Excessive enzyme activity often has pathological consequences. This for example is the case in thrombosis and hereditary angioedema, where serine proteases of the coagulation system and kallikrein-kinin system are excessively active. Serine proteases are controlled by SERPINs ( se rine p rotease in hibitors). We here describe the basic biochemical mechanisms behind SERPIN activity and identify key determinants that influence their function. We explore the clinical phenotypes of several SERPIN deficiencies and review studies where SERPINs are being used beyond replacement therapy. Excitingly, rare human SERPIN mutations have led us and others to believe that it is possible to refine SERPINs toward desired behavior for the treatment of enzyme-driven pathology.

Targeting natural anticoagulant proteins as a means to rebalance the hemostatic system is an emerging trend in the development of innovative therapeutic strategies for hemophilia. These 2 articles develop these concepts in the areas of hemostasis and contact activation. In the study by Aymonnier and colleagues, simple amino acid substitutions converted a serpin elastase inhibitor, α1-antitrypsin (α1AT), into a potent antithrombin, activated protein C inhibitor, or anti-PKa/FXIIa inhibitor. In the study by de Maat and colleagues, redesign of α1AT strongly altered its inhibitory behavior and enables it to be used for the treatment of contact system–mediated thrombosis and inflammation.

Factor XII (FXII) zymogen activation requires cleavage after arginine 353 located in the activation loop. This cleavage can be executed by activated FXII (autoactivation), plasma kallikrein (PKa), or plasmin. Previous studies proposed that the activation loop of FXII is shielded to regulate FXII activation and subsequent contact activation. In this study, we aimed to elucidate this mechanism by expressing and characterizing seven consecutive N-terminally truncated FXII variants as well as full-length wild-type (WT) FXII. As soon as the fibronectin type II domain is lacking (FXII Δ1–71), FXII cleavage products appear on Western blot. These fragments display spontaneous amidolytic activity, indicating that FXII without the fibronectin type II domain is susceptible to autoactivation. Additionally, truncated FXII Δ1–71 is more easily activated by PKa or plasmin than full-length WT FXII. To exclude a contribution of autoactivation, we expressed active-site incapacitated FXII truncation variants (S544A). FXII S544A Δ1–71 is highly susceptible to cleavage by PKa, indicating exposure of the activation loop. In surface binding experiments, we found that the fibronectin type II domain is non-essential for binding to kaolin or polyphosphate, whereas the following epidermal growth factor-like domain is indispensable. Binding of full-length FXII S544A to kaolin or polyphosphate increases its susceptibility to cleavage by PKa. Moreover, the activation of full-length WT FXII by PKa increases approximately threefold in the presence of kaolin. Deletion of the fibronectin type II domain eliminates this effect. Combined, these findings suggest that the fibronectin type II domain shields the activation loop of FXII, ensuring zymogen quiescence.