Antibody Structure Problem Set

Problem 8. Water and antigen binding


The crystal structure of the Fab complexed to antigen reveals a number of water molecules that are fixed in the binding area. Many of these water molecules form hydrogen bond bridges between antigen and antibody, contributing to the strength of binding. There is no evidence for the exclusion of water from the antibody/antigen complex. The affinity for binding can be expressed by the equation describing free energy in a system: ΔG = ΔH - TΔS, where G = free energy or energy available for work, H = total energy in a system (enthalpy), T = absolute temperature (in Kelvin) and S = entropy. Which of the following statements are correct concerning the generation of a negative ΔG (indicating a spontaneous reaction) for high affinity antibody/antigen binding?

A. Hydrophobic attractions likely plays little role in the affinity, since a number of water molecules are inside the structural complex formed by antibody and antigen.
B. Combining water molecules into the crystal structure associated with antigen binding should have an unfavorable (negative) entropy.
C. The formation of hydrogen bonds creates a negative ΔH, and would lead to a favorable negative ΔG.
D. The TΔS term showing a decrease in entropy due to reduced molecular freedom must be less quantitatively than the enthalpy change (ΔH) coming from the formation of hydrogen bonds and van der Waals contacts.

All of the above are correct.

The decrease in entropy (ΔS) due to water molecules becoming more ordered is quantitatively less than the decrease in enthalpy (ΔH) due to the formation of hydrogen bonds.

The Biology Project
The University of Arizona
Tuesday, June 20, 2000
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