The number of cooperative regions (energetic domains) in a pepsin molecule depends on the pH of the medium

The technique of scanning microcalorimetry was used to study the effect exerted by ethanol and by the pH of the medium on the number and size of cooperative regions in a pepsin molecule. Ethanol addition lowered the temperature of protein denaturation, but did not change the number of energetic domains. The number of thermodynamic cooperative units (determined as a ΔHcal to ΔHeff ratio) was reduced from four to two when the pH changed from 6.7 to 2.0. As was demonstrated using the CD technique, this process involved no changes either in the secondary structure or in the local surroundings of aromatic amino acids. Therefore, variations in the cooperative properties of a pepsin globule at different pH values are associated with the electrostatic interactions of individual parts of the molecule.