Thorelli E
Department of Clinical Chemistry, Lund University, University Hospital Malmö, Sweden.
Scand J Clin Lab Invest Suppl. 1999;229:19-26. doi: 10.1080/00365519950185913.
Coagulation factor V is composed of domains A1-A2-B-A3-C1-C2 and is activated by thrombin through proteolytic cleavage at Arg 709, Arg 1018 and Arg 1545. Upon thrombin activation, the B-domain is released and the active factor Va is formed by the heavy (A1-A2) and light chains (A3-C1-C2). Factor Va functions as an essential cofactor to factor Xa in the conversion of prothrombin to thrombin during coagulation. Recently it was shown that coagulation factor V, apart from being a precursor form to the procoagulant factor Va, also has anticoagulant properties, as it functions as a cofactor to activated protein C (APC). APC is a member of the anticoagulant pathway and downregulates the coagulation process through proteolytic inactivation of factors VIII/VIIIa and factors V/Va. In a factor VIIIa degradation assay, the APC-mediated inactivation of factor VIIIa is potentiated by the synergistic cofactors protein S and factor V. Protein S alone has little cofactor activity, whereas in the presence of factor V it is dramatically enhanced. This study provides insights into the molecular mechanisms that regulate the anticoagulant activity of factor V. Thrombin cleavage of factor V occurs in a sequential order. The thrombin cleavage site Arg 1545 is kinetically less favored than the other two sites, and cleavage at this site is the last to occur during thrombin activation of factor V As a consequence of this, different activation intermediates exist that express different levels of procoagulant activity. The anticoagulant activities of these intermediates have now been studied. It was found that factor V could be cleaved by thrombin at both Arg 709 and Arg 1018 and still work fully as a cofactor to APC, whereas cleavage at Arg 1545 completely abolished the anticoagulant activity of factor V. This suggests that the APC cofactor function of factor V depends on the B-domain remaining attached to the A3 domain. This study further shows that APC converts coagulation factor V into a member of the anticoagulant pathway by cleaving factor V in the A2 domain at Arg 506. By cleavage of factor V, APC not only produces an anticoagulant cofactor, but at the same time eliminates the pool of procoagulant factor V, since APC cleaved factor V will have no future as a cofactor in the coagulation. The unique way by which APC and thrombin, through proteolytic cleavage, can convert factor V into either an anticoagulant or a procoagulant adds to the intriguing mechanisms that balance the procoagulant and anticoagulant forces.
凝血因子V由A1 - A2 - B - A3 - C1 - C2结构域组成，通过凝血酶在精氨酸709、精氨酸1018和精氨酸1545处的蛋白水解切割而被激活。凝血酶激活后，B结构域被释放，活性因子Va由重链（A1 - A2）和轻链（A3 - C1 - C2）形成。在凝血过程中，因子Va作为因子Xa将凝血酶原转化为凝血酶的必需辅因子发挥作用。最近研究表明，凝血因子V除了作为促凝因子Va的前体形式外，还具有抗凝特性，因为它作为活化蛋白C（APC）的辅因子发挥作用。APC是抗凝途径的成员，通过对因子VIII / VIIIa和因子V / Va的蛋白水解失活来下调凝血过程。在因子VIIIa降解试验中，蛋白S和因子V作为协同辅因子可增强APC介导的因子VIIIa失活。单独的蛋白S几乎没有辅因子活性，而在因子V存在时其活性显著增强。本研究深入探讨了调节因子V抗凝活性的分子机制。凝血酶对因子V的切割按顺序发生。凝血酶切割位点精氨酸1545在动力学上比其他两个位点更不易被切割，在因子V的凝血酶激活过程中该位点的切割是最后发生的。因此，存在表达不同水平促凝活性的不同激活中间体。现在已经研究了这些中间体的抗凝活性。发现因子V可被凝血酶在精氨酸709和精氨酸1018处切割，并且仍可作为APC的完全正常工作的辅因子，而在精氨酸1545处的切割则完全消除了因子V的抗凝活性。这表明因子V的APC辅因子功能取决于B结构域与A3结构域保持连接。本研究进一步表明，APC通过在精氨酸506处切割因子V的A2结构域将凝血因子V转化为抗凝途径的成员。通过切割因子V，APC不仅产生一种抗凝辅因子，同时消除了促凝因子V的储备，因为APC切割后的因子V在凝血过程中不能再作为辅因子发挥作用。APC和凝血酶通过蛋白水解切割将因子V转化为抗凝剂或促凝剂的独特方式，进一步丰富了平衡促凝和抗凝力量的有趣机制。
