The chemical modification of alpha-chymotrypsin with both hydrophobic and hydrophilic compounds stabilizes the enzyme against denaturation in water-organic media

We considered a-chymotrypsin (CT) in homogeneous water-organic media as a m odel system to examine the influence of enzyme chemical modification with h ydrophilic and hydrophobic substances on its stability, activity and struct ure. Both types of modifying agents may lead to considerable stabilization of the enzyme in water-ethanol and water-DMF mixtures: (i) the range of org anic cosolvent concentration at which enzyme activity (Vm) is at least 100% of its initial value is broadened and (ii) the range of organic cosolvent concentration at which the residual enzyme activity is observed is increase d. We found that for both types of modification the stabilization effect ca n be correlated with the changes in protein surface hydrophobicity/hydrophi licity brought about by the modification. Circular dichroism studies indica ted that the effects of these two types of modification on CT structure and its behavior in water-ethanol mixtures are different. Differential scannin g calorimetry studies revealed that after modification two or three fractio ns or domains, differing in their stability, can be resolved. The least sta ble fractions (or domains) have properties similar to native CT.