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 without factor Va and 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]
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
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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