Structure of an antibody
Antibody: a globular glycoprotein with antigen-binding sites that are specific to a particular antigen.
An antibody is built from four polypeptide chains: a pair of heavy chains and a pair of light chains. Disulfide bonds join these chains to produce the characteristic Y-shaped quaternary structure.
The molecule contains regions with different roles:
- The variable regions form the antigen-binding sites near the ends of the two arms.
- Differences in the amino acid sequence of the variable regions give different antibodies differently shaped antigen-binding sites.
- The constant region is the same for antibodies within a particular class and is associated with the way the antibody brings about destruction.
- Where present, a hinge region in the heavy chains gives flexibility so the binding sites can attach to antigens at different angles.
Each antibody has two antigen-binding sites, one at the end of each arm.
Specificity
The part of an antigen recognised by an antibody is called an epitope. An antigen-binding site can bind when its shape is complementary to that epitope.
Because the amino acid sequence of the variable region differs between antibodies, their antigen-binding sites also differ. This gives each antibody its particular antigen specificity.
Exam Tip: When explaining antibody specificity, link the sequence clearly: different amino acid sequence in the variable region → different antigen-binding-site shape → only a complementary antigen or epitope binds.