kcat is the enzyme turnover number: the number of substrate molecules converted per active site per second under the measured conditions.

What inputs does the calculator accept?

It accepts Vmax with enzyme amount, Vmax with enzyme concentration, or Vmax with protein concentration and molecular weight.

What units are used for the result?

The calculator reports kcat in min⁻¹ and s⁻¹.

Should enzyme concentration represent active sites?

Yes. Use the total concentration of active sites when the enzyme has multiple active-site-bearing subunits.

Is this calculator suitable for clinical or production decisions?

No. It is intended for education and experiment planning, not as a substitute for validated laboratory analysis.

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Enzyme Turnover Number Calculator (kcat)

kcat = Vmax ÷ [E]total in min⁻¹ and s⁻¹, entered as enzyme amount, enzyme concentration, or protein concentration + molecular weight; bands results from very slow to catalase-class. Educational use only.

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Vmax 12 µmol/min with 0.02 µmol enzyme → kcat = 600 min⁻¹ = 10 s⁻¹

A saturated assay of a 50 kDa monomeric enzyme gives Vmax = 12 µmol/min with 0.02 µmol (20 nmol, i.e. 1 mg) of active sites in the cuvette. kcat = 12 ÷ 0.02 = 600 min⁻¹ = 10 s⁻¹ — a moderate, thoroughly typical metabolic enzyme (each active site converts 10 substrate molecules per second).

{"result":"kcat ≈ 600 min⁻¹ = 10 s⁻¹ (Vmax 12 µmol/min ÷ 0.02 µmol enzyme) — moderate, typical of metabolic enzymes.","metadata":{"input":{"mode":"amount","vmaxUmolMin":12,"enzymeUmol":0.02},"result":{"kcatPerMin":600,"kcatPerSec":10}}}

Protein route: 1 mg/mL of a 50 kDa enzyme → [E] = 20 µM; Vmax 40 µM/min → kcat = 0.0333 s⁻¹

You know the stock as 1 mg/mL protein and 50 kDa subunits, so [E] = 1000 × 1 ÷ 50 = 20 µM. A Vmax of 40 µM/min then gives kcat = 40 ÷ 20 = 2 min⁻¹ = 0.0333 s⁻¹ — a slow enzyme. This route skips molarity bookkeeping entirely and uses only numbers you already have from the protein assay.

{"result":"kcat ≈ 2 min⁻¹ = 0.0333 s⁻¹ ([E] = 20 µM from 1 mg/mL ÷ 50 kDa; Vmax 40 µM/min) — slow.","metadata":{"input":{"mode":"protein","vmaxUMLMin":40,"proteinMgPerMl":1,"molecularWeightKda":50},"result":{"enzymeUM":20,"kcatPerMin":2,"kcatPerSec":0.0333}}}

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8 options

SettingsAdjust formats, ranges, numbers, and modes.8

Calculation ModeselectRequiredHow you know the enzyme: as an amount, a concentration, or protein concentration plus molecular weight.Vmax (µmol/min)numberRequiredMaximal velocity from a saturated assay (amount mode).Enzyme Amount (µmol)numberRequiredTotal enzyme (better: active-site) amount in the assay (amount mode).Vmax (µM/min)numberRequiredMaximal velocity as a concentration rate (concentration and protein modes).Enzyme Concentration (µM)numberRequiredEnzyme (active-site) concentration (concentration mode).Protein Concentration (mg/mL)numberRequiredProtein concentration of the enzyme stock (protein mode).Molecular Weight (kDa)numberRequiredMolecular weight of one active-site-bearing subunit (protein mode).Decimal PlacesnumberOptional

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Enzyme Turnover Number Calculator (kcat)

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Examples

1. Enzyme amount route: 12 µmol/min Vmax with 0.02 µmol enzyme

2. Protein route: 1 mg/mL of a 50 kDa enzyme

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