Ionic interactions in the formation of the thrombin-hirudin complex.

Site-directed mutagenesis has been used to examine the importance of each of the acidic C-terminal residues of hirudin in the formation of its complex with alpha-thrombin. The contribution to binding energy of acidic residues in the 11 C-terminal amino acids varied from 2.3 to 5.9 kJ.mol-1. The differences between the contributions of individual residues were smaller than would be expected from the crystal structures of the thrombin-hirudin complex. In particular, the small effect (2.4 kJ.mol-1) for the replacement of Asp-55 was surprising in view of the two salt bridges made by this residue. The results of studies involving multiple mutations indicated that the additivity of the effects varied with the position of the mutation. Whereas the effect of mutations involving the glutamic acid residues at positions 61 and 62 were additive, non-additivity was observed with the glutamic acid residues at positions 57 and 58.