(DEC 2001)
38. Which statement is CORRECT regarding specific activity of enzyme?
(1) Depends on concentration of substrate and amount of free enzyme
(2) The number of enzyme units per ml divided by the concentration of protein in mg/ml
(3) Affinity for one particular substrate
(4) Activity against more than one substrate

The correct answer is (2) The number of enzyme units per ml divided by the concentration of protein in mg/ml.

Introduction

Specific activity is a key parameter in enzyme biochemistry representing the purity and catalytic effectiveness of an enzyme preparation. It quantifies the number of enzymatic units per milligram of protein, providing a normalized measure of enzyme activity relative to protein concentration. This article defines specific activity, explains how it is calculated, and clarifies misconceptions about its dependence on substrate concentration or specificity.

What is Specific Activity?

• Definition:
  The specific activity of an enzyme is the number of enzyme units (units of catalytic activity) per milliliter of enzyme solution, divided by the amount of protein in milligrams per milliliter (mg/ml). It shows how much enzyme activity exists per unit protein mass.

Specific Activity=Enzyme Units/mlProtein mg/ml=units/mgSpecific Activity=Protein mg/mlEnzyme Units/ml=units/mg

• Units: usually expressed as units per mg of protein (U/mg).

Why is Specific Activity Important?

• Measure of Purity:
  As enzyme preparations become purer, specific activity increases because less total protein corresponds to the same or more enzymatic activity.

• Comparison Between Samples:
  Allows comparison of enzyme efficiency across batches or treatments, regardless of total protein concentration.

What Specific Activity Does NOT Depend On

• Substrate Concentration:
  Specific activity is intrinsic to the enzyme prep and reflects concentration-normalized catalytic activity; substrate concentration influences raw enzyme activity but is accounted for in units.

• Enzyme Affinity or Specificity for Multiple Substrates:
  Specific activity is not an affinity measure nor does it describe substrate spectrum.

Incorrect Options Clarified

• (1) Depends on concentration of substrate and amount of free enzyme: Incorrect, as specific activity normalizes activity to protein content, independent of substrate concentration or free enzyme amount.

• (3) Affinity for one particular substrate: Incorrect, affinity is measured by Km or binding constants, not specific activity.

• (4) Activity against more than one substrate: Incorrect, specific activity is measured for one defined substrate in a given assay.

Calculation Example

If 1 ml enzyme solution has 100 enzyme units and protein concentration is 2 mg/ml, then:

Specific Activity=100 units/ml2 mg/ml=50 U/mgSpecific Activity=2mg/ml100units/ml=50U/mg

Summary Table

Conclusion

Specific activity, defined as the number of enzyme units per ml divided by protein concentration in mg/ml, is a fundamental measure of enzyme purity and activity. It normalizes enzymatic activity to protein content, ensuring consistent comparison across samples and experimental conditions. It is distinct from substrate affinity or substrate specificity.

This detailed explanation enables students and researchers to accurately calculate and interpret specific activity, driving better understanding of enzymatic preparations and experimental results.