[17] Meizothrombin: Active intermediate formed during prothrombinase-catalyzed activation of prothrombin

This chapter focuses on the active intermediate formed during the prothrombinase-catalyzed activation of prothrombin. The activation of prothrombin to thrombin proceeds via the cleavage of two peptide bonds in the prothrombin molecule. These bonds are cleaved by the prothrombinase enzyme complex that is composed of the enzyme, factor Xa, and a cofactor, factor Va, assembled on a phospholipid surface in the presence of calcium ions. Further studies on the activation of human prothrombin indicate that the formation of meizothrombin as an intermediate is a consequence of the association of the cofactor, human factor Va, with the enzyme, human factor Xa, on the phospholipid surface. The absence of factor Va causes the activation to proceed via the prethrombin-2 intermediate. The observation that meizothrombin binds dansylarginine-N-(3-ethyl-l,5-pentanediyl)amide (DAPA) indicates that the intermediate has expressed an active site, and therefore could possibly have activity similar to thrombin. To determine the activity of meizothrombin, it first becomes necessary to obtain the intermediate in a stable form suitable for study.