The Reaction of Serpins with Proteinases Involves Important Enthalpy Changes

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Abstract
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 α1-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 (ΔH = −53 to −63 kcal mol-1). The former reaction also has a large and negative heat capacity (ΔCp = −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 α1-proteinase inhibitor and that of pancreatic trypsin and coagulation factor Xa by antithrombin are accompanied by a ΔH 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.