A Plausible Mechanism for Prothrombin Activation by Factor X,, Factor V,,

Recent work from this laboratory has been concerned with elucidating the function of the non-thrombin-forming half or propiece of prothrombin in the zymogen activation process. These studies have demonstrated that Fragment 1, the NH2-terminal region of the propiece (Fragment 1.2), is responsible for the Ca2+-mediated binding of prothrombin to phospholipid vesicles (1) and that Fragment 2, the COOH-terminal region of the propiece, is required for acceleration of prothrombin activation by Factor V,i (2). Investigations of the kinetic consequences of the functioning of the two propiece regions (3) have led to the following proposal for the principal pathway of prothrombin conversion to thrombin.’