Q.15 Michaelis-Menten constant (Km) is related to the
- Enzyme’s affinity for the substrate
- Enzyme’s activity for the substrate
- Enzyme’s specificity for the substrate
- Enzyme’s selectivity for the substrate
The Michaelis-Menten constant (Km) measures the enzyme’s affinity for its substrate. The correct answer is option 1.
Introduction
The Michaelis-Menten constant (Km) is related to the enzyme’s affinity for the substrate, defined as the substrate concentration yielding half-maximal velocity (Vmax/2). This key parameter from enzyme kinetics is essential for life sciences students studying biochemistry and molecular biology exams like NEET/GATE/CSIR NET.
Option Analysis
Each option tests distinct aspects of enzyme kinetics parameters.
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Enzyme’s affinity for the substrate
Correct: Km inversely reflects binding strength—low Km means high affinity (tight substrate binding at low concentrations); high Km indicates low affinity. -
Enzyme’s activity for the substrate
Incorrect: Activity relates to Vmax (turnover number, kcat), not Km, which measures binding without directly indicating catalytic rate. -
Enzyme’s specificity for the substrate
Wrong: Specificity constant (kcat/Km) combines affinity and activity; Km alone doesn’t assess substrate discrimination among alternatives. -
Enzyme’s selectivity for the substrate
Invalid: Selectivity involves competitive inhibition or multiple substrates; Km measures intrinsic binding for a single substrate type.
Michaelis-Menten Kinetics
The equation v=Vmax[S]Km+[S] shows Km as the [S] where v = Vmax/2. Derived from Km=k−1+kcatk1, low dissociation (k₋₁) lowers Km, enhancing affinity.
Biological Applications
Km guides physiological efficiency: glucose kinase (high Km) responds to elevated glucose; hexokinase (low Km) works at basal levels. Drug design targets Km to create competitive inhibitors mimicking substrates.
Exam Tip
Remember: Low Km = High affinity (enzyme grabs substrate easily). Common trick: distinguish Km (affinity) from Vmax (activity).
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