Inhibition of six serine proteinases of the human coagulation system by mutants of bovine pancreatic trypsin inhibitor

series of 12 bovine pancreatic trypsin inhibitor variants mutated in the P- 4 and P-3 positions of the canonical binding loop containing additional R15 R and M52L mutations mere used to probe the role of single amino acid subst itutions on binding to bovine trypsin and to the following human proteinase s involved in blood clotting: plasmin, plasma kallikrein, factors X-a and X IIa, thrombin, and protein C. The mutants were expressed in Escherichia col i as fusion proteins with the LE1413 hydrophobic polypeptide and purified f rom inclusion bodies; these steps were followed by CNBr cleavage and oxidat ive refolding, The mutants inhibited the blood-clotting proteinases with as sociation constants in the range of 10(3)-10(10) M-1. Inhibition of plasma kallikrein, factors X-a and XIIa, thrombin, and protein C could be improved by up to 2 orders of magnitude by the K15R substitution The highest increa se in the association constant for P-3 mutant was measured for factor XIIa; P13S substitution increased the K-a value 58-fold, Several other substitut ions at P-3 resulted in about 10-fold increase for factor X-a, thrombin, an d protein C. The cumulative P-3 and P-1 effects on K-alpha values for the s trongest mutant compared with the wild type bovine pancreatic trypsin inhib itor were in the range of 2,2- (plasmin) to 4,000-fold (factors XIIa and X- a). The substitutions at the P-4 site always caused negative effects (a dec rease in the range from over 1.000- to 1.3-fold) on binding to all studied enzymes, including trypsin, Thermal stability studies showed a very large d ecrease of the denaturation temperature (about 22 degreesC) for all P-4 mut ants, suggesting that substitution of the mild type Gly-12 residue leads to a change in the binding loop conformation manifesting itself in non-optima l binding to the proteinase active site.