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.^[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.