Number of cooperative regions (energy domains) in the pepsin molecule depends on the pH of the medium

: Ethanol and pH influence on the number and dimensions of cooperative regions in pepsin molecule was studied by scanning microcalorimetry. It is shown that ethanol solution causes a decrease of temperature of protein denaturation but does not influence the number of energetic domains. While changing pH from 6.7 to 2.0 the number of thermodynamic cooperative units (defined as the ratio delta Hcal/delta Heff) decreases from four to two. This process, as shown by CD technique, is followed by changes neither in the secondary structure, nor in the local environment of aromatic amino acids. A conclusion is made that the distinctions in cooperative characteristics of the protein globule at different pH are determined by electrostatic interactions of separate parts of the molecule.