Enzyme Activity Calculators

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Enzyme activity is a measure of the catalytic capacity of an enzyme preparation — how many substrate molecules are converted to product per unit time under defined conditions. It is measured in units (μmol substrate converted per minute) or katal (mol product formed per second), and expressed as total activity, specific activity (activity per mg protein), or turnover number (kcat). Accurate enzyme activity measurement is essential in biochemistry research, clinical diagnostics (serum enzyme levels diagnose organ damage), industrial biotechnology, and drug development.

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Units of Enzyme Activity

  • International Unit (U or IU): 1 U = amount of enzyme catalyzing conversion of 1 μmol substrate per minute under defined conditions (temperature, pH, substrate concentration)
  • Katal (kat): SI unit. 1 katal = 1 mol substrate converted per second. 1 U = 16.67 nkat
  • Specific activity: U/mg protein — activity per unit mass of total protein. Used to assess enzyme purity during purification.
  • Turnover number (kcat): Substrate molecules converted per enzyme active site per second. Intrinsic catalytic rate; requires knowledge of active enzyme concentration.

Enzyme Assay Methods

Spectrophotometric (Most Common)

Measures absorbance change as substrate or product absorbs at a specific wavelength:

  • NADH assays: NADH absorbs at 340 nm; many dehydrogenases can be coupled to NADH-producing or consuming reactions (linked enzyme assays)
  • pNP assays: para-nitrophenol released from pNP-substrates absorbs at 405 nm

Fluorometric

Higher sensitivity; uses fluorescent substrates or products (AMC, MU-substrates)

Colorimetric

Color-forming reagents detect product accumulation (DNS for reducing sugars, Coomassie for protein)

Radiometric

Radiolabeled substrate; most sensitive; separated from substrate by TLC or scintillation counting

Factors Affecting Enzyme Activity

  • Temperature: Rate increases with temperature until the enzyme denatures at its Tm. Each 10°C rise roughly doubles rate (Q10 ≈ 2) below the optimum.
  • pH: Optimum pH for each enzyme (e.g., pepsin pH 2; trypsin pH 8; alkaline phosphatase pH 10). Extreme pH denatures enzyme or alters active site ionization.
  • Substrate concentration: Rate approaches Vmax hyperbolically with [S] — described by Michaelis-Menten kinetics
  • Inhibitors: Competitive (increase apparent Km), non-competitive (decrease Vmax), uncompetitive (decrease both)

Glossary

Enzyme Activity Unit (U)
The amount of enzyme catalyzing conversion of 1 μmol substrate per minute under standard conditions. The SI equivalent is the katal (1 kat = 1 mol/s; 1 U = 16.67 nkat).
Specific Activity
Enzyme activity per unit mass of total protein (U/mg). Increases as enzyme is purified. Used to track purification efficiency and assess enzyme purity in biochemical purification procedures.
Turnover Number (kcat)
The maximum number of substrate molecules converted per enzyme active site per second at saturating substrate concentration. An intrinsic catalytic rate constant. Related to Vmax by: kcat = Vmax / [E]total.

Frequently Asked Questions

The International Unit (U or IU): 1 U = 1 μmol substrate converted per minute under defined conditions (standard temperature, pH, saturating substrate). The SI unit katal (kat): 1 kat = 1 mol/second; 1 U = 16.67 nkat. Specific activity = U/mg total protein — increases as enzyme is purified; 100% purity gives the maximum specific activity. Turnover number (kcat) = substrate molecules converted per active site per second — an intrinsic property of the enzyme.

Most enzyme assays measure the rate of change in absorbance as a substrate is consumed or product formed. The most common system uses NADH: dehydrogenase reactions produce or consume NADH, which absorbs at 340 nm (ε = 6,220 M⁻¹cm⁻¹). Activity (U/mL) = ΔA₃₄₀/min × total volume (mL) / (ε × path length × enzyme volume). Coupled assays link non-UV-absorbing reactions to NADH production/consumption for spectrophotometric detection.

Specific activity = total enzyme activity (U) / total protein mass (mg). It measures enzyme purity and enrichment during purification. As enzyme is purified from crude cell extract, specific activity increases because non-enzyme protein is removed. A purification table tracks: total activity, total protein, specific activity, fold purification, and % yield at each step. A highly pure enzyme preparation has specific activity close to the theoretical maximum for that enzyme.

Temperature: enzyme activity increases with temperature (Q10 ≈ 2) until the enzyme begins to denature near its thermal optimum. Above Tm, denaturation causes rapid activity loss. Each enzyme has a temperature optimum (37°C for human enzymes; 60–70°C for thermophilic bacterial enzymes). pH: each enzyme has an optimal pH that maintains correct active site ionization state. Deviation from optimum impairs catalysis and extreme pH denatures the protein. Pepsin optimum pH 2; most intracellular enzymes pH 6–8; alkaline phosphatase pH 10.