Inputs
Tip: When [S] = Km, v = 0.5·Vmax. Ensure Km and [S] use the same concentration units (pick from the dropdowns—conversion is automatic).
Result
Initial rate
— (rate)
- v / Vmax —
- Fractional saturation (θ) —
- Half‑saturation [S] (where v = 0.5·Vmax) — —
Kinetic Plots
Auto-range: 0.05×Km → 10×Km (excluding 0)
Slope ≈ Km/Vmax; y‑intercept ≈ 1/Vmax; x‑intercept ≈ −1/Km.
Slope ≈ −Km; y‑intercept ≈ Vmax.
Example Data Table
| # | [S] | Unit | Predicted v | Observed v (optional) | Residual (Obs − Pred) |
|---|---|---|---|---|---|
| RMSE (observed vs predicted) | — | ||||
Formula Used
The Michaelis–Menten rate law describes the initial velocity v as a function of substrate concentration [S]:
v = (Vmax · [S]) / (Km + [S])
Vmax: maximal rate when the enzyme is saturated with substrate.Km: substrate concentration at whichv = 0.5 · Vmax; a lower value generally indicates higher apparent affinity.[S]: substrate concentration.
Assumptions: steady‑state for ES complex, initial‑rate conditions, constant enzyme concentration, negligible product inhibition or reverse reaction, and single‑substrate non‑cooperative kinetics.
How to Use This Calculator
- Enter Vmax in any rate unit (e.g., µM/s). The output v uses the same unit.
- Enter Km and select its concentration unit.
- Enter substrate [S] and select its concentration unit.
- Click Compute to calculate
v,v/Vmax, and echo Km at half‑saturation. - Use the Kinetic Plots tabs to visualize double‑reciprocal and Eadie–Hofstee relationships. The range auto‑scales around Km.
- Use the Example Data Table to generate predicted rates across multiple [S] values and optionally type observed rates to view residuals and RMSE.
- Export your table using Export CSV or Export PDF.
FAQs
Yes. v and Vmax must share the same rate unit; [S] and Km must share the same concentration unit. This tool normalizes [S] and Km units internally for accurate ratios.
Generally, a lower Km implies higher apparent affinity because half‑maximal velocity is achieved at a lower substrate concentration.
This calculator is for the classic single‑substrate model without inhibitors or cooperativity. For competitive, noncompetitive, or Hill kinetics, use specialized models.
Yes. Pick units for [S] and Km; the calculator converts them to molar (M) internally, so the ratio
[S]/Km is dimensionless.They quantify how well the model fits your experimental data. Smaller residuals and RMSE indicate better agreement between predicted and observed rates.
That may indicate substrate inhibition or other complexities beyond the simple model. Consider alternative kinetics that include an inhibition term or consult literature for appropriate analysis.