Heparin activation of protein Z-dependent protease inhibitor (ZPI) allosterically blocks protein Z activation through an extended heparin-binding site

Abstract

Protein Z (PZ)-dependent protease inhibitor (ZPI) is a plasma anticoagulant protein of the serpin superfamily, which is activated by its cofactor, PZ, to rapidly inhibit activated factor X (FXa) on a procoagulant membrane surface. ZPI is also activated by heparin to inhibit free FXa at a physiologically significant rate. Here, we show that heparin binding to ZPI antagonizes PZ binding to and activation of ZPI. Virtual docking of heparin to ZPI showed that a heparin-binding site near helix H close to the PZ-binding site as well as a previously mapped site in helix C was both favored. Alanine scanning mutagenesis of the helix H and helix C sites demonstrated that both sites were critical for heparin activation. The binding of heparin chains 72 to 5-saccharides in length to ZPI was similarly effective in antagonizing PZ binding and in inducing tryptophan fluorescence changes in ZPI. Heparin binding to variant ZPIs with either the helix C sites or the helix H sites mutated showed that heparin interaction with the higher affinity helix C site most distant from the PZ-binding site was sufficient to induce these tryptophan fluorescence changes. Together, these findings suggest that heparin binding to a site on ZPI extending from helix C to helix H promotes ZPI inhibition of FXa and allosterically antagonizes PZ binding to ZPI through long-range conformational changes. Heparin antagonism of PZ binding to ZPI may serve to spare limiting PZ and allow PZ and heparin cofactors to target FXa at different sites of action.

Keywords: anticoagulant; factor Xa; heparin; protein Z; protein Z-dependent protease inhibitor; serpin.