made up of
predominant of cysteine and proline residues. The hinge region permits
flexibility between the two Fab arms of the Y-shaped antibody molecule.
Hypervariable Region
Consists of relatively invariant stenches of 15-30 amino acids, separated
by shorter region of extreme variability called hypervariable region that are
each 9-12 amino acids long. Hypervariable regions are also called
complementarity-determining regions.
J Chain and Secretary Components
J Chain
The secreted form of IgM and IgA generally exist as polymers of the
basic four-chain unit that include a single additional polypeptide called the J
chain.
The Secretary
Secretary component is a single glycopeptide with a peptide
molecular weight of approximately 70,000 and a high carbohydrate content. The function
of secretary component is to facilitate the trasepithelial passage of IgA.