The thrombin mutant, des-ETW-thrombin, lacking Glu146, Thr147 and Trp148 within a unique insertion loop located at the extreme end of the primary specificity pocket, has been shown previously to exhibit reduced catalytic activity with respect to macromolecular and synthetic thrombin substrates and reduced or enhanced susceptibility to inhibition. Investigation of the hydrolysis of peptidyl p-nitroanilide substrates by des-ETW-thrombin showed increased activity in the presence of heparin and other sulphated glycosaminoglycans. No effect was observed upon the activity of wild-type thrombin. Heparin was found to decrease the Km for cleavage of four thrombin-specific substrates by des-ETW-thrombin, by 3–4-fold. Similarly, pentosan polysulphate (PPS) decreased the Km with these substrates by 8–10-fold. Heparin also increased the rate of inhibition of des-ETW-thrombin by antithrombin III and D-phenylalanyl-prolyl-arginylchloromethane (PPACK). The inhibition of des-ETW-thrombin by a number of thrombin-specific peptide boronic acids also showed significant reduction in the final Ki in the presence of heparin, due to reduction in the off-rate. A peptide analogue of a sequence of hirudin which binds thrombin tightly to exosite 1 (fibrinogen recognition site) potentiated the activity of des-ETW-thrombin against peptide p-nitroanilide substrates in a manner similar to heparin. The Ki for the inhibition of des-ETW-thrombin by p-aminobenzamidine was decreased by these ligands from 9.7 mM to 7.5 mM, 5.1 mM and 2.5 mM in the presence of heparin, hirudin peptide and PPS respectively, suggesting the increased catalytic activity is due to enhanced access to the primary specificity pocket. The positive influence of these ligands on des-ETW-thrombin was reversed in the presence of ATP or ADP; the latter has previously been shown to inhibit thrombin activity by blocking initial interaction with fibrinogen at exosite 1. Because the effect of heparin and PPS is similar to that of hirudin peptide, it is proposed that the most likely mechanism is that binding at the heparin-binding site (thrombin exosite 2) facilitates interaction at exosite 1 causing a conformational change which partially corrects the defective ground-state binding of the mutant thrombin. Although no effect was observed upon the activity of wild-type thrombin, our findings do provide further evidence of an allosteric property of thrombin which may control the geometry of, and access to, the primary specificity pocket.
- The Biochemical Society, London © 1996