Localization of the heparin binding exosite of factor IXa

Factor IXa (FIXa) is known to have a binding site for heparin that has not been mapped by a mutagenesis study. By homology modeling based on structural data, we identified eight basic residues in the catalytic domain of FIXa that can potentially bind to heparin. These residues, Lys(98), Lys(126), Arg(165), Arg(170), Lys(173), Lys(230), Arg(233), and Lys(239) (chymotrypsin numbering) were substituted with Ala in separate constructs in Gla-domain-less forms. Following activation, it was found that all FIXa derivatives cleaved the chromogenic substrate CBS 31.39 with near normal catalytic efficiencies. Similarly, antithrombin inactivated FIXa derivatives with a similar second-order association rate constant (k(2)) in both the absence and presence of pentasaccharide. In the presence of a full-length heparin, however, k(2) values were dramatically impaired with certain mutants. Direct binding studies revealed that the same mutants lost their affinities for binding to heparin-Sepharose. Both kinetic and direct binding data indicated that five basic residues of FIXa in the following order of importance, Arg(233) > Arg(165) > Lys(230) > Lys(126) > Arg(170) are critical for binding to heparin. Consistent with these results, examination of the crystal structure of the catalytic domain of FIXa indicated that all five basic residues are spatially aligned in a manner optimal for interaction with heparin.