All calculatorsEnzyme Activity Calculator
Compute enzyme activity (U), rate per mL, and optional specific activity (U/mg) from either a spectrophotometric assay (ΔA/min) or product formed over time. See clear steps, a mini bar chart, and an activity gauge.
Background
By convention, 1 unit (U) of enzyme activity is defined as the amount of enzyme that converts 1 μmol of substrate per minute under specified conditions. In spectrophotometric assays, we often convert a measured ΔA/min to concentration change using the Beer–Lambert relationship and then to μmol/min. If you know how many milligrams of enzyme were present, you can also compute specific activity (U/mg).
How this calculator works
- Enzyme unit (U): 1 U = 1 μmol of substrate converted per minute under the assay conditions.
- Spectrophotometric route: we use Δc/Δt = ΔA / (ε·ℓ) to convert absorbance change per minute to concentration change (mM/min), then multiply by reaction volume (mL) and dilution factor to get μmol/min (U).
- Product route: if you know how many μmol of product formed over a given time, we compute μmol/min directly and correct for dilution.
- Specific activity: if you give enzyme mass (mg), we divide total activity (U) by mg to report U/mg.
Formula & Equations Used
Unit definition: U = μmol product per minute
Spectrophotometric route (mM and μmol):
Δc/Δt (mM/min) = ΔA/min ÷ (ε·ℓ)
rate (μmol/min) = Δc/Δt (mM/min) × volume (mL) × dilution factor
Product route:
rate (μmol/min) = product (μmol) ÷ time (min) × dilution factor
Specific activity: U/mg = rate (U) ÷ enzyme mass (mg)
Example Problems & Step-by-Step Solutions
Example 1 — NADH assay: ΔA/min = 0.12; ε = 6.22 mM⁻¹·cm⁻¹; 1 mL; no dilution
Δc/Δt = 0.12 ÷ (6.22 × 1.0) ≈ 0.0193 mM/min.
rate = 0.0193 mM/min × 1.00 mL = 0.0193 μmol/min.
So total activity ≈ 0.019 U in the cuvette.
Example 2 — 4.0 μmol product in 5.0 min, dilution factor 1
rate = product ÷ time = 4.0 μmol ÷ 5.0 min = 0.80 μmol/min.
So total activity ≈ 0.80 U in the assay.
Example 3 — Specific activity from Example 2 with 0.10 mg enzyme
Total activity = 0.80 U; enzyme mass = 0.10 mg.
Specific activity = 0.80 U ÷ 0.10 mg = 8.0 U/mg.
Frequently Asked Questions
Q: What units should I use for ε and volume?
Enter ε in mM⁻¹·cm⁻¹, path length in cm, and volume in mL. With these units, 1 mM corresponds to 1 μmol/mL, so the calculator naturally reports μmol/min (U).
Q: What is the difference between activity and specific activity?
Activity (U) is the total rate of product formation in the assay. Specific activity (U/mg) normalizes this to how much enzyme protein was present, which lets you compare enzyme purity and efficiency across samples.
Q: What if I do not know the exact enzyme mass?
That is fine: the calculator will still compute the total activity in U and rate per minute. Specific activity is only reported when you provide enzyme mass in mg.
