= Factor V
{c}
<Factor V> is a <coagulation factor> whose activated form, <factor Va>, is a <protein> <cofactor (biochemistry)> in the <prothrombinase complex>. <Limited proteolysis>, including cleavage by <thrombin>, converts <Factor V> into the more active cofactor. In the assembled complex, <factor Xa> performs the cleavage of <prothrombin> to <thrombin>; <factor Va> helps the reaction occur efficiently on a <phospholipid> surface in the presence of <calcium ions>.<Factor-V activation by thrombin and its role in prothrombin conversion by J. C. Giddings and A. L. Bloom>[Abstract: thrombin pretreatment and reconstitution from purified components]{ref}
The distinction between precursor and activated cofactor matters experimentally. Investigators mixed purified <coagulation factors>, separated the products by gel <chromatography>, and identified components with specific <antibodies>. Pretreating <Factor V> with small amounts of <thrombin> increased the rate and yield of subsequent <thrombin> generation. A little product could therefore promote production of more product: a <positive feedback> mechanism within <coagulation>.<Factor-V activation by thrombin and its role in prothrombin conversion by J. C. Giddings and A. L. Bloom>[Abstract: column fractions, immunological analysis, and kinetic experiments]{ref}
A useful reaction sketch is “<Factor V> becomes <factor Va>; <factor Xa> plus <factor Va> helps convert <prothrombin> into <thrombin>”. The two activated factors have different jobs: adding a cofactor changes the performance of a catalytic assembly without making the cofactor itself the <protease>.
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