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| Immunoglobulin G is built of 4 polypeptide chains. |
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Two heavy chains of molecular weight 50 kDa and two light chains
of 25 kDa, which are four polypeptide chains of two kinds consist IgG, of which
molecular weight 150 kDa. |
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Two heavy chains and two light chains consisting one IgG molecule
have the same structure respectively. |
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Two heavy chains are linked by disulfide bonds, and each heavy
chain is linked with each light chain also by a disulfide bond. (Fig. 1) |
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There are two types of light chain: lambda (λ) and kappa (κ), but
only one type is present in each antibody. No functional differences of two
chains are known so far. |
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| Fig. 1 Antibody structure. Heavy and light chains combine to
form an antibody |
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| There are domains in immunoglobulins. |
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There are repeating sequences of 110 amino acids in each chain. They are called
protein domains, which have firmly folded structures. There are two domains in
a light chain, four in a heavy one. |
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| Each chain has constant and variable regions |
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The amino-terminals of the first domains of heavy and light chains have highly
variable amino acid sequence, which make it possible to bind specifically to
various antigens. They are called variable regions, designated as vH,
vL for those in heavy and light chains. On the other hand, the
domains keeping constant structure are called constant domains. They are
designated as CH, CI and since there are three constant
regions in heavy chains numbers are added like CH1, CH2
in the order of nearness to the amino-terminal (Fig. 2). |
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| Fig. 2 Antibodies are composed of polypeptides with variable and
constant regions |
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| Antibodies are made up of functional units. |
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The following conclusions were reached as results of structural analysis of
antibodies with proteolytic enzymes (Fig. 3). |
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The enzyme papain cleaves an antibody into three fragments. Two Fab(fragment
antigen binding) fragments having antigen binding regions are made up of VH
and CH1 domains of heavy chain and light chain. An easily
crystallizable Fc(fragment crystallizable) fragment with no antigen binding
capacity is made up of CH2, CH3 domains. It is the result
of cleavage at the animo-terminal side of the disulfide bond by papain. |
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The enzyme pepsin cleaves disulfide bond at the carboxy-terminal side, to make
F(ab')₂ fragment and small fragments of heavy chains. |
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| Fig. 3 Antibodies are composed of polypeptides with variable and
constant regions |
