The Local Phospholipid Environment Modulates the Activation of Blood Clotting*

Examples abound of membrane-bound enzymes for which the local membrane environment plays an important role, including the ectoenzyme that triggers blood clotting, the plasma serine protease, factor VIIa, bound to the integral membrane protein, tissue factor. The activity of this enzyme complex is markedly influenced by lipid bilayer composition and further by tissue factor partitioning into membrane microdomains on some cell surfaces. Unfortunately, little is known about how membrane microdomain composition controls factor VIIa-tissue factor activity, as reactions catalyzed by membrane-tethered enzymes are typically studied under conditions in which the experimenter cannot control the composition of the membrane in the immediate vicinity of the enzyme. To overcome this problem, we used a nanoscale approach that afforded complete control over the membrane environment surrounding tissue factor by assembling the factor VIIa·tissue factor complex on stable bilayers containing 67 ± 1 phospholipid molecules/leaflet (Nanodiscs). We investigated how local changes in phospholipid bilayer composition modulate the activity of the factor VIIa·tissue factor complex. We also addressed whether this enzyme requires a pool of membrane-bound protein substrate (factor X) for efficient catalysis, or alternatively if it could efficiently activate factor X, which binds directly to the membrane nanodomain adjacent to tissue factor. We have shown that full proteolytic activity of the factor VIIa·tissue factor complex requires extremely high local concentrations of anionic phospholipids and further that a large pool of membrane-bound factor X is not required to support sustained catalysis.