Enzyme activity is a measure of the quantity of active enzyme present and is thus dependent on various physical conditions, which should be specified.
It is calculated using the following formula:
where
: = Enzyme activity
: = Moles of substrate converted per unit time
: = Rate of the reaction
: = Reaction volume
The SI unit is the katal, 1 katal = 1 mol s (mole per second), but this is an excessively large unit. A more practical and commonly used value is enzyme unit (U) = 1 μmol min (micromole per minute). 1 U corresponds to 16.67 nanokatals.
Enzyme activity as given in katal generally refers to that of the assumed natural target substrate of the enzyme. Enzyme activity can also be given as that of certain standardized substrates, such as gelatin, then measured in gelatin digesting units (GDU), or milk proteins, then measured in milk clotting units (MCU). The units GDU and MCU are based on how fast one gram of the enzyme will digest gelatin or milk proteins, respectively. 1 GDU approximately equals 1.5 MCU.
An increased amount of substrate will increase the rate of reaction with enzymes, however once past a certain point, the rate of reaction will level out because the amount of active sites available has stayed constant.