Km of Enzyme-Catalysed Reaction: Substrate Concentration at Half Maximum Velocity Explained

✅ Correct Answer

Substrate concentration at which reaction rate is half of the maximum.

Km in enzyme kinetics is the substrate concentration at which the reaction rate reaches half of its maximum velocity (Vmax).

📋 Detailed Option Analysis

✅ Option 1: Substrate concentration at which reaction rate is half of the maximum

Correct. Km, or the Michaelis constant, is defined by the Michaelis-Menten equation as the [S] where velocity v = ½ Vmax, reflecting enzyme-substrate affinity.

❌ Option 2: Product concentration at which reaction rate is half of the maximum

Incorrect. Km measures substrate ([S]) binding, not product accumulation, which follows reverse kinetics in detailed models.

❌ Option 3: Substrate concentration at which reaction rate is double of the maximum

Incorrect. Reaction rate cannot exceed Vmax, the saturation limit; doubling Vmax is impossible under standard conditions.

❌ Option 4: Maximum kinetic constant

Incorrect. Km is not maximal; Vmax is the maximum rate, while Km indicates affinity (low Km = high affinity). Kinetic constants include kcat (turnover number), but Km specifically denotes half-Vmax [S].

🔬 Michaelis-Menten Kinetics Basics

The Michaelis-Menten model describes enzyme velocity:

v = Vmax [S] / (Km + [S])

At [S] = Km, v = ½ Vmax, as substitution yields Vmax Km / (Km + Km) = ½ Vmax.

  • Low Km signals high substrate affinity; enzymes saturate quickly at low [S].

🎯 Practical Significance

Km guides drug design and metabolic studies:

  • Hexokinase (low Km) thrives in low-glucose tissues
  • Glucokinase (high Km) functions in liver

Plotting velocity vs. [S] gives a hyperbolic curve; Lineweaver-Burk plots (1/v vs. 1/[S]) linearize data to find Km as x-intercept.

📊 Core Kinetic Parameters Comparison

Parameter Definition Biological Insight
Km [S] at ½ Vmax Enzyme-substrate affinity measure
Vmax Maximum rate Enzyme saturation limit
kcat/Km Specificity constant Catalytic efficiency

📚 Exam Preparation Tips

  • Remember: Km = [S] when v = ½ Vmax
  • Low Km = High affinity
  • Lineweaver-Burk plot: Km = -1/x-intercept