title: The interaction of human factor VIIa with tissue factor.
abstract: The interaction of factor VIIa with tissue factor (TF) results in an increase in the catalytic efficiency for the hydrolysis of several synthetic peptidyl p-nitroanilide substrates by factor VIIa. The binding of human recombinant factor VIIa to recombinant human TF incorporated into vesicles containing phosphatidylcholine (TF/PC) or phosphatidylcholine/phosphatidylserine (TF/PCPS) was studied using the increased rate of H-D-phenylalanyl L-pipecoyl L-arginine p-nitroanilide (S2238) hydrolysis as a signal for the interaction. The saturable dependence of rate on increasing concentrations of factor VIIa or TF/PCPS yielded no obvious evidence for cooperativity and could be analyzed according to the interaction of factor VIIa with independent noninteracting sites (Kd = 259 +/- 60 pM, n = 1.05 +/- 0.12 mol of factor VIIa/mol of TF at saturation). Identical titration curves and equilibrium parameters were derived from titrations using TF/PC or TF in the absence of phospholipids, indicating that possible protein-membrane interactions do not further stabilize the extrinsic Xase complex. The dissociation constant for the interaction of factor VIIa with TF/PCPS inferred from measurements of factor X activation (Kd = 197 +/- 38 pM) was comparable with the values obtained from measurements of S2238 hydrolysis. In contrast to the membrane-independent nature of the enzyme-cofactor interaction, the rate of factor X activation was reduced by approximately 50-fold when the enzyme complex was assembled using solution-phase TF. Collectively, the result indicate that the membrane dependence of extrinsic Xase function primarily results from an influence of the membrane surface on factor X utilization.
authors.affiliation: Department of Medicine, Emory University School of Medicine, Atlanta, Georgia 30322.
authors.forename: S
authors.initials: S
authors.lastname: Krishnaswamy
mesh_terms: D000595:Amino Acid Sequence; D015942:Factor VIIa / Q000378:metabolism*; D005170:Factor X / Q000378:metabolism*; D015951:Factor Xa / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D008081:Liposomes; D008969:Molecular Sequence Data; D009842:Oligopeptides / Q000138:chemical synthesis / Q000378:metabolism; D010713:Phosphatidylcholines; D010718:Phosphatidylserines; D011485:Protein Binding; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000378:metabolism; D013379:Substrate Specificity; D013925:Thromboplastin / Q000378:metabolism*
chemical_list: D008081:Liposomes; D009842:Oligopeptides; D010713:Phosphatidylcholines; D010718:Phosphatidylserines; D011994:Recombinant Proteins; D005170:Factor X; D013925:Thromboplastin; D015942:Factor VIIa; D015951:Factor Xa