Conformational Equilibria and the Salt Bridge in Chymotrypsin

5-Chymotrypsin is stabilized in the active conformation by a salt bridge between the a-ammonium group of Ile 16 and the carboxylate ion of Asp 194. At ne.utral pH the enzyme exists mainly in the active conformation but at high pH where the salt bridge is deprotonated only 12 % of the enzyme takes up the active conformation. The stabilization energy of the salt bridge is 2.9 kcal/mole. The formation is entropy driven and involves an adverse nthalpy change. The pKa of Ile 16 is increased from 7.9 when unconstrained in the inactive conformation to 10 when in the salt bridge in the active conformation (see Fig. 1). I will now briefly describe how these results were obtained. Chymotrypsinogen is activa, ted to chymo-