Question

Describe the basic parts of an antibody ( ovarll general shape and regions ) and where antigens bind. Which region of an antibody determines its epitope specifically?

Answer 1

Basic structure of Antibody is given below:

Antibodies are the antigen binding glycoproteins. These are synthesized exclusively in B-clls.

The simplest antibodies are Y-shaped molecules with two identical antigen-binding sites, one at the tip of each arm of the Y-shape. Antibodies are described as bivalents because of their two antigen-binding sites.

Antibodies have four polypeptide chains (two light (L) chains and two identical heavy (H) chains).

Each light chain is bound to heavy chain by disulphide bridges and other non-covalent linkages. Antibody is a dimer of H-L chain.

All the species have two major classes of light chains: κ (kappa) and λ (lambda). In any one antibody molecule, light chain are always either both κ and both λ, never each one of each.

There are five differnet types of heavy chains, α (alpha), γ (lambda), δ (delta), ε (epsilon) and μ (mu).

These heavy chains of given antibody molecule determine the class of antibody: IgM (μ), IgG (γ), IgA (α), IgD (δ) and IgE (ε).

Both light and heavy chains have a variable sequence at their N-terminal ends but a constant sequence at their C-terminal ends. Light chains have constant region about 110 amino acids long and variable region of the same size.

The variable region of heavy chains is also about 110 amino acids long but heavy chain constant region is about three to four time longer (330 - 440 amino acids), depending on the class.

Paratope (antigen-binding site) is the part of an antibody which recognizes an antigen. It is a small region having 15–22 amino acids of the antibody's Fv region. Fv represent the variable region of antibody.

It contains parts of the antibody's heavy and light chains.

The specific region on an antigen which is recognized by an antibody and binds to is called the epitope. This is also called as antigenic determinant. It is usually 5-8 amino acids long on the surface of the protein.