Factor Va is the activated form of Factor V that serves as a protein cofactor 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.[ref]
Crystal Structure of Prothrombin Reveals Conformational Flexibility and Mechanism of Activation by Nicola Pozzi and others. Introduction: prothrombinase components and substrate interactions
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 to . Other vesicle compositions gave different relationships between binding and apparent kinetics.[ref]
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
The ratio 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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