Enzyme Activity Calculator
From a cuvette’s slope to the numbers a paper would print: units in the assay, specific activity per milligram of protein, and the katal beside them — with the assay conditions named.
Enzyme Activity Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Spectrophotometric assays with a known ε only
- ε values quoted approximate — pH and T shift them
In short: A dehydrogenase assay at ΔA340 = 0.100 per minute in a 1.000 mL cuvette (1.000 cm path, NADH ε = 6.22 mM⁻¹cm⁻¹) converts 0.016077 µmol of substrate per minute — 0.016077 U in the cuvette. With 0.100 mg of protein in the beam, the specific activity is 0.160772 U/mg; in katal the same activity reads 2.679528e-10 kat, and 1 kat = 6e7 U exact. Every one of those numbers means what it means only at the assay’s pH and temperature — quote them or the units are decoration.
Formula
rate = ΔA/(ε·l) mM/min · U = rate × V(mL) · specific = U/mg · kat = U/6×10⁷
Beer–Lambert turns the absorbance slope into a concentration slope: ΔA per minute divided by the extinction coefficient and the path gives mM/min. Multiplying by the assay volume in mL converts to µmol/min — which is the unit of enzyme activity by definition: 1 U converts 1 µmol of substrate per minute under the declared conditions. Dividing by the protein mass prices purity; dividing by 6×10⁷ converts to the SI katal.
Worked Example
- Enter the absorbance slope per minute.
- Enter ε, the path and the assay volume.
- Enter the protein mass for the specific activity.
- Read U, U/mg and the katal — then cite the conditions.
Defaults: ΔA340 0.100/min, ε 6.22, 1 cm, 1 mL → 0.016077 U, 0.160772 U/mg, 2.679528e-10 kat. A pNP assay (ΔA405 0.483/min, ε 18.3, 3 mL) → 0.079180 U and 1.319672e-9 kat per mg-scale run.
Strengths & Limits Of This Model
Where this engine is strong
- Units, U/mg and katal from one slope
- Condition-bound doctrine, not bare numbers
Where it stops
- No coupled-assay bookkeeping
- No endpoint (non-slope) assays
Practical Use Cases
Assay QC
units per preparation, tracked lot to lot
Purity audits
U/mg across a purification table
Teaching
Beer–Lambert into units
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Enzyme Activity Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
What exactly is one unit of enzyme activity?
By definition: the amount that converts one micromole of substrate per minute under the conditions you declare. The definition is deliberately condition-bound — change the pH, the temperature or the substrate and the same enzyme reads different units, which is why papers state ‘U/mg at pH 8.0, 25 °C’ and why this page makes the assay’s context part of the answer rather than an afterthought.
Why does NADH get 6.22 everywhere?
Because reduced nicotinamide absorbs at 340 nm with ε = 6.22 mM⁻¹cm⁻¹ (approximate; it drifts slightly with pH and temperature), while NAD⁺ barely absorbs there. That single fact turns any dehydrogenase — LDH, MDH and hundreds more — into a spectrophotometer readout: watch the absorbance fall and you are watching substrate convert. It is the most exploited number in enzymology.
What is the katal, and why does anyone use it?
The SI unit: one katal converts one mole per second, so 1 kat = 6×10⁷ U exactly — a conversion, not a convention. Journals increasingly ask for it; benches keep speaking units because micromoles per minute match how assays are actually pipetted. The page prints both, which is the whole bridge: same activity, two unit systems, exact arithmetic between them.
Why is specific activity the purity ruler?
Because it divides what the enzyme does by everything that is present: contaminating protein adds mass without activity, so as a preparation purifies, U/mg climbs and then plateaus at the enzyme’s intrinsic value. Purification tables are exactly this number tracked fraction by fraction — which is why the card refuses to divide by zero milligrams of protein: no protein, no ruler.
My ΔA is negative — the absorbance falls.
Then the slope’s magnitude is what counts: NADH-consuming assays watch absorbance fall, NADH-producing assays watch it rise, and the rate is the absolute slope either way. Enter the magnitude and the units describe the conversion honestly. What the page refuses is a slope of exactly zero — an assay that goes nowhere converts nothing, and dividing by an extinction coefficient to dress that up as 0.000000 U would be decoration, not measurement.
Can I use this for any enzyme?
For any enzyme with a spectrophotometric handle — a chromophore with a known extinction coefficient that changes as the reaction proceeds. Coupled assays extend the trick by wiring a product to NADH or pNP. Enzymes without such a handle need endpoints, radiolabels or HPLC — different rulers the page does not pretend to hold.
Why does the volume matter but the concentration of substrate not appear?
The volume converts a rate per millilitre into an amount per minute — it is the geometry of the cuvette, not the chemistry of the enzyme. Substrate concentration matters enormously (it decides whether you are measuring Vmax or something on the Michaelis–Menten curve), but it enters the protocol, not this arithmetic: the slope already contains whatever the substrate was doing. Assay at saturation if you want activity to mean capacity.
Where do the other extinction coefficients come from?
The same kind of careful spectrophotometry that fixed NADH: p-nitrophenol at 405 nm (ε ≈ 18.3 mM⁻¹cm⁻¹) underlies phosphatase and lipase assays; TNB at 412 (ε ≈ 14.15) underlies thiol assays — values approximate, each condition-dependent. The chip presets carry the common three; your assay’s paper carries the number you should actually type.