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The interaction between alpha-chymotrypsin and pancreatic trypsin inhibitor (Kunitz inhibitor). Kinetic and thermodynamic properties.

1. The association constant, Ka, of the 1:1 complex formed between α-chymotrypsin and the pancreatic trypsin inhibitor is 110 μM−1 at pH 8.0 and 25 °C. The second-order rate constant for the association, ka, is 110 mM−1 s−1 and the first-order rate constant for the dissociation, kd, is 10−3 s−1 under the same conditions. Thermodynamic parameters for complex formation at 25 °C, pH 8.0 are ΔGa0= -11.0 kcal×mol−1, ΔHa0= 3 kcal×mol−1 and ΔSa0= 48 cal. · mol−1· K−1. 
 
 
 
2. Temperature and pH-dependences of the rate constants ka and kd have been studied. 
 
 
 
3. The difference in stability between the α-chymotrypsin · inhibitor complex and the trypsin · inhibitor complex (Ka= 16 pM−1, ΔGa0=–18.1 kcal×mol−1) is due essentially to differences of kd values. The dissociation of the α-chymotrypsin · inhibitor complex is 1.5×104 times faster than that of the trypsin · inhibitor complex. Differences in Ka and kd values are discussed in terms of differences in stabilizing interactions. 
 
 
 
4. The Cys14-Cys38 disulfide bridge of the inhibitor, which is highly succeptible to reduction when the inhibitor is free, is masked in the α-chymotrypsin · inhibitor complex. Reduction of the Cys14-Cys38 bridge in the free inhibitor does not prevent association with α-chymotrypsin. Values of ka and kd for the formation of the 1:1 α-chymotrypsin · reduced-inhibitor complex and for the complex formed with the native inhibitor are very similar. This situation differs from that observed with the trypsin · pancreatic-inhibitor complex. In that case, reduction of Cys14-Cys38 bridge considerably decreases the stability of the complex formed with trypsin; Ka is decreased by a factor of 3×104 and kd is increased by a factor of 8.6×103.