Modification of α1-antitrypsin Pittsburgh’s RCL for contact system enzyme blockade. (A) α1-Antitrypsin reactive center loop grafting strategy. The reactive center is indicated by a scissors symbol. (B-E) Screening of target enzymes inhibition by α1AT variants and their controls; table insets show second-order rate constants (k₂: 10⁴ M⁻¹·s⁻¹; ND = not detected). (B) Inhibition of 25 nM αFXIIa by 96.2 nM SERPIN. (C) Inhibition of 2.3 nM PKa by 28.9 nM SERPIN. (D) Inhibition of 24.1 nM plasmin by 24.1 nM SERPIN. (E) Inhibition of 2.5 nM FXIa by 9.6 nM SERPIN. (F) Inhibition of dilute aPTT clotting times in normal plasma by 384 nM SERPIN or buffer (vehicle). (G) Inhibition of kaolin-driven thrombin generation in normal plasma by 384 nM SERPIN or buffer (vehicle). Data represent the mean ± standard deviation (SD) of 3 separate experiments, each performed in duplicate. #P < .005; §P < .0001, compared with pdC1INH by 1-way analysis of variance (ANOVA). pdC1INH, plasma-derived C1INH; Pitts, α1AT-Pittsburgh; SLLR/S, α1AT-SLLR/S; SMTR/S, α1AT-SMTR/S; WT, wild-type α1AT.