= Prothrombin
<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.<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>[1984, abstract: prothrombin activation with and without factor Va and phospholipid vesicles]{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.<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 containing 20 mole percent phosphatidylglycerol and 80 mole percent phosphatidylcholine]{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.
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