Michaelis-Menten Calculator
Velocity, saturation and catalytic efficiency.
Work out Michaelis-Menten. Velocity, saturation and catalytic efficiency. Says where the method stops working.
Maximum velocity, in whatever rate unit you measured — µmol/min, ΔA/min.
The substrate concentration giving half of Vmax. Same units as [S] below.
Optional — gives kcat and catalytic efficiency.
Reaction velocity
50
50.0% of Vmax · [S]/Km = 1
Km is a concentration, not a rate — it is the substrate level at which the enzyme reaches half its maximum speed. A low Km means the enzyme saturates early, which is about affinity rather than speed, and reading it as "faster" is the standard misinterpretation. Velocity approaches Vmax asymptotically and never reaches it, which is why kinetic assays are run well below saturation where the curve still has slope. kcat/Km is the number to compare enzymes on. It measures how efficiently the enzyme works at low substrate, which is the physiologically relevant condition, and it has an upper limit around 10⁸–10⁹ M⁻¹s⁻¹ set by how fast substrate can diffuse in at all.
How the Michaelis-Menten Calculator works
Enter Vmax, Km and a substrate concentration to get the reaction velocity, how close the enzyme is to saturation, and which kinetic regime you are in. Add an enzyme concentration for kcat and catalytic efficiency.
Also known as: enzyme kinetics calculator · km vmax calculator · kcat km calculator · enzyme reaction velocity calculator
Frequently asked questions
What is Km?
The substrate concentration at which the enzyme runs at half its maximum velocity. It has units of concentration, not rate, and it describes affinity rather than speed.
Does a low Km mean a fast enzyme?
No, and this is the standard misreading. A low Km means the enzyme saturates at low substrate — it reaches its maximum sooner, but that maximum may still be slow. Speed is kcat; efficiency is kcat/Km.
What is catalytic efficiency?
kcat/Km, the best single number for comparing enzymes. It describes performance at low substrate, which is the physiologically relevant condition, and it has a ceiling around 10⁸–10⁹ M⁻¹s⁻¹ set by how fast substrate can diffuse in.
Why does velocity never quite reach Vmax?
Because the relationship is a hyperbola that approaches its asymptote without arriving. At ten times Km you are at 91% of Vmax, and at a hundred times you are at 99% — which is why assays are run below saturation where the curve still responds.
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