Calculator inputs
Use the mode selector to solve for velocity, Vmax, Km, or substrate concentration.
Example data table
The table below uses an example enzyme with Vmax = 120 µmol/min and Km = 2.5 mM. It shows how rate rises with substrate concentration.
| Substrate [S] (mM) | Velocity v (µmol/min) | Fraction of Vmax |
|---|---|---|
| 0.25 | 10.9091 | 0.0909 |
| 0.5 | 20 | 0.1667 |
| 1 | 34.2857 | 0.2857 |
| 2 | 53.3333 | 0.4444 |
| 4 | 73.8462 | 0.6154 |
| 8 | 91.4286 | 0.7619 |
| 12 | 99.3103 | 0.8276 |
Formula used
Primary Michaelis-Menten relationship
v = (Vmax × [S]) / (Km + [S])
This equation estimates enzyme reaction velocity for a given substrate concentration, assuming classic steady-state kinetics with one substrate and no cooperativity.
Rearranged forms used by the calculator
Vmax = v(Km + [S]) / [S]
Km = [S](Vmax − v) / v
[S] = vKm / (Vmax − v)
Additional derived metrics
Fractional saturation = v / Vmax = [S] / (Km + [S])
kcat = Vmax / [E]t
Catalytic efficiency = kcat / Km
Use one internally consistent unit system. For example, if substrate is in mM and enzyme is in µM, interpret derived turnover values with care unless the units are intentionally compatible.
How to use this calculator
- Select the variable you want to solve for.
- Enter the remaining known kinetics values.
- Supply optional enzyme concentration to estimate kcat.
- Set the substrate range and number of curve points.
- Click Calculate Kinetics to generate the result summary.
- Review the computed table, interpretation note, and saturation graph.
- Export the output as CSV for spreadsheets or PDF for reports.
FAQs
1) What does Km mean in enzyme kinetics?
Km is the substrate concentration giving half of Vmax in the simple Michaelis-Menten model. Lower Km usually suggests stronger apparent substrate affinity under the tested conditions.
2) Why must Vmax be greater than velocity?
In this model, instantaneous velocity approaches Vmax asymptotically but does not exceed it. Rearranged formulas for Km and substrate concentration therefore require Vmax to remain larger than v.
3) Can I use any concentration units?
Yes, as long as you stay consistent. If substrate uses mM, Km should also use mM. The calculator lets you label units, but it does not convert them automatically.
4) What does the saturation curve show?
The graph shows how reaction velocity changes across the chosen substrate range. It helps you see the low-substrate region, the transition near Km, and the near-saturation region toward Vmax.
5) When is kcat meaningful here?
kcat becomes meaningful only when Vmax and total enzyme concentration are entered in compatible units. Otherwise, the numeric ratio appears, but the physical interpretation may be misleading.
6) Does this model include inhibition effects?
No. This page focuses on the classic single-substrate equation. Competitive, noncompetitive, uncompetitive, and allosteric cases require modified models and separate parameter fitting.
7) Why is Km also shown as half-saturation substrate?
For the classical equation, v equals one-half Vmax when substrate concentration equals Km. That gives Km a practical interpretation directly linked to the curve midpoint.
8) Can I use this for teaching and lab notes?
Yes. The calculator includes a clear result table, exported files, an example dataset, and a visual curve, which makes it useful for demonstrations, assignments, and reporting.
Notes for interpretation
The Michaelis-Menten equation is a simplified model. Real systems may show inhibition, cooperativity, substrate depletion, product accumulation, pH effects, temperature sensitivity, and multi-step mechanisms.