= Phosphatidylglycerol
<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>.<Phosphatidylglycerol in ChEBI>[CHEBI:17517, structure and definition; CHEBI:60523, conjugate-base 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.<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>[Biochemistry 23, pp. 4557–4564; abstract, 20:80 vesicle mixture and factor Va comparison]{ref}
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