The reaction of serpins with proteinases involves important enthalpy changes.

When active serpins are proteolytically inactivated in a substrate-like reaction, they undergo an important structural transition with a resultant increase in their conformational stability. We have used microcalorimetry to show that this conformational alteration is accompanied by an important enthalpy change. For instance, the cleavage of alpha(1)-proteinase inhibitor by Pseudomonas aeruginosa elastase, Staphylococcus aureus V8 proteinase, or papain and that of antithrombin by leukocyte elastase are characterized by large enthalpy changes (DeltaH = -53 to -63 kcal mol(-1)). The former reaction also has a large and negative heat capacity (DeltaC(p)() = -566 cal K(-1) mol(-1)). In contrast, serpins release significantly less heat when they act as proteinase inhibitors. For example, the inhibition of pancreatic elastase, leukocyte elastase, and pancreatic chymotrypsin by alpha(1)-proteinase inhibitor and that of pancreatic trypsin and coagulation factor Xa by antithrombin are accompanied by a DeltaH of -20 to -31 kcal mol(-1). We observe no heat release upon proteolytic cleavage of inactive serpins or following inhibition of serine proteinases by canonical inhibitors or upon acylation of chymotrypsin by N-trans-cinnamoylimidazole. We suggest that part of the large enthalpy change that occurs during the structural transition of serpins is used to stabilize the proteinase in its inactive state.