Pentasaccharide Enhances the Inactivation of Factor Xa by Antithrombin by Promoting the Assembly of a Michaelis-Type Intermediate Complex. Demonstration by Rapid Kinetic, Surface Plasmon Resonance, and Competitive Binding Studies

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Abstract
It has been demonstrated that a unique pentasaccharide fragment of heparin (H5) activates AT by exposing an exosite on the serpin that is a recognition site for interaction with the basic autolysis loop (residues 143−154) of fXa. In support of this, the substitution of Arg-150 of fXa with Ala (R150A) impaired the reactivity of the mutant with AT by 1 order of magnitude specifically in the presence H5. To understand the mechanism by which heparin activation of AT improves the reactivity of the serpin with fXa, the H5-catalyzed reaction of AT with fXa, fXa R150A, and fXa S195A was studied using rapid kinetic, surface plasmon resonance, and competitive binding methods. The pseudo-first-order rate constants for the H5-catalyzed AT inhibition of both fXa and fXa R150A exhibited a linear dependence on the serpin concentration, thereby yielding second-order rate constants of 1.0 × 106 and 1.5 × 105 M-1 s-1, respectively. On the other hand, an ∼70-saccharide, high-affinity heparin-catalyzed AT inhibition of both fXa derivatives showed a saturable dependence on the inhibitor concentration, yielding an identical rate constant of ∼20 s-1, but different ternary fXa−heparin−AT dissociation constants (KE,ATH) of ∼130 and ∼1780 nM for wild-type and R150A fXa, respectively. Competitive kinetic and surface plasmon resonance binding studies using the catalytically inactive S195A mutant of fXa yielded dissociation constants of 255 and 610 nM, respectively, for the mutant protease interaction with the AT−H5 complex. These results suggest that H5 enhances the reactivity of AT with fXa primarily by lowering the KE,ATH for the formation of a Michaelis-type serpin−protease encounter complex.

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