Question

1. An enzyme has a molecular weight of 25,000. If V_max= 80umol/min and you use 500uL of a 1mg/mL solution of enzyme, what is the turnover number?

2. In an enzyme activity assay, a 100uL assay sample containing 3.2 x 10^-5 M enzyme produced an absorbance change per minute of 0.42. The molecular weight of the enzyme is 17,000 g/mol and the extinction coefficient is 6220 M^-1 cm^-1.

What is the activity of the enzyme?

What is the specific activity of the enzyme?

Answer 1

(1) Turnover number-

The units of sp. act. are, µmoles (unit time)-1 (mg enzyme)-1. Here µmoles s-1 mg-1 is used, and the value is 31.3 µmole s- 1 mg-1. The turnover number, kcat, is effectively the activity in terms of (µmole active site)-1, instead of (mg enzyme)-1 as in sp. act., and its units are, µmoles s-1 (µmole active site)-1 = s-1.

(2) Activity of the enzyme-

An enzymatic reaction is the conversion of one molecule into another; a chemical reaction catalyzed at the reactive sites on the enzyme. Considering the complex nature of the enzyme itself, it is not unreasonable to expect that many parameters will affect the rate of this catalytic activity.

Enzyme activity = moles of substrate converted per unit time = rate × reaction volume. Enzyme activity is a measure of the quantity of activeenzyme present and is thus dependent on conditions

Specific activity of the enzyme-

specific activity is defined in terms of enzyme units per mg enzyme protein. An enzymeunit is the amount of substrate converted to product per unit time underspecific reaction conditions for pH and temperature.

Specific enzyme activity = moles of substrate converted per unit time = rate × reaction volume. Enzyme activity is a measure of the quantity of activeenzyme present and is thus dependent on conditions