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 (k(2D)) of 4.9 (± 0.8) x 106 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.
|Original language||English (US)|
|Number of pages||4|
|Journal||Journal of Biological Chemistry|
|State||Published - 1993|
ASJC Scopus subject areas
- Molecular Biology
- Cell Biology