Page 509 - Veterinary Immunology, 10th Edition

VetBooks.ir Receptor-Antigen Binding

When an antigen and its receptor bind, they interact through the
chemical groups on the antigen and on the complementarity

determining regions (CDRs) of the receptor. In classic chemical
reactions, molecules are assembled through the establishment of
firm, covalent bonds. These bonds can be broken only by the input
of a large amount of energy; energy that is not readily available. In
contrast, the formation of non-covalent bonds provides a rapid and

reversible way of forming complexes and permits reuse of
molecules in a way that covalent bonding would not allow.
However, noncovalent bonds act over short intermolecular

distances and, as a result, form only when two molecules approach
each other very closely. The binding of an antigen to a BCR or TCR
is exclusively non-covalent, so the strongest binding occurs when
the shape of the antigen and the shape of the receptor perfectly
match. This requirement for a close conformational fit has been

likened to the specificity of a key for its lock.
The major bonds formed between an antigen and its receptor are
hydrophobic (Fig. 17.1). When antigen and antibody molecules

come together, they exclude water molecules from the area of
contact. This frees some water molecules from constraints imposed
by the proteins and is therefore energetically stable. (The bond can
be likened to two wet glass microscope slides stuck together.
Anyone who has tried to separate two wet glass slides can confirm

the effectiveness of this type of bonding.)

509