= Factor X
{c}
<Factor X> is the <zymogen> of <factor Xa>, a <serine protease> involved in <coagulation>. <Limited proteolysis> activates <Factor X>; the resulting <factor Xa> cleaves <prothrombin> to form <thrombin>. Efficient membrane-associated activation uses the <prothrombinase complex>, containing <factor Xa>, <factor Va>, <calcium ions> and <phospholipids>.<Coagulation factor Xa EC 3.4.21.6 in IUBMB Enzyme Nomenclature>[Accepted name, reaction and comments: precursor activation and prothrombin conversion]{ref}
The human <protein> contains a heavy chain and a light chain joined by <disulfide bonds>. This chain structure and activation are different distinctions: having two linked chains does not by itself mean that the <zymogen> has become <factor Xa>.<Coagulation factor Xa EC 3.4.21.6 in IUBMB Enzyme Nomenclature>[Comments: precursor processing and limited proteolysis]{ref}
For example, a preparation can contain abundant <Factor X> yet require an activating cleavage before efficiently supporting <thrombin> production. Conversely, detecting <factor Xa> activity reports an activated <enzyme>, not merely the presence of its precursor. Purified-component experiments establish the downstream reaction independently of a whole <blood> clot, allowing the catalytic role of <factor Xa> to be distinguished from the assisting role of <factor Va>.<Factor-V activation by thrombin and its role in prothrombin conversion by J. C. Giddings and A. L. Bloom>[Abstract: prothrombin conversion by the reconstituted complex]{ref}
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