= Prothrombinase complex
The <prothrombinase complex> is a membrane-associated assembly that converts <prothrombin> into <thrombin>. Its catalytic <enzyme> is <factor Xa>, assisted by the protein <cofactor (biochemistry)> <factor Va>, <calcium ions>, and a <phospholipid> surface. The <protease> and its assembled reaction system are distinct: <factor Va> helps organize efficient <prothrombin> conversion without replacing the catalytic role of <factor Xa>.<Coagulation factor Xa EC 3.4.21.6 in IUBMB Enzyme Nomenclature>[Comments; factor Xa, factor Va, calcium and phospholipids in prothrombin conversion]{ref}
Experiments in 1975 combined purified coagulation components and used <chromatography> to separate the material formed. Tests with specific <antibodies> helped identify the factors in the collected fractions, and kinetic measurements established that the assembled preparation converted purified <prothrombin> into <thrombin>. These supplied different kinds of evidence: association of components, and catalytic activity.<Factor-V activation by thrombin and its role in prothrombin conversion by J. C. Giddings and A. L. Bloom>[British Journal of Haematology 29, pp. 349–364; abstract, Sephadex fractionation, antibody analysis and purified-factor kinetics]{ref}
Pretreating <Factor V> with trace amounts of <thrombin> increased the subsequent rate and yield of <thrombin> production. This illustrates <positive feedback>: a product of the reaction helps activate a component that promotes further product formation. A single observed rate increase would not establish complex composition; the fractionation and identification experiments addressed that separate question.<Factor-V activation by thrombin and its role in prothrombin conversion by J. C. Giddings and A. L. Bloom>[Abstract; thrombin pretreatment of factor V and subsequent thrombin generation]{ref}
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