factor Va + factor Xa -> Va:Xa complex (prothrombinase)

Factors Va and Xa associate on a membrane surface to form a complex in which the activity of factor Xa on prothrombin is greatly increased (Mann et al. 1988). The presence of negatively charged phospholipid in the membrane greatly facilitates this process, a feature that may contribute to its localization, as such phospholipids are normally on the cytosolic face of the plasma membrane (Devaux 1992), but could be exposed to the extracellular space following platelet activation or mechanical injury to endothelial cells.

Va因子与Xa因子在细胞膜表面结合，形成一种复合物，显著增强了Xa因子对凝血酶原的活性（Mann等人，1988年）。膜上存在带负电荷的磷脂质大大促进了这一过程，这一特性可能有助于其定位，因为此类磷脂质通常位于质膜的内质面（Devaux，1992年），但在血小板激活或内皮细胞机械损伤后，也可能暴露于细胞外空间。