The physical significance of K_m in the prothrombinase reaction

Key kinetic parameters for the prothrombinase complex formed on membranes of phosphatidylserine (PS)/phosphatidylcholine (PC) ( 40 60) (K_m = 0.12 μM, k_cat = 11 s^-1) or PS/PC ( 2 98) (K_m = 0.40 μM, k_cat = 11 ^-1) differed only slightly. In contrast, the density of proteins on the membrane surface at the K_m differed greatly for the two membranes. The kinetics appeared unaffected by conditions where the number of phospholipid vesicles (2% PS) exceeded the number of protein molecules. These results establish that the K_m for the prothrombinase reaction is determined by the concentration of prothrombin in solution rather than its density at the membrane surface. This system can be treated as a dissociable enzyme acting on a soluble substrate.