Background: Structural changes in the antithrombin reactive center loop P14 residue are thought to mediate heparin allosteric activation. Results: P14 residue mutations produced antithrombin activating effects indicative of multiple allosteric activation states. Conclusion: Heparin allosterically activates antithrombin by a minimal three-step mechanism involving intermediate and fully activated states. Significance: New insights into the allosteric activation mechanism of a critical anticoagulant protein are gained.

Background: Exosites are known to mediate heparin allosteric activation of antithrombin. Results: Mutagenesis revealed that an exosite differentially contributes to antithrombin reactivity with factors Xa/IXa in unactivated and heparin-activated states. Conclusion: Heparin allosteric activation of antithrombin results from alterations in an exosite interaction with protease induced by core conformational changes. Significance: The findings support our recently proposed model of antithrombin allosteric activation.

Background:The heparin activation mechanism of antithrombin as a factor IXa and Xa inhibitor is not established. Results: Mutations adjacent to helix D result in full activation of antithrombin without heparin. Conclusion: Activation is largely dependent on Tyr-131 and Ala-134 and minimally on reactive center loop hinge expulsion. Significance: This changes the understanding of the activation mechanism of antithrombin.

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