Mapping of the thrombin des-ETW conformation by using site-directed mutants of hirudin. Evidence for the induction of nonlocal modifications by mutagenesis

Abstract

Deletion of Glu146, Thr147, and Trp148 from thrombin dramatically alters its interactions with substrates, ligands, and inhibitors, and the changes appear to result from nonlocal modification of thrombin's structure [Le Bonniec, B. F., Guinto, E. R., & Esmon, C. T. (1992) J. Biol. Chem. 267, 19341-19348]. In an attempt to localize conformational change in this thrombin mutant (des-ETW), its affinity for 43 site-directed mutants of hirudin have been compared with that of native thrombin. The inhibition constants (KI) of recombinant and natural hirudin with des-ETW were found to be approximately 2300-fold higher than those with thrombin. The reduced affinity was predominantly the result of an increase in the dissociation rate constant rather than a decrease in the association rate constant. For most of the hirudin mutants tested, the difference in binding energy between thrombin and des-ETW [delta delta Gbo(IIa-ETW)] was about 19.9 kJ mol-1 and comparable to that of hirudin; in particular, all mutations in the C-terminal region of the inhibitor did not affect delta delta Gbo(IIa-ETW). Thus, the conformation of thrombin's anion-binding exosite seems unaltered by the des-ETW mutation. In contrast, hirudin substitutions involving Val1', Val2', Thr4', Asp5', and Ser19', as well as the addition of an N-terminal glycine, resulted in values of delta delta Gbo(IIa-ETW) which were 2 to 10 kJ mol-1 lower than that for hirudin. Furthermore, a Leu15'-->Ala hirudin mutant behaved with des-ETW, as a partial competitive inhibitor rather than a tight-binding inhibitor, in contrast to all other hirudin variants tested.(ABSTRACT TRUNCATED AT 250 WORDS)