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3. Turnover Number

Enzyme Activity 3 — Turnover Number

 
For a pure sample of enzyme of known subunit molecular weight you can use the specific activity to calculate a fundamental constant for the enzyme — the turnover number — the number of molecules of substrate converted to product per second by one active site.

Turnover Number:  per sec   
 
These calculations are not really suitable for doing in your head, but working them out on paper is recommended to ensure you understand the logic. This is not really difficult, it’s just that it involves division by large numbers:
  • The turnover number is expressed as molecules converted per enzyme molecule active site (equivalent to moles per mole) per sec, whereas the specific activity of a pure enzyme is in units (µmoles per min) per mg enzyme. The key step is therefore to convert the mg into moles.
  • First, remember that 1 mole contains the molecular weight (MW) in grams, hence 1 g = 1/MW moles, and 1 mg = 1/(1000 x MW) moles
  • Enzyme units are expressed in µmoles, so we need to divide the specific activity by a million to convert to moles.
  • Now if we divide the units per mole by the number of moles we get the turnover number per min.
  • Dividing this by 60 gives the turnover number per sec.
  • In summary, the turnover number is calculated as:
    (Specific Activity/1000000) / (1/(1000 x MW)) / 60, or
    (Specific Activity x MW) / (1000 x 60).