Mutations within the reactive-site region of human pancreatic secretory trypsin inhibitor confer alpha-thrombin and factor Xa inhibitory activities

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

A series of 12 bovine pancreatic trypsin inhibitor variants mutated in the P(4) and P(3) positions of the canonical binding loop containing additional K15R and M52L mutations were used to probe the role of single amino acid substitutions on binding to bovine trypsin and to the following human proteinases involved in blood clotting: plasmin, plasma kallikrein, factors X(a) and XII(a), thrombin, and protein C. The mutants were expressed in Escherichia coli as fusion proteins with the LE1413 hydrophobic polypeptide and purified from inclusion bodies; these steps were followed by CNBr cleavage and oxidative refolding. The mutants inhibited the blood-clotting proteinases with association constants in the range of 10(3)-10(10) m(-)(1). Inhibition of plasma kallikrein, factors X(a) and XII(a), thrombin, and protein C could be improved by up to 2 orders of magnitude by the K15R substitution. The highest increase in the association constant for P(3) mutant was measured for factor XII(a); P13S substitution increased the K(a) value 58-fold. Several other substitutions at P(3) resulted in about 10-fold increase for factor X(a), thrombin, and protein C. The cumulative P(3) and P(1) effects on K(a) values for the strongest mutant compared with the wild type bovine pancreatic trypsin inhibitor were in the range of 2.2- (plasmin) to 4,000-fold (factors XII(a) and X(a)). The substitutions at the P(4) site always caused negative effects (a decrease 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 decrease of the denaturation temperature (about 22 degrees C) for all P(4) mutants, suggesting that substitution of the wild type Gly-12 residue leads to a change in the binding loop conformation manifesting itself in non-optimal binding to the proteinase active site.

Inhibition of serine proteinases from human blood clotting system by squash inhibitor mutants

A series of six CMTI I variants mutated in the P(2)-P(4)' region of the canonical binding loop were used to probe the role of single amino acid substitutions on binding to the following human proteinases involved in blood clotting: plasmin, plasma kallikrein, factors X(a) and XII(a). The mutants were expressed as fusion proteins with the LE1413 hydrophobic polypeptide in Escherichia coli, purified from inclusion bodies, followed by cyanobromide cleavage and refolding. The mutants inhibited the proteinases with the association constants in the range 10(3)-10(9) M(-1). Inhibition of plasma kallikrein and factors X(a) and XII(a) could be improved up to 30-fold by single mutations. In contrast, neither of the introduced mutations increased inhibitory properties of CMTI I against plasmin. Additionally, using two inhibitors of natural origin, CMTI I (P(1) Arg) and CPTI II (P(1) Lys), we determined the effect of Lys-->Arg on binding to four proteinases. With the exception of plasmin (no effect), P(1) Arg resulted in up to 30-fold stronger binding than P(1) Lys.