Question

A pure enzyme has a specific activity of 120 units/mg protein. (a) calculate the turnover number if MW=260,000. (b) Calculate the time required for one catalytic cycle
The answer for (a) is Turnover = 4.32x10^4 molecules S/(molecules E - min)
The answer for (b) is 2.315x10^-5 min per molecule of S converted to P

I'm confused over how the answer for (a) was reached. Can you please explain?

Answer 1

Turnover number is the number of molecules of substrate converted to product per unit time by one active site. It is expressed as molecules converted per enzyme molecule active site (equivalent to moles per mole) per min, whereas the specific activity of a pure enzyme is in units (µmoles per min) per mg enzyme. So, the key step is therefore to convert the mg into moles.

As we know, 1 mole contains the molecular weight (MW) in grams, hence 1 g = 1/MW moles, and 1 mg = 1/(1000 x MW) moles. and the Enzyme units are expressed in µmoles, so we need to divide the specific activity by a million to convert to moles.

So, the turnover number is calculated as

(Specific Activity/1000000) / (1/(1000 x MW))  
or (Specific Activity x MW) / 1000

120 X 260000/1000

=31200

=3.12 X 10⁴