Antibody Structure Problem Set

Problem 7. Weak forces/ strong binding

Tutorial to help answer the question

The interaction between an antigen and antibody can be very strong, and yet all of the forces involved are considered to be relatively weak. How can weak hydrogen bonds, electrostatic attractions, hydrophobic forces, and van der Waals contacts lead to a high affinity?

A. Weak attractions lead to the formation of covalent bonds.

B. Since the antigen is contacted by the antibody over a very limited surface area, several weak forces can form a strong attraction.

C. Contact between antigen and antibody occurs over a wide surface area, allowing multiple weak interactions that give a strong affinity.

Tutorial

The hypervariable regions of the antibody (purple) maximize contact with antigen (green) over a wide surface area.

Hydrogen bonds form between amino acids in the antibody's HV regions and the antigen. The antigen and antibody are brought into close proximity, allowing the formation of more hydrogen bonds and van der Waals interactions. While the individual attractions are relatively weak (about 1/20 of a typical covalent bond), the three dimensional structure and the presence of appropriate amino acids in the antibody HV regions allow for the formation of numerous interactions. This combination means that the binding of antibody to antigen is both specific and very strong.

The usual four weak forces for interactions of biological molecules are hydrogen bonds, electrostatic attractions, van der Waals interactions, and hydrophobic forces. The next question will illustrate that hydrophobic forces are probably not involved in this antibody and hen egg white lysozyme interaction.