title: Localization of factor IXa and factor VIIIa interactive sites.
abstract: The contribution of the catalytic and noncatalytic domains of factor IXa to the interaction with its cofactor, factor VIIIa, was evaluated. Two proteolytic fragments of factor IXa, lacking some or all of the serine protease domain, failed to mimic the ability of factor IXa to enhance the reconstitution of factor VIIIa from isolated A1/A3-C1-C2 dimer and A2 subunit. Both fragments, however, inhibited this factor IXa-dependent activity. Selective thermal denaturation of the factor IXa serine protease domain eliminated its effect on factor VIIIa reconstitution. Modification of factor IXa with dansyl-Glu-Gly-Arg chloromethyl ketone (DEGR-IXa) stabilized this domain, and heat-treated DEGR-IXa retained its ability to enhance factor VIIIa reconstitution. These results indicate the importance of the serine protease domain as well as structures residing in the factor IXa light chain (gamma-carboxyglutamic acid and/or epidermal growth factor domains) for cofactor stabilizing activity. In the presence of phospholipid, the A1/A3-C1-C2 dimer produced a saturable increase in the fluorescence anisotropy of fluorescein-Phe-Phe-Arg chloromethyl ketone-modified factor IXa (Fl-FFR-IXa). This effect was inhibited by a factor IXa fragment comprised of the gamma-carboxyglutamic acid and epidermal growth factor domains. The difference in Fl-FFR-IXa anisotropy in the presence of A1/A3-C1-C2 dimer (delta r = 0.043) compared with factor VIIIa (delta r = 0.069) represented the contribution of the A2 subunit, A peptide corresponding to factor VIII A2 domain residues 558-565 decreased the factor VIIIa dependent-anisotropy of Fl-FFR-IXa to a value similar to that observed with the A1/A3-C1-C2 dimer. These results support a model of multiple interactive sites in the association of the enzyme-cofactor complex and localize sites for the A1/A3-C1-C2 dimer and the A2 subunit to the factor IXa light chain and serine protease domain, respectively.
authors.affiliation: Department of Medicine, University of Rochester School of Medicine and Dentistry, New York 14642, USA.
authors.forename: L M
authors.initials: LM
authors.lastname: O'Brien
authors.forename: L V
authors.initials: LV
authors.lastname: Medved
authors.forename: P J
authors.initials: PJ
authors.lastname: Fay
mesh_terms: D000590:Amino Acid Chloromethyl Ketones / Q000737:chemistry; D000595:Amino Acid Sequence; D001665:Binding Sites; D003619:Dansyl Compounds / Q000737:chemistry; D015949:Factor IXa / Q000737:chemistry / Q000378:metabolism*; D015944:Factor VIIIa / Q000737:chemistry / Q000378:metabolism*; D005454:Fluorescence Polarization; D005456:Fluorescent Dyes / Q000737:chemistry; D006801:Humans; D066298:In Vitro Techniques; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000737:chemistry / Q000378:metabolism; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D012697:Serine Endopeptidases / Q000737:chemistry / Q000378:metabolism
chemical_list: D000590:Amino Acid Chloromethyl Ketones; D003619:Dansyl Compounds; D005456:Fluorescent Dyes; D010446:Peptide Fragments; D011994:Recombinant Proteins; C030824:dansylglutamyl-glycyl-arginine chloromethyl ketone; D015944:Factor VIIIa; C027587:phenylalanyl-phenylalanyl-arginine chloromethyl ketone; D012697:Serine Endopeptidases; D015949:Factor IXa