Question

Biochemistry: Enzyme Units

Can relative activity be added together? For example, if fraction A has 0.2 U/mL and fraction B has 0.4 U/mL, will combining the two fractions produce a solution with a relative activity of 0.6 U/mL?

Answer 1

Ans. No.

Specific activity of enzyme is defined as “enzyme activity per unit mass (in mg) of protein”.

Fraction A: specific activity = 0.2 U / mL   = 0.2 U enzyme activity / mg protein

Fraction B: specific activity = 0.2 U / mL   = 0.4 U enzyme activity / mg protein

# In the mixture –

Sum of enzyme activity = 0.2 U + 0.4 U = 0.6 U

            Sum of mass of protein = 1.0 mg + 1.0 mg = 2.0 mg

Now, specific activity = Total enzyme activity / Mass of protein

                                    = 0.6 U / 2.0 mg

                                    = 0.3 U/ mg

Therefore, the specific activity of mixture of two fractions is the average, but NOT the sum, of the two fractions.

Note: It’s assumed that relative specific in this context refers to “specific activity”.