47. The concentration of a purified enzyme is 10 mg/mL. Ten microlitres of the enzyme solution in a total reaction volume of 1 mL catalyses the formation of 20 nanomoles of product in one minute under optimum conditions. The specific activity of the enzyme is unit/mg.

How to Calculate the Specific Activity of an Enzyme?

Correct Answer

0.20 U/mg

Introduction

One of the most important parameters used to evaluate enzyme purity and catalytic efficiency is specific activity. Specific activity represents the amount of enzymatic activity present per milligram of total protein and is widely used during enzyme purification to determine how much of the protein sample consists of the desired enzyme. As purification proceeds, contaminating proteins are removed while catalytic activity is retained, resulting in an increase in specific activity.

In enzymology, one enzyme unit (U) is defined as the amount of enzyme that catalyzes the conversion of 1 micromole (µmol) of substrate into product per minute under optimal assay conditions. Therefore, calculating specific activity requires determining both the enzyme activity (in units) and the amount of enzyme protein used in the assay.

Understanding the Concept Behind the Question

Specific activity is defined as:

Specific Activity = Enzyme Activity (Units) / Amount of Enzyme Protein (mg)

To solve this problem, two quantities must first be calculated:

The amount of enzyme protein used in the reaction.
The enzyme activity expressed in standard enzyme units (U).

Once these values are known, the specific activity is obtained by simple division.

Step 1. Calculate the Amount of Enzyme Used

The enzyme concentration is given as:

10 mg/mL

The volume of enzyme added is:

10 µL

Convert microlitres into millilitres:

10 µL = 0.01 mL

Now calculate the amount of enzyme:

Protein = Concentration × Volume

Protein = 10 mg/mL × 0.01 mL

Protein = 0.10 mg

Thus,

Amount of enzyme used = 0.10 mg

Step 2. Calculate the Enzyme Activity

The enzyme produces:

20 nanomoles of product per minute

One enzyme unit is defined as:

1 Unit = 1 µmol product formed per minute

Convert nanomoles into micromoles:

1 µmol = 1000 nmol

Therefore,

20 nmol = 20 / 1000 µmol

= 0.020 µmol

Hence,

Enzyme activity = 0.020 U

Step 3. Calculate the Specific Activity

The formula is:

Specific Activity = Activity / Protein

Substitute the calculated values:

Specific Activity = 0.020 U / 0.10 mg

= 0.20 U/mg

Final Calculation

Specific Activity = 0.20 U/mg

Why Is Specific Activity Important?

Specific activity is one of the most useful indicators of enzyme purity because it measures the catalytic activity associated with each milligram of protein. During enzyme purification, contaminating proteins are gradually removed while the enzyme remains active. As a result, the amount of catalytic activity per milligram of protein increases. A purified enzyme therefore exhibits a much higher specific activity than a crude cell extract.

Specific activity also allows researchers to compare enzyme preparations obtained from different purification procedures and determine whether purification has been successful. Since it is independent of sample volume, it serves as a standardized measure of enzyme quality.

Biological Importance

Enzymes function as biological catalysts that accelerate biochemical reactions without being consumed. Measuring enzyme activity provides insight into catalytic efficiency, while measuring specific activity helps evaluate the purity of enzyme preparations. In research laboratories, biotechnology industries, pharmaceutical production, and clinical diagnostics, specific activity is routinely used to monitor enzyme purification and assess enzyme quality before further biochemical characterization.

Formula Summary

Enzyme Activity

1 Unit (U) = 1 µmol product formed per minute

Protein Used

Protein (mg) = Concentration (mg/mL) × Volume (mL)

Specific Activity

Specific Activity = Enzyme Activity (U) / Protein (mg)

Unit Conversions Used

Quantity	Conversion
1 mL	1000 µL
10 µL	0.01 mL
1 µmol	1000 nmol
20 nmol	0.020 µmol

High-Yield Exam Points

Specific Activity = Units/mg protein
1 Enzyme Unit = 1 µmol product/minute
Higher specific activity indicates greater enzyme purity.
Specific activity generally increases during enzyme purification.
Always convert:
- µL → mL
- nmol → µmol
Reaction volume does not affect the calculation unless concentration is specifically required.

Frequently Asked Questions

What is meant by one enzyme unit?

One enzyme unit (U) is the amount of enzyme that converts 1 micromole of substrate into product per minute under defined experimental conditions.

Why is reaction volume not used in this calculation?

The reaction volume is irrelevant because the question provides the total amount of product formed per minute, not its concentration. Only enzyme activity and the amount of enzyme protein are needed to calculate specific activity.

Why does specific activity increase during purification?

As contaminating proteins are removed, the proportion of active enzyme increases. Consequently, the catalytic activity per milligram of total protein becomes higher.

Key Takeaways

Specific activity is a measure of enzyme purity expressed as enzyme units per milligram of protein. In this problem, 10 µL of a 10 mg/mL enzyme solution contains 0.10 mg of protein. The enzyme forms 20 nmol of product per minute, equivalent to 0.020 enzyme units. Dividing enzyme activity by protein amount gives:

Specific Activity = 0.020 / 0.10 = 0.20 U/mg

This calculation demonstrates the importance of unit conversion and reinforces one of the most fundamental numerical concepts in enzyme biochemistry.

Final Answer

Specific Activity = 0.20 U/mg

Explanation

The enzyme concentration is 10 mg/mL, and 10 µL (0.01 mL) of enzyme solution is used. Therefore, the amount of enzyme protein present is:

Protein = 10 × 0.01 = 0.10 mg

The enzyme produces 20 nanomoles of product per minute, which is equivalent to:

20 nmol = 0.020 µmol/min = 0.020 U

Using the formula:

Specific Activity = Enzyme Activity / Protein

= 0.020 U / 0.10 mg