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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]

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.^[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.

Factor Xa is the activated serine protease derived from Factor X by limited proteolysis. It catalyses cleavage of prothrombin to generate thrombin, an important activation step in coagulation. Efficient conversion is organized by factor Va, calcium and phospholipids.^[ref]

Purified-component experiments varied the phospholipid surface and the presence of factor Va, showing that apparent kinetic parameters depend on how the components assemble. The same amount of factor Xa can therefore produce different reaction rates in different surroundings.^[ref]

For an invented catalytic accounting example, 1 nmol of active factor Xa generates 100 nmol of activated product during an interval. That is 100 product molecules per initial enzyme molecule, demonstrating catalytic reuse rather than one-to-one consumption. The value is not a measured turnover number: a true rate constant requires a specified time, substrate availability and activation pathway.

J. L. van Rijn is the first author of a 1984 study of how membrane composition and an accessory protein affect activation of prothrombin. The investigators varied phospholipid concentration and composition, with and without factor Va, and measured the action of factor Xa on bovine prothrombin. This tested an enzyme in a membrane-associated reaction system rather than assuming that its behavior in solution determines the whole process.^[ref]

For vesicles containing 20 mole percent phosphatidylglycerol and 80 mole percent phosphatidylcholine, the reported apparent Michaelis constant for prothrombin was 0.04 micromolar with factor Va, compared with 2.2 micromolar without it. The ratio is $2.2/0.04=55$. That comparison shows the scale of the change in an apparent kinetic parameter; it is not a claim that the reaction rate increases 55-fold at every substrate concentration. The experiment illustrates how a surface and an accessory protein can change the effective behavior of a biochemical reaction.^[ref]

Phosphatidylglycerol is a glycerophospholipid whose usual membrane form has two fatty acid chains attached by ester linkages to a glycerol backbone, with a phosphate linking that backbone to a second glycerol. Its name describes a family: the fatty acid chains can vary. In the deprotonated form, the phosphate-containing head carries a net negative electric charge.^[ref]

The two chains and the charged head give phosphatidylglycerol different interactions with water and membrane-bound proteins. A useful controlled example comes from enzyme kinetics: J. L. van Rijn and colleagues activated bovine prothrombin using factor Xa on liposomes containing 20% phosphatidylglycerol and 80% phosphatidylcholine by mole fraction. Adding factor Va changed the apparent Michaelis constant from $2.2\mu \mathrm{M}$ to $0.04\mu \mathrm{M}$. This compares the same stated membrane composition with and without a protein cofactor; it does not imply that adding phosphatidylglycerol alone universally multiplies the reaction rate by 55.^[ref]

Prothrombin is the zymogen, or inactive precursor, from which the protease thrombin is generated during coagulation. Controlled proteolysis converts a circulating precursor into an active enzyme. factor Xa catalyses activation, with factor Va and a phospholipid surface greatly affecting the reaction's efficiency.^[ref]

A purified bovine system varied both vesicle composition and the presence of factor Va. With one defined mixture, the apparent Michaelis constant for prothrombin was $2.2\mu \mathrm{M}$ without factor Va and $0.04\mu \mathrm{M}$ with it, a 55-fold difference. The result connects the enzyme reaction to assembly on a phospholipid surface, rather than treating the surrounding lipids as inert solvent.^[ref]

An apparent Michaelis constant in this assembled system summarizes several interacting steps; it need not equal a single molecular binding constant. In an activation assay, detecting the newly generated thrombin's activity also differs from merely measuring how much prothrombin remains.