The mechanism by which heparin promotes the inhibition of coagulation factor XIa by protease nexin-2

Previous kinetic studies have shown that protease nexin-2 is a potent, reversible, and competitive inhibitor of factor XIa. Here we show that high molecular weight heparin potentiates the ability of protease nexin-2 to inhibit factor XIa with a parabolic concentration dependence, predominantly because of an increase of the association rate constant with little perturbation of the dissociation rate constant. No effect on factor XIa inhibition by protease nexin-a was observed with heparin preparations of 6-22 saccharide units (0.1 nM-10 mu M), whereas heparin preparations with 32-64 saccharide units potentiated factor XIa inhibition by protease nexin-2 in a size-and concentration-dependent manner. We propose a model wherein heparin exerts this effect by providing a template for the assembly of factor XIa-protease nexin-a complexes, and only heparin polymers consisting of greater than 32 saccharide units (M-r similar to 10,000) are sufficiently long to provide a template to which factor XIa and protease nexin-a molecules can bind simultaneously. Heparin-mediated enhancement of factor XIa inhibition by protease nexin-2 was partially abrogated by high molecular weight kininogen, suggesting that high molecular weight kininogen may play a role in regulating factor XIa activity.