Antibodies are protein molecules, which belong to a class called globulins. When serum (serum is a clear fluid formed after the coagulation of the blood) is placed in an electric file Proteins present in it, migrate, based on their molecular weights. They segregate into two classes.
Smaller molecules move faster and are termed as albumins. Larger molecules move slower and are called globulins. Globulins are divided into three classes: alpha (a), beta (b) and gamma (g). M. Heidelberger, A. Tiselius and E. Kabat (between late 1930s and early 1940s) showed that antibodies segregated with gamma globulin class of serum proteins.
Antibodies are synthesized by plasma cells. Plasma cells are the maturation products of B-lymphocytes. A B-lymphocyte undergoes maturation, when it is stimulated by an antigen. An antibody is synthesized in response to an antigen that enters into the body.
Like is the antigenic stimulus, like is the antibody synthesized. Therefore, an antibody is antigen specific. This antigen- antibody specificity property of the immune system makes the system function flawlessly. A typical antibody molecule is a glycoprotein (carbohydrate + protein) having a molecular weight of around 1, 50,000. It is a tetramer of four polypeptides. Of the four, two are identical light (L) chains and two heavy (H) chains. This four chain structure of an antibody was proposed by Rodney Porter (1962).
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Light Chain
Each light chain has 220 amino acid residues and heavy chain, 440 residues. A light chain is linked to a heavy chain of its side by one disulfide (S – S) bond. The light chain has a variabie region (VL) of 1 -108 amino acid residues at the N-terminus followed by a constant region (C,) of 109-220 residues. There are two intra-chain disulfide bonds, one each in the variable and constant region of each chain.
Heavy Chain
Similarly, each heavy chain has a variable region (VH) of 1 -118 amino acid residues at the N-terminus followed by a constant region (CH) of 119-440 residues. The heavy chain constant region is divided into three equal parts: Cm, CH2. And CH3. The VH, CH|, CH2 and CH3 regions have one intra-chain disulfide bond each. The two heavy chains are held together by one or more disulfide bonds. Carbohydrate residues are attached to each heavy chain.
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The four polypeptides are so arranged that the antibody molecule has two identical, w separated antigen binding sites. Each site can combine with an antigen. Circulating antibodie ‘ Y’ or’T’ shaped molecules with one antigen binding site at the end of each arm. This shaj possible due to the presence of a relatively more flexible region between the CH, and CH2 doom of the heavy chain. This region is known as the hinge. It allows the bending of the circular antibodies. Some information about the antibody structure and functions were obtained by dirge the antibody molecule with proteolysis enzymes, papain and pepsin.
Proteolytic enzyme cleavage products of immunoglobulin (IgG) (a) Papain cleavage products [two antigen binding fragments (Fab) and acrystallizable fragment (F,)] and (b) pepsin cleavage products [F(ab’)2 with two inter-chain disulfide bonds and fragments of the Fc fragment].
The stem of the ‘ Y’ shaped antibody molecule (Fc) is designated to carry out the effectors functions. The antigen-antibody complexes must interact with phagocytes in order to be ingested and digested. This interaction is mediated by the Fc. It also interacts with the complement system facilitating the destruction of microorganism.
Hyper variable Regions:
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The constant regions of light and heavy chains in antibodies have minor variations in the amino acid sequence. However, the variable regions vary considerably in amino acid sequence from antibody to antibody. This variability makes an antibody very specific in its antigen binding.
The variability is not evenly distributed in both the chains. It is concentrated in three segments called hyper variable regions (HYRs) or complementarily determinig regions (CDlj. The hypervariable regions line the antigen binding sites of antibodies. The relative less variable regions are called the framework regions. Elvin Kabat (1970) proposed™ three hypervariable regions form the antigen binding site. The antibody specificity is determin by the amino acid sequence of these regions.
Immunoglobulin Domains
X-ray crystallographic studies of immunoglobulin suggested that the polypeptides a folded into domains. A domain is a compactly folded part of a polypeptide. The regions of the light and heavy chains, marked VL, CL, VH, CH|, CH2 and CR3 are the immunoglobulin domain! Each domain is stabilized by an intra-chain disulfide bond. There will be as many domains in each immunoglobulin chain as the number of intra-chain disulfide bonds. The number of light chalk domains is universally two, whereas, the number of heavy chain domains varies from 3 to 4.
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The complete amino acid sequence of an immunoglobulin was worked out by Gerald Edelman (1968). He found a considerable homology among the various regions of the light an: heavy chains, which can be listed as follows: