= Factor Xa
<Factor Xa> is the activated <serine protease> derived from <Factor X> by limited <proteolysis>. It catalyses cleavage of <prothrombin> to generate <thrombin>, an important activation step in <coagulation>. Efficient conversion is organized by <factor Va>, <calcium> and <phospholipids>.<Coagulation factor Xa EC 3.4.21.6 in IUBMB Enzyme Nomenclature>[Reaction and comments: precursor, activation and prothrombin conversion]{ref}
Purified-component experiments varied the <phospholipid> surface and the presence of <factor Va>, showing that apparent kinetic parameters depend on how the components assemble. The same amount of <factor Xa> can therefore produce different <reaction rates> in different surroundings.<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: vesicle composition and cofactor comparisons]{ref}
For an invented catalytic accounting example, 1 nmol of active <factor Xa> generates 100 nmol of activated product during an interval. That is 100 product molecules per initial enzyme molecule, demonstrating catalytic reuse rather than one-to-one consumption. The value is not a measured <turnover number>: a true rate constant requires a specified time, substrate availability and activation pathway.
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