Prothrombinase complex assembly. Kinetic mechanism of enzyme assembly on phospholipid vesicles

The kinetics of the assembly of the prothrombinase complex was studied using factor Xa modified with the active site-directed fluorophore dansylglutamylglycylarginyl chloromethyl ester (DEGR.Xa) as a reporter for the process. Stopped-flow kinetic studies of prothrombinase assembly were undertaken at saturating concentrations of calcium ion, using vesicles composed of phosphatidylcholine and phosphatidylserine (PCPS), factor Va, and DEGR.Xa. The rate of complex formation, studied under pseudo first-order conditions, depended on the premixing protocol used to initiate complex assembly and was most rapid when prothrombinase assembly was initiated by reacting Va with the preformed DEGR.Xa.PCPS complex. Under these conditions, reaction rate was linearly dependent on the concentration of PCPS with no evidence for saturation and was independent of the concentration of VA. Essentially identical reaction rates were observed when the association of Va with PCPS was independently assessed by stopped-flow light scattering. The data indicate that prothrombinase assembly initiated by mixing Va with the DEGR.Xa.PCPS complex is rate limited by the initial reaction between Va and available combining sites on the vesicle surface (k = 5.7 x 10(7) M-1.s-1) followed by extremely rapid reactions (k greater than 1 x 10(9) M-1.s-1) between PCPS-bound DEGR.Xa and PCPS-bound Va. The formation of a separate Va.PCPS complex is therefore required for prothrombinase assembly to proceed. Reaction rates obtained when prothrombinase assembly was initiated by reacting DEGR.Xa with the preformed Va.PCPS complex or by reacting PCPS with a mixture of DEGR.Xa and Va were lower. Under these initiation conditions, reaction rates were inhibited by increasing concentrations of factor Va. These data indicate that the formation of the DEGR.Xa.PCPS binary complex is also a prerequisite for prothrombinase assembly. The second-order rate constant for the reaction between factor Xa and PCPS (k = 2.9 x 10(7) M-1.s-1) and the first-order rates of dissociation of the Va.PCPS complex (koff = 0.17 s-1) and the Xa.PCPS complex (koff = 3.3 s-1) were independently determined by stopped-flow light scattering. Collectively, the data are consistent with the interpretation that prothrombinase assembly proceeds via the initial formation of separate Xa.PCPS and Va. PCPS binary complexes at near diffusion-limited rates on the same vesicle. The lipid-bound constituents then react rapidly via rearrangements on the vesicle surface to form prothrombinase.