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Antibody Structure made easy


ANTIBODY STRUCTURE

Introduction

Recognition of antigens is accomplished by two distinct type of molecules:

Immunoglobulins (Igs)

T-cell antigen receptors (TCRs)

These molecules are diverse and heterogeneous. Extensive gene rearrangements lead to generation of Igs and TCRs capable of recognising many different antigens. Antibodies are proteins (glycoproteins).


They are found in the blood/serum and tissue fluids of immunized animals/humans, mainly in the g fraction of serum. These serum proteins are members of the immunoglobulin (Ig) family, Some Igs are carried on the surface of B-cells where they act as receptors for specific antigens, Others are free in blood or lymph (Antibodies).


Contact between B-cells and antigen lead to development of these cells into antibody-producing cells (Plasma cells). The Igs represent the functional products of the humoral arm of the specific immune system.


Immunoglobulin Structure

The structural and functional properties of individual Ig molecules vary enormously. All have a common unit structure, which forms the molecular basis for their immunological activities. Are bi-functional molecules having antigen binding activity on one region and mediate effector functions on the other. All are governed by the same rules of structure as those for the IgG molecule. IgG has been described by the four-chain model Antibody molecules are composed of Two identical heavy chains and Two identical light chains.


The N-terminus of each heavy and light chain is juxtaposed in three-dimensional terms to generate the antigen combining site (Variable region). Inter-chain disulphide bonds link the heavy and light chains and the two heavy chains together. Hinge region is the location of the inter-heavy chain disulphide bonds and is an exposed region vulnerable to attack by the proteolytic enzymes.



The Hinge region of the Ig

Located between the Cg1 (CH1) and Cg2 (CH2) domains, is rich in proline and cysteine residues, has considerable segmental flexibility and is important in expression of biological activities. It allows the Fab arms of the immunoglobulin to adopt a wide range of orientation to each other and is responsible for the Y-shape of the Ig molecule



Variable regions of Ig molecule

The N-terminal regions possess considerable variability from one chain to another. Variable regions (N-terminal regions) are contained within the amino (NH2) terminal end of the polypeptide chain (amino acids 1-110) The amino acid sequences are quite distinct from one antibody to another. The V-regions are hypervariable explaining the antibody specificity at molecular level. They also give rise to the serological marker of antibody molecules. Function as the complementarity determining regions (CDRs) due to the specificity they confer to the antibody molecule. Both the heavy and light chains possess three CDRs.


Primary Structure of Ig

Amino acid sequence. The linear arrangement of amino acids in the polypeptide chain. L-chains are composed of 220 amino acid residues while the H-chains have 440-550 amino acids. Primary sequence analysis revealed that the chains have variable (V) and constant (C) regions basing on the variability of the amino acid sequence within the protein. One Constant domain, CL, is associated with the L-chains, while 3 constant domains, CH1, CH2 & CH3, are associated with the H-chain.


Secondary structure

Folding of the amino acid chain into anti-parallel pleated sheets. Is the relationship between amino acids located some distance apart in the primary structure as revealed in the folded protein molecule


Tertiary structure

Folding of pleated sheets into globular domains connected to one another. Describes the spatial relationship between amino acids which are separated by great distances in the primary sequence, mainly the cysteine residues which form the intra-chain disulphide bridges


Quaternary Ig structure

Interaction of globular domains between adjacent light and heavy chains. This reflects interaction between distinct polypeptide subunits of a multi-domain protein, e.g. VH and VL domains yield the correct conformation for generation of the antigen-binding site


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