The Michaelis-Menten constant (Km) of an enzyme is defined as:

Correct option is A

The correct answer is: (A) The substrate concentration at which the reaction rate is half of Vmax

• The Michaelis-Menten constant (Km) is a key parameter in enzyme kinetics.

• It is defined as the substrate concentration at which the reaction velocity is half of the maximum velocity (Vmax).

• Km gives an idea of the affinity of the enzyme for its substrate:

  • Lower Km → higher affinity (enzyme reaches half-max rate at low substrate concentration)

  • Higher Km → lower affinity

• The Michaelis-Menten equation is:  $$V = \frac{V_{\text{max}} [S]}{K_m + [S]}$$

where:

The concept is valid under the steady-state assumption in enzyme kinetics.

Km has units of molar concentration (e.g., mM or µM).

It is specific to each enzyme-substrate pair and is affected by pH, temperature, and ionic strength.

• • V = reaction velocity

  • Vmax = maximum velocity

  • [S] = substrate concentration

  • Km = Michaelis constant

Additional Knowledge:

• Option B (Vmax): Represents the maximum rate of the enzyme-catalyzed reaction when the enzyme is fully saturated with substrate.

• Option C: Refers to enzyme concentration, not Km. Km is independent of enzyme concentration.

• Option D: Describes IC50, which is the concentration of an inhibitor that reduces enzyme activity by 50%—unrelated to Km.

• Enzyme kinetics is crucial in drug design, metabolic engineering, and diagnostics, as Km helps predict how enzymes will behave under different substrate levels.