The Role of Phospholipids and Factor Va in the Mechanism of Prothrombin Activation

Abstract

The kinetic parameters of the conversion of prothrombin into thrombin by activated clotting factor X (factor Xa) have been determined in the absence and presence of Ca2+, phospholipid (phosphatidyl serine/phosphatidylcholine vesicles) and activated blood clotting factor V (factor Va). In free solution the Km for prothrombin is 298 μM which is well above its plasma concentration of 4μM. Under these conditions the Vmax of thrombin formation is 1.25 Moles min-1 Mole Xa -1. When phospholipid is present the km for prothrombin drops to 0.1μM while the Vmax is only slightly affected (3 Moles min-1 Mo Le Xa -1). For the complete prothrombin activating complex consisting of factor Xa, factor Va, Ca2+ and phospholipids the kinetic constants greatly favour thrombin formation. A for prothrombin of 0.26μM and a Vmax of 2130 Moles min-1 Mole xa -1 are measured under these conditions. These results help to elucidate the role of phospholipid and factor Va in prothrombin activation. The earlier observed rate enhancements caused by phospholipid and factor Va are explained as effects on the Km for prothrombin and the Vmax of thrombin formation, respectively. The changes of the kinetic parameters for prothrombinase complexes of various composition will be considered with respect to the function of the accessory components in the mechanism of prothrombin activation. Implications of these data for in vivo blood coagulation will be discussed.

Rheological study on coagulation of blood with special reference to the triggering mechanism of venous thrombus formation

“…7, it is clear that the propagation reaction on platelet membranes is much faster than that on RBC membranes, and almost independent of the individuals examined. It is known that platelet surface plays an important role in the promotion and regulation of thrombin generation [43,44]. The difference in the propagation reaction on RBC membranes may be due to the difference in the local structure or properties of the RBC surface that offers the propagation reaction.…”

On the molecular mechanism of flippase- and scramblase-mediated phospholipid transport

“…Conversely, cell-surface exposure of PtdSer may trigger recognition of apoptotic cells by macrophages or activation of the blood coagulation cascade [6,7].…”

Selective Inhibition of the Prothrombinase Complex: Factor Va Alters Macromolecular Recognition of a Tick Anticoagulant Peptide Mutant by Factor Xa,

“…This enzyme complex converts prothrombin to the clot forming protease thrombin by two successive cleavages (Jackson & Nemerson, 1980;Mann et al, 1988Mann et al, , 1990. Although factor Xa, the catalytic component of prothrombinase, is also capable of activating prothrombin in solution, incorporation into prothrombinase amplifies its catalytic activity by a factor of ∼10 5 Nesheim et al, 1979;Rosing et al, 1980). The full catalytic activity of prothrombinase may result from a cumulative effect of the interaction of all accessory components with factor Xa.…”