Describe the structure of an antibody molecule and how this structure enables it to bind to a specific antigen. Include the following terms in your description:

Question

heavy chain (H chain), light chain (L chain), variable region, constant region, Fab, Fc, antigen-binding site, hypervariable region, and framework region.

What will be an ideal response?

Answer 1

An antibody molecule is made of four polypeptide chains—two identical heavy chains and two identical and smaller light chains, with a total molecular weight of approximately 150 kDa. Each chain is made up of a series of structurally similar domains known as immunoglobulin domains. The amino-terminal portion of each H chain combines with one L chain, and the two carboxy-terminal portions of the H chains combine with each other, forming a Y-shaped quaternary structure. Disulfide bonds hold the H and L chains together, hold the two H chains together (interchain disulfide bonds), and stabilize the domain structure of the chains (intrachain disulfide bonds). The arms of the antibody molecule are called Fab (fragment antigen binding) and interact with antigen. The stalk is called Fc (fragment crystallizable) and is made up of H chains only. The amino-terminal domains of an H and an L chain together make up a site that binds directly to antigen and varies greatly between different antibodies. These domains are referred to as the variable region, and each antibody has two identical antigen-binding sites. The remaining domains of both H and L chains are the same in all antibodies of a given class (isotype). These domains are referred to as the constant region. The variable region of each chain includes hypervariable regions of amino acid sequences that differ the most between different antibodies. These are nested within less variable sequences known as the framework regions. The hypervariable regions make loops at one end of the domain structure and are also known as complementarity-determining regions because they confer specificity on the antigen-binding site.