Question 35:
The number of enzyme units per milligram of total protein is called:
Specific activity is the number of enzyme units per milligram of total protein, making option (B) correct for this biochemistry question.
Question Solution
Enzyme units measure catalytic activity (typically µmol substrate converted per minute), while total protein quantifies all proteins in a sample. Specific activity normalizes activity to protein mass (units/mg), indicating enzyme purity during purification—higher values signal purer enzyme.
Option Analysis
-
(A) Activity: Total enzyme units per volume (e.g., U/mL), not normalized to protein; measures raw output without purity info.
-
(B) Specific activity: Enzyme units per mg protein (U/mg); directly matches question, used to track purification progress.
-
(C) Kcat: Turnover number (s⁻¹); molecules converted per enzyme molecule per second, requires known enzyme concentration.
-
(D) Km: Michaelis constant (substrate concentration at half Vmax); measures enzyme-substrate affinity, not activity per protein mass.
The number of enzyme units per milligram of total protein is called specific activity—a key metric in enzyme kinetics for GATE Life Sciences prep. It reveals enzyme purity by normalizing activity to total protein content.
Specific Activity Explained
Defined as enzyme units (U/mg protein), where 1 U = 1 µmol substrate/min under standard conditions. Formula: Specific activity = (total activity, U) / (protein mass, mg). Example: 100 U in 2 mg protein = 50 U/mg.
Why It Matters
Rises during purification as contaminants drop; pure enzyme shows constant high value. Differs from total activity (not normalized) and constants like Kcat/Km.
| Term | Definition | Units |
|---|---|---|
| Activity | Raw enzyme output | U/mL |
| Specific Activity | Units per mg protein | U/mg |
| Kcat | Turnover per enzyme site | s⁻¹ |
| Km | Substrate affinity | mM |
Exam Tips
For GATE, remember: specific activity tracks purity. Practice: If activity halves but protein quarters, specific activity doubles.
