= Factor Va
<Factor Va> is the activated form of <Factor V> that serves as a <protein> <cofactor (biochemistry)> for <factor Xa> during <prothrombin> activation. In the membrane-associated complex, <factor Xa> supplies the proteolytic catalyst while <factor Va> helps organize efficient conversion to <thrombin>.<Crystal Structure of Prothrombin Reveals Conformational Flexibility and Mechanism of Activation by Nicola Pozzi and others>[Introduction: prothrombinase components and substrate interactions]{ref}
The cofactor's effect can be measured by reconstituting the reaction from purified components. In a bovine study with one defined <phospholipid> mixture, adding <factor Va> reduced the apparent <Michaelis constant> for <prothrombin> from $2.2\,\mu\mathrm M$ to $0.04\,\mu\mathrm M$. Other vesicle compositions gave different relationships between binding and apparent kinetics.<Kinetic studies of prothrombin activation: effect of factor Va and phospholipids on formation of the enzyme-substrate complex by J. L. van Rijn and others>[Abstract: vesicles with 20 mole percent phosphatidylglycerol and 80 mole percent phosphatidylcholine]{ref}
The ratio $2.2/0.04=55$ describes the change in that apparent kinetic parameter. It does not imply that every reaction rate rises 55-fold: the observed rate also depends on <substrate> abundance and the limiting rate. The example shows how a noncatalytic <protein> can strongly regulate an <enzyme> reaction by participating in its molecular assembly.
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