title: Factor VII autoactivation proceeds via interaction of distinct protease-cofactor and zymogen-cofactor complexes. Implications of a two-dimensional enzyme kinetic mechanism.
abstract: Tissue factor (TF), an integral membrane protein, enhances the feedback activation of factor VII by factor VIIa (factor VII autoactivation). We found that, in contrast to the other known membrane-dependent coagulation activation reactions, TF-dependent factor VII autoactivation occurred preferentially on neutral phospholipid vesicles relative to negatively charged vesicles containing phosphatidylserine. This reaction was best described by a novel mechanism in which the enzyme and substrate are each bound to separate cofactor (TF) molecules. This unusual mechanism of substrate presentation to a membrane-bound protease predicts that the reaction rate will be directly dependent on the surface density, and hence lateral diffusion, of factor VII.TF and factor VIIa.TF complexes, obeying obligatorily two-dimensional enzyme kinetics. This prediction was confirmed, yielding a two-dimensional second-order rate constant (k2D) of 4.9 (+/- 0.8) x 10(6) m2 mol-1 s-1. Since intact cells normally sequester acidic phospholipids away from the outer leaflet of the plasma membrane, this reaction mechanism should permit factor VII autoactivation to predominate on unactivated/undamaged cell surfaces when other clotting reactions are dormant.
authors.affiliation: Oklahoma Medical Research Foundation, Oklahoma City 73104.
authors.forename: P F
authors.initials: PF
authors.lastname: Neuenschwander
authors.forename: M M
authors.initials: MM
authors.lastname: Fiore
authors.forename: J H
authors.initials: JH
authors.lastname: Morrissey
mesh_terms: D002462:Cell Membrane / Q000201:enzymology; D003067:Coenzymes / Q000378:metabolism; D010450:Endopeptidases / Q000378:metabolism*; D004792:Enzyme Precursors / Q000378:metabolism*; D005167:Factor VII / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D010718:Phosphatidylserines / Q000494:pharmacology; D013925:Thromboplastin / Q000378:metabolism*
chemical_list: D003067:Coenzymes; D004792:Enzyme Precursors; D010718:Phosphatidylserines; D005167:Factor VII; D013925:Thromboplastin; D010450:Endopeptidases