1. Enzyme Units

Enzyme Activity 1 — Enzyme Units

Calculate the number of (international) enzyme units in a sample from the amount of substrate converted to product per minute per ml, measured in an aliquot of the sample.
« 1 enzyme unit = the enzyme activity that catalyses the conversion of
1 µmol substrate into product in 1 minute »

Enzyme units:    
You should aim to do these calculations in your head or on a scrap of paper (i.e. without a calculator). The following logic may be helpful:
  • Enzyme units are expressed as µmol substrate converted per min.
  • If the question gives enzyme activity in nmol per min, divide by 1000 to convert to µmol.
  • Then multiply by the volume to get the total number of units.
  • To summarize, the total number of enzyme units is: volume x substrate conversion rate/(1000 if nmol, or 1 if µmol)
Always do a ‘sanity check’ on your answer. Is it the right order of magnitude? E.g. with volumes of a couple of ml, for a rate of hundreds of nmol per min the final answer is likely to be in the range 0.1–0.5 enzyme units. If the rate is x µmol per min, then the answer will be of the order of magnitude of x.