Role of regulatory exosite I in binding of thrombin to human factor V, factor Va, factor Va subunits, and activation fragments.
The blood coagulation proteinase, thrombin, converts factor V into factor Va through a multistep activation pathway that is regulated by interactions with thrombin exosites. Thrombin exosite interactions with human factor V and its activation products were quantitatively characterized in equilibrium binding studies based on fluorescence changes of thrombin covalently labeled with 2-anilinonaphthalene-6-sulfonic acid (ANS) linked to the catalytic site histidine residue by Nalpha-[(acetylthio)acetyl]-D-Phe-Pro-Arg-CH2Cl ([ANS]FPR-thrombin). Exosite I was shown to play a predominant role in the binding of factor V and factor Va from the effect of the exosite I-specific ligand, hirudin54-65, on the interactions. Factor V and factor Va bound to exosite I of [ANS]FPR-thrombin with similar dissociation constants of 3.4 +/- 1.3 and 1.1 +/- 0.4 microM and fluorescence enhancements of 182 +/- 41 and 127 +/- 17%, respectively. Native thrombin and labeled thrombin bound with similar affinity to factor Va. Among factor V activation products, the factor Va heavy chain was shown to contain the site of exosite I binding, whereas exosite I-independent, lower affinity interactions were observed for activation fragments E and C1, and no detectable binding was observed for the factor Va light chain. The results support the conclusion that the factor V activation pathway is initiated by exosite I-mediated binding of thrombin to a site in the heavy chain region of factor V that facilitates the initial cleavage at Arg709 to generate the heavy chain of factor Va. The results further suggest that binding of thrombin through exosite I to factor V activation intermediates may regulate their conversion to factor Va and that similar binding of thrombin to the factor Va produced may reflect a mode of interaction involved in the regulation of prothrombin activation.
Dharmawardana
K R
KR
Department of Pathology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232, USA.
Olson
S T
ST
Bock
P E
PE
D000817:Anilino Naphthalenesulfonates / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D002417:Cattle; D005165:Factor V / Q000378:metabolism*; D015943:Factor Va / Q000378:metabolism*; D005456:Fluorescent Dyes / Q000378:metabolism; D006801:Humans; D008956:Models, Chemical; D010446:Peptide Fragments / Q000378:metabolism*; D011487:Protein Conformation; D013917:Thrombin / Q000378:metabolism*
D000817:Anilino Naphthalenesulfonates; D005456:Fluorescent Dyes; D010446:Peptide Fragments; C047136:2-anilinonaphthalene-6-sulfonic acid; D015943:Factor Va; D005165:Factor V; D013917:Thrombin