Quantitative evaluation of the contribution of ionic interactions to the formation of the thrombin-hirudin complex

The effect of ionic strength on the kinetics of inhibition of human alpha-thrombin has been examined by using genetically engineered forms of hirudin that differed only in the number of negatively charged residues in the carboxyl-terminal region of the molecule. Analysis of the data obtained allowed the binding energy for the thrombin-hirudin complex to be divided into contributions from ionic and nonionic interactions. The contribution of nonionic interactions to the binding energy was the same for each of the forms whereas the ionic contribution varied with the charge of the molecule. Each of the negatively charged residues made an approximately equal contribution of -4kJ mol-1 to the binding energy. For native hirudin, ionic interactions accounted for 32% of the binding energy at an ionic strength of zero.