Complex Assembly
Prothrombinase assembly begins with the binding of Factor Xa and Factor Va to negatively charged phospholipids on plasma membranes. Activated Factor Xa and Factor Va bind to the plasma membranes of a variety of different cell types, including monocytes, platelets, and endothelial cells. Both Factor Xa and Va bind to the membrane independently of each other, but they both bind to mutually exclusive binding sites. Both Factor Xa and Factor Va associate with the membrane via their light chains, with Factor Xa binding via its Gla-domain in a calcium-dependent manner and Factor Va via its C2 and C1 domains. Once bound to the plasma membrane, Factor Xa and Factor Va rapidly associate in a 1:1 stoichiometric ratio to form the prothrombinase complex. Assembly of the prothrombinase complex is calcium dependent. When associated with the prothrombinase complex, the catalytic efficiency of Factor Xa is increased 300,000-fold compared to its efficiency alone. Factor Xa and Factor Va interact tightly with each other when associated on the plasma membrane. Further, membrane-bound Factor Va provides a strong catalytic advantage to the prothrombinase complex. Factor Va strengthens the affinity of Factor Xa for the membrane and also increases the kcat of Factor Xa for prothrombin. Factor Va also decreases the Km of the reaction by enhancing the binding of prothrombin to the prothrombinase complex.
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