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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]