title: Localization of an antithrombin exosite that promotes rapid inhibition of factors Xa and IXa dependent on heparin activation of the serpin.
abstract: We have previously shown that exosites in antithrombin outside the P6-P3' reactive loop region become available upon heparin activation to promote rapid inhibition of the target proteases, factor Xa and factor IXa. To identify these exosites, we prepared six antithrombin-alpha 1-proteinase inhibitor chimeras in which antithrombin residues 224-286 and 310-322 that circumscribe a region surrounding the reactive loop on the inhibitor surface were replaced in 10-16-residue segments with the homologous segments of alpha1-proteinase inhibitor. All chimeras bound heparin with a high affinity similar to wild-type, underwent heparin-induced fluorescence changes indicative of normal conformational activation, and were able to form SDS-stable complexes with thrombin, factor Xa, and factor IXa and inhibit these proteases with stoichiometries minimally altered from those of wild-type antithrombin. With only one exception, conformational activation of the chimeras with a heparin pentasaccharide resulted in normal approximately 100-300-fold enhancements in reactivity with factor Xa and factor IXa. The exception was the chimera in which residues 246-258 were replaced, corresponding to strand 3 of beta-sheet C, which showed little or no enhancement of its reactivity with these proteases following pentasaccharide activation. By contrast, all chimeras including the strand 3C chimera showed essentially wild-type reactivities with thrombin after pentasaccharide activation as well as normal full-length heparin enhancements in reactivity with all proteases due to heparin bridging. These findings suggest that antithrombin exosites responsible for enhancing the rates of factor Xa and factor IXa inhibition in the conformationally activated inhibitor lie in strand 3 of beta-sheet C of the serpin.
authors.affiliation: Center for Molecular Biology of Oral Diseases, College of Dentistry, University of Illinois at Chicago, Chicago, Illinois 60612, USA.
authors.forename: Gonzalo
authors.initials: G
authors.lastname: Izaguirre
authors.forename: Weiqing
authors.initials: W
authors.lastname: Zhang
authors.forename: Richard
authors.initials: R
authors.lastname: Swanson
authors.forename: Tina
authors.initials: T
authors.lastname: Bedsted
authors.forename: Steven T
authors.initials: ST
authors.lastname: Olson
mesh_terms: D000595:Amino Acid Sequence; D000990:Antithrombin III / Q000737:chemistry* / Q000502:physiology; D015949:Factor IXa / Q000037:antagonists & inhibitors* / Q000378:metabolism; D015951:Factor Xa / Q000378:metabolism; D065427:Factor Xa Inhibitors; D006493:Heparin / Q000494:pharmacology*; D006801:Humans; D007700:Kinetics; D011485:Protein Binding; D011487:Protein Conformation; D011993:Recombinant Fusion Proteins; D015842:Serine Proteinase Inhibitors / Q000378:metabolism; D015843:Serpins / Q000187:drug effects / Q000378:metabolism*; D013917:Thrombin / Q000378:metabolism
chemical_list: D065427:Factor Xa Inhibitors; D011993:Recombinant Fusion Proteins; D015842:Serine Proteinase Inhibitors; D015843:Serpins; D000990:Antithrombin III; D006493:Heparin; D015949:Factor IXa; D013917:Thrombin; D015951:Factor Xa