Role of thrombin anion-binding exosite-I in the formation of thrombin-serpin complexes

Site-directed mutagenesis was used to investigate the role of basic residues in the thrombin anion-binding exosite-I during formation of thrombin-antithrombin III (ATIII), thrombin-protease nexin 1 (PN1), and thrombin-heparin cofactor II (HCII) inhibitor complexes, in the absence and presence of glycosaminoglycans. In the absence of glycosaminoglycan, association rate constant (kon) values for the inhibition of the mutant thrombins (R35Q, K36Q, R67Q, R73Q, R75Q, R77(a)Q, K81Q, K109Q, K110Q, and K149(e)Q) by ATIII and PN1 were similar to wild-type recombinant thrombin (rIIa), whereas kon values were decreased 2-3-fold for HCII against the majority of the exosite-I mutants. The exosite-I mutants did not have a significant effect on heparin-accelerated inhibition by ATIII with maximal kon values similar to rIIa. A small effect was seen for PN1/heparin inhibition of the exosite-I mutants R35Q, R67Q, R73Q, R75Q, and R77(a)Q, where kon values were decreased 2-4-fold, compared with rIIa. For HCII/heparin, kon values for inhibition of the exosite-I mutants (except R67Q, R73Q, and K149(e)Q) were 2-3-fold lower than rIIa. Larger decreases in kon values for HCII/heparin were found for R67Q and R73Q thrombins with 441- and 14-fold decreases, respectively, whereas K149(e)Q was unchanged. For HCII/dermatan sulfate, R67Q and R73Q had kon values reduced 720- and 48-fold, respectively, whereas the remaining mutants were decreased 3-7-fold relative to rIIa. The results suggest that ATIII has no major interaction with exosite-I of thrombin with or without heparin. PN1 bound to heparin uses exosite-I to some extent, possibly by utilizing the positive electrostatic field of exosite-I to enhance orientation and thrombin complex formation. The larger effects of the thrombin exosite-I mutants for HCII inhibition with heparin and dermatan sulfate indicate its need for exosite-I, presumably through contact of the "hirudin-like" domain of HCII with exosite-I of thrombin.