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Antibodies, or immunoglobulines (Ig),
are Y shaped proteins complexes composed of four
polypeptide chains, two identical light
chains and two identical heavy
chains. Each light chain is bound to a heavy
chain via disulfide bridges to form a heterodimer. Two
identical heterodimers are linked to each other by
disulfide bridges to form the basic antibody molecule
(Fig. 1). The constant domains of the heavy chain (Fc)
determine the biological function of the antibody
creating five major classes of antibodies found in higher
verterbrates (IgG, IgA, IgD, IgE and IgM). The variable
domains (Fab), however, are involved in
antigen binding. Within the Fab regions
there are six hypervariable loops (3 on the surface of
the light chain and 3 on the surface of the heavy chains)
that directly interact with the antigen named the
Complementarity Determining Regions
(CDRs) (Fig. 2).
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| Fig 1. The basic
structure of antibodies. Antibodies are Y
shaped molecules. The heavy chains are colored in
purple and green and the light chains are colored in
pink and yellow. |
Fig 2. The Fab region
and the Complementarity Determining Regions (CDRs) of
an antibody - The Fab region of the antibody
is composed of the antibody light chain (colored in
white) and variable part of the heavy chain (in red).
The 6 loops defining the CDRs according to Kabat
identification scheme are colored grey (the heavy
chain CDRs) and blue (the light chain
CDRs). |
Antigenic determinants, or epitopes,
are particular surface areas of a protein that are
specifically recognized by immunoglobulin molecules.
Epitopes are commonly classified as either linear or
conformational. Linear epitopes are continuous amino acid
sequences of five to ten residues. When a protein is
denatured or digested into several segments, peptides
corresponding to the linear epitope amino acid sequences
can sometimes be recognized by the antibody (Fig. 3a ).
Conformational epitopes are however discontinuous and
occur as a result of the higher order of the structure
(Fig. 3b). After denaturation or digestion into small
fragments, the peptides corresponding to the amino acid
sequences of the conformational epitopes can no longer
bind the antibody (some conformational epitopes were
found to be composed of a number of linear epitopes).
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Fig 3. Schematic
representation of two antibodies interacting with
linear and conformational epitopes.
a. Linear epitopes are short and
continuous. After denaturation the linear epitopes
may still be able to bind the
antibody.
b. Conformational epitopes are domains
of proteins composed of specific regions of protein
chains. After denaturation the discontinuous
epitope can no longer bind the
antibody.
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