The dual role of thrombin's anion-binding exosite-I in the recognition and cleavage of the protease-activated receptor 1

The role of thrombin anion-binding exosite-I in the recognition and cleavage of the extracellular domain of the seven transmembrane domain thrombin receptor (PAR1) was determined using site-directed mutagenesis. Basic residues in anion-binding exosite-I (Arg35, Arg36, Arg67, Arg73, Arg75, Arg77A, Lys81, Lys109, Lys110 and Lys149E) were substituted with glutamines and the resultant recombinant mutant thrombins were used to determine kinetic parameters for the cleavage of a peptide (PAR38-60) based on the PAR1 extracellular domain. Compared with wild-type thrombin, replacement of Arg67 and Arg73 had a dramatic effect on the cleavage of PAR38-60 (k(cat)/K-m = 1.8 x 10(6) and 4.6 x 10(6) vs 9.2 x 10(7) M-1 s(-1)), whereas the remaining mutations of the anion-binding exosite-I of thrombin had a less pronounced effect, with k(cat)/K-m values ranging from 3.3 x 10(7) M-1.s(-1) (R77(a)Q) to 5.8 x 10(7) M-1.s(-1) (K109Q). The ability of thrombin mutants to activate platelets paralleled that of PAR38-60 cleavage, whereas their ability to clot fibrinogen differed profoundly, as did their susceptibility to hirudin inhibition. Results are interpreted with respect to known interactions of thrombin with thrombomodulin, hirudin, rhodniin and heparin cofactor II. We conclude that the basic residues of anion-binding exosite-I contribute significantly to enhancing the rate of complex formation in two ways; the first (general) ensures electrostatic steering of ligands with complementary electrostatic fields, the second (specific) involves a combination of molecular contacts within the complex that is unique for each ligand.