Title

Heparin modulates the 99-loop of factor IXa: Effects on reactivity with isolated Kunitz-type inhibitor domains

Document Type

Article

Publication Date

2006

Abstract

Reactivity of factor IXa with basic pancreatic trypsin inhibitor is enhanced by low molecular weight heparin (enoxaparin). Previous studies by us have suggested that this effect involves allosteric modulation of factor IXa. We examined the reactivity of factor IXa with several isolated Kunitz-type inhibitor domains: basic pancreatic trypsin inhibitor, the Kunitz inhibitor domain of protease Nexin-2, and the first two inhibitor domains of tissue factor pathway inhibitor. We find that enhancement of factor IXa reactivity by enoxaparin is greatest for basic pancreatic trypsin inhibitor (>10-fold), followed by the second tissue factor pathway inhibitor domain (1.7-fold) and the Kunitz inhibitor domain of protease Nexin-2 (1.4-fold). Modeling studies of factor IXa with basic pancreatic trypsin inhibitor suggest that binding of this inhibitor is sterically hindered by the 99-loop of factor IXa, specifically residue Lys98. Slow-binding kinetic studies support the formation of a weak initial enzyme-inhibitor complex between factor IXa and basic pancreatic trypsin inhibitor that is facilitated by enoxaparin binding. Mutation of Lys98 to Ala in factor IXa results in enhanced reactivity with all inhibitors examined, whereas almost completely abrogating the enhancing effects of enoxaparin. The results implicate Lys98 and the 99-loop of factor IXa in defining enzyme inhibitor specificity. More importantly, these results demonstrate the ability of factor IXa to be allosterically modulated by occupation of the heparin-binding exosite.

Publication Title

Journal of Biological Chemistry

Volume

281

Issue

32

First Page

23066

Last Page

23074

Comments

This article was published in Journal of Biological Chemistry, Volume 281, Issue 32, Pages 23066-23074.

The published version is available at http://dx.doi.org/10.1074/jbc.M603743200.

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