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Different levels of structural organization of an immunoglobulin or of a T cell receptor
The three dimensional (3D) structure of an immunoglobulin (made of two identical heavy chains and two identical light chains) or that of a T cell receptor (heterodimer) is determined as for any other protein, by the amino acid sequences of the polypetides chains and can be described in terms of different levels of folding, each of which is constructed from the preceding one in hierarchical fashion.
Primary structure. The chain amino acid sequence(s) is (are) termed the primary structure.
Secondary structure. Regular hydrogen-bond interactions between amino acids
belonging to contiguous stretches of a chain give rise to beta sheets, which constitute the first
folding level or secondary structure.
There are only a limited number of ways of combining beta sheets to make a globular structure, these combinations are called motifs. As an example, the hairpin beta motif consists of two antiparallel beta strands joined by a sharp turn formed by a loop of the polypeptide chain.
Beta sheets pack together to form compactly folded globular units, each of which is called a domain. The domains are the modular units of the extracellular regions of the immunoglobulin and T cell receptor chains. These globular regions are constructed from a section of the polypeptide chain that contains 100-110 amino acids.
Domains are formed from a polypeptide chain that winds back and forth, making sharp turns or protruding loops at the protein surface. The loop regions, which vary in length and have an irregular shape often form the binding sites for other molecules. Because the loop regions are exposed to water, they are rich in hydrophilic amino acids. The amino acids with hydrophobic side chains tend to cluster in the interior of the molecule.
Tertiary structure. The three dimensional structure of the each chain is termed the tertiary structure.
Quaternary structure. Individual polypeptides serve as subunits for the formation of larger molecules. This assembly of different chains (protein subunits) is the quaternary structure. The chains are bound to one another by a large number of weak, noncovalent interactions, stabilized by disulphide bonds.
The domains have often a binding site that is complementary to a region of the domains of the other chain. Thus, heavy and light chains assemble spontaneously to form immunoglobulins.
Examples of quaternary structure: The immunoglobulin (IG) is made of two identical heavy chains and two identical light (kappa or lambda) chains. The T cell receptor (TR) is made of two chains, alpha and beta, or gamma and delta.
Last updated: Friday, 17-Jan-2020 21:00:15 CET
Author: Marie-Paule Lefranc Marie-Paule.Lefranc@igh.cnrs.fr
Editors: Amina Mrani and Chantal Ginestoux