Structure and Function of Antibodies

Recall:

• B-cells can recognize an antigen via their surface immunoglobulin (Ig) molecules
• T-cells can only recognize an antigen that is associated with a major histocompatibility complex (MHC) molecule

The immunoglobulins (Ig) are glycoprotein molecules that are produced by plasma cells in response to an immunogen and function as antibodies. The immunoglobulins derive their name from the finding that they migrate with globular proteins when antibody-containing serum is placed in an electrical field

STRUCTURE OF THE ANTIBODY

Heavy and Light Chains

Typical Antibody Structure (Kuby)

All immunoglobulins have a four chain structure as their basic unit. They are composed of two identical light chains (23 kD) and two identical heavy chains (50-70kD)


Disulfide bonds

1. Inter-chain disulfide bonds → the heavy and light chains and the two heavy chains are held together by inter-chain disulfide bonds and by non-covalent interactions The number of inter-chain disulfide bonds varies among different immunoglobulin molecules.
2. Intra-chain disulfide binds → within each of the polypeptide chains there are also intra-chain disulfide bonds.

Variable (V) and Constant (C) Regions

After the amino acid sequences of many different heavy chains and light chains were compared, it became clear that both the heavy and light chain could be divided into two regions based on variability in the amino acid sequences. These are the:

1. Light Chain → V_L (110 amino acids) and C_L (110 amino acids)
2. Heavy Chain → V_H (110 amino acids) and C_H (330-440 amino acids)

Hinge Region

This is the region at which the arms of the antibody molecule forms a "Y". It is called the hinge region because there is some flexibility in the molecule at this point. It is rich in proline and cysteine. Proline gives it extended polypeptide conformation making it vulnerable to cleavage by proteolytic enzymes whereas cysteine forms interchain disulphide bonds to hold the two heavy chains together.


Domains

Three dimensional images of the immunoglobulin molecule show that it is not straight, but is rather folded into globular regions, each of which contains an intra-chain disulfide bond called "domains".

1. Light Chain Domains → V_L and C_L
2. Heavy Chain Domains → V_H, C_H1― C_H3 (or C_H4)

Oligosaccharides

Carbohydrates are attached to the C_H2 domain in most immunoglobulins. However, in some cases carbohydrates may also be attached at other locations


Variable and Constant Regions of Antibodies

• The C-terminus of both H and L chains is invariant and is defined as C (constant) region. Its length is approximately 330 amino acids in H chains and ~ 110 amino acids in L chains. The constant domain of both heavy and light chains may contribute to antibody diversity.
• The N-terminal segment substantially differs in different antibodies and is thus named V (variable) region. It is approximately 110 amino acids in length and contains regions that show maximal variation between different antibodies and is further divided into: 

CDRs

1. Hypervariable regions now designated as complementarity determining regions (CDRs) → regions of antigen-binding site that show complementarity to the epitope. Six loops of the V_H (H1, H2 and H3) and V_L (L1, L2 and L3) domains create a great variety of surfaces. H3 is the most variable of the loops and in all crystallographically solved antibody-antigen complexes makes several contacts with antigen
2. Framework Regions (FRs) → these are the remainder of V_H and V_L domains and show lesser variation than the former

FUNCTION OF THE ANTIBODY

Antibodies recognize:

• proteins (conformational determinants, denatured or proteolyzed determinants)
• nucleic acids
• polysaccharides
• some lipids
• small chemicals (haptens)

The Antigen═Antibody (Ag═Ab) Complex

• Antibodies bind to antigens by recognizing a large surface, and through surface complementarity. Thus, these complexes have a very high affinity for each other.
• The interaction between an antigen and antibody can be very strong, and yet all of the forces involved are considered to be relatively weak.

☼ So how can weak hydrogen bonds, electrostatic attractions, hydrophobic forces, and van der Waals contacts lead to a high affinity?

Contact between antigen and antibody occurs over a wide surface area, allowing multiple weak interactions that give a strong affinity. The hydrogen bonds join the antibody and antigen over a wide surface area. Other weak forces, including van der Waals forces, electrostatic attractions and hydrophobic forces, add to the strength and specificity of antibody═antigen binding