The **Michaelis Constant (Km) Calculator** finds the **Michaelis constant (Km)** and **substrate affinity** from your choice of **calculation method** — enter either the **rate constants** (**k₁**, **k₋₁**, **k₂**) or plug in **Vmax**, **initial reaction velocity (V₀)**, and **substrate concentration [S]** to also get the **kcat/Km ratio**.
Michaelis Constant (Km)
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Substrate Affinity
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kcat/Km Ratio
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The Michaelis constant (Km) is a fundamental parameter in enzyme kinetics that represents the substrate concentration at which the reaction rate is half of its maximum value (Vmax). A lower Km indicates higher enzyme affinity for its substrate.
Km can be calculated from rate constants using the formula: Km = (k₋₁ + k₂) / k₁, where k₁ is the substrate binding rate, k₋₁ is the unbinding rate, and k₂ is the product formation rate.
When you have kinetic data, Km can be calculated using: Km = (Vmax × [S] / V₀) - [S], where Vmax is the maximum reaction rate, [S] is substrate concentration, and V₀ is the initial velocity.
A high Km value indicates low enzyme affinity for the substrate, meaning a higher substrate concentration is needed to reach half-maximal velocity. This suggests weaker enzyme-substrate binding.
A low Km value indicates high enzyme affinity for the substrate, meaning the enzyme can achieve half-maximal velocity at low substrate concentrations. This suggests strong enzyme-substrate binding.
Km is the Michaelis constant related to enzyme kinetics, while Kd is the dissociation constant for equilibrium binding. Km equals Kd only when k₂ << k₋₁ (rapid equilibrium assumption).
Km is typically expressed in concentration units such as μM (micromolar), mM (millimolar), or M (molar), depending on the substrate concentration range in the experiment.
Enzyme efficiency is measured by the kcat/Km ratio, also called the specificity constant. Higher kcat/Km values indicate more efficient enzymes, with the theoretical maximum being around 10⁸-10⁹ M⁻¹s⁻¹.