Immunology

Bonds in Antigen-Antibody Reaction

Chemical bonds in Antigen-Antibody Reactions

Antigen- antibody interaction is bimolecular irreversible association between antigen and antibody. Association between antigen and antibody includes various non-covalent interactions between epitope (antigenic determinant) and variable region (VH/VL) domain of antibody.

Chemical Bonds Responsible for the Antigen–Antibody Reaction

An interaction between  Ab-binding site and the epitope involves exclusively non-covalent bonds, in a similar manner to that in which proteins bind to their cellular receptors, or enzymes bind to their substrates. These bindings are reversible and can be prevented or dissociated by high ionic strength or extreme pH. The following intermolecular forces are involved in Ag–Ab binding:

Electrostatic bonds

Valence bonds in which two atoms attracted by electrostatic force transfers one or more electrons between atoms.

Electrostatic attractive forces between charged sites on antigens (e.g. COO) and oppositely charged sites on antibodies (e.g. NH3+).

This result from the attraction between oppositely charged ionic groups of two protein side chains; for example, an ionized amino group (NH4+) on a lysine in the Ab, and an ionized carboxyl group (COO) on an aspartate residue in the Ag.

 

Chemical bonds in Antigen-Antibody Reactions

 

Hydrophobic interactions

Attractive force between molecules due to close positioning of non-hydrophilic portions of two molecules.

When two hydrophobic surfaces on an antigen and antibody are brought close together, water molecules between them are excluded and they come together to decrease total surface area exposed.

Hydrophobic groupsin amino acids (such as the side chains of valine, leucine, and phenylalanine) tend to associate due to Van der Waals bonding and coalesce in an aqueous environment, excluding water molecules from their surroundings. As a consequence, the distance between them decreases, enhancing the energies of attraction involved. This type of interaction is estimated to contribute up to 50% of the total strength of the Ag–Ab bond.

 

Hydrogen bonding

Chemical bond in which hydrogen atom of one molecule is attracted to an electronegative atom of another molecule.

It arises when proton donors (OH, NH) and acceptors (COO) on antigens and antibodies share hydrogen atoms.

When the Ag and Ab are in very close proximity, relatively weak hydrogen bonds can be formed between hydrophilic groups (e.g., OH and C=O, NH and C=O, and NH and OH groups).

 

Van der Waals bond

Forces acting between non-bonded atoms or molecules.

These forces depend upon interactions between the electron clouds that surround  Ag and Ab molecules. This interaction has been compared to that which might exist between alternating dipoles in two molecules, alternating in such a way that, at any given moment, oppositely oriented dipoles will be present in closely apposed areas of the Ag and Ab molecules.

 

Each of these non-covalent interactions operates over very short distance (generally about 1 Å) so, Ag-Ab interactions depends on very close fit between antigen and antibody.

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