Activity is the rate at which a sample of protein (more specifically,
an enzyme) catalyzes a reaction. It is a measure of the total
catalytic potential of the sample. It has units of (moles)/(time). By
convention, activity is typically reported in units of ?Units?
(confusing terminology). One ?Unit? is defined as the amount of
protein that will catalyze one micromole of substrate to product in
one minute at 25 degrees C and at optimal pH.
Specific activity is defined as the activity divided by the mass of
the protein sample. It is a measure of how much of the protein in the
sample consists of the enzyme of interest. It has units of
moles/time/(mass protein), or, in terms of ?Units?, Units/(mass
protein). One typically monitors the effectiveness of a protein
purification process by monitoring the specific activity. As the
sample becomes increasingly enriched in the enzyme of interest, the
specific activity of the sample increases.
A third, related, measure is ?Molecular Activity?, which is defined as
the rate at which a unit quantity of a pure sample of the enzyme of
interest catalyzes a reaction. It has units of moles/time/(mole of
pure enzyme), or, in terms of ?Units?, Units/(mole pure enzyme).
For a pure sample of an enzyme, the Specific Activity is equal to the
Molecular Activity times the molecular weight of the enzyme. A
mixture of proteins will has a specific activity less than this. |
