Standard enzyme unit (U, unit) or activity:
One unit (U) is the amount of enzyme activity which will catalyse the transformation of 1 micromole of the substrate per minute under standard conditions.
One katal (kat) is that catalytic activity which will raise the rate of reaction by one mole per second in a specified assay system.
Specific enzyme activity:
activity per unit protein amount (U/mg). It is a useful term to follow purification of enzymes
Catalytic constant or turnover number:
a maximum number of substrate molecules that an enzyme can convert to product per catalytic site per unit time: Its dimension is s-1. It shows how fast the enzyme is when its active centre is saturated with substrate.
Environmental parameters
Due to the fact that the enzymes are proteins, the parameters which are affecting the activity of the enzymes are:
- pH
- temperature
DETERMINATION OF KM AND Vmax - THE MEANING OF KM
It is the substrate concentration when the rate is half of the Vmax.
A typical example of the difference in the KM value: hexokinase - glucokinase (Figure 3).