An enzyme assay is a laboratory method for measuring enzyme activity. By quantifying the rate of an enzyme-catalyzed reaction, we can determine how much enzyme is present, how active it is, and how it responds to conditions like pH, temperature, or inhibitors .
Key insight: Enzyme assays measure the appearance of product or disappearance of substrate over time. The rate must be measured under conditions where it's proportional to enzyme concentration—the initial rate .
Enzyme activity is expressed in standardized units:
One unit (U) is the amount of enzyme that catalyzes the conversion of 1 micromole of substrate per minute under specified conditions .
The SI unit; 1 katal = 1 mole per second. 1 U = 16.67 nanokatals .
Units per milligram of protein. A measure of enzyme purity—higher specific activity means purer enzyme .
Measure reaction progress continuously, usually by spectrophotometry. The change in absorbance is monitored over time, giving a direct rate measurement .
Reaction is stopped at specific time points, and product is measured later. Used when there's no convenient continuous method .
Used when the primary reaction doesn't produce a detectable change. A second enzyme links the product to a detectable reaction (e.g., NADH production) .
Hexokinase activity is often measured by coupling to glucose-6-phosphate dehydrogenase:
NADPH production is measured at 340 nm. The rate of NADPH increase is proportional to hexokinase activity .
A well-designed enzyme assay requires careful consideration of several factors:
| Factor | Consideration | Typical approach |
|---|---|---|
| pH | Enzymes have optimal pH; activity drops rapidly away from optimum | Use buffer at known optimal pH; test if unknown |
| Temperature | Rate increases with T until denaturation | Usually 25°C, 30°C, or 37°C; must be controlled |
| Substrate concentration | Must be saturating ([S] >> Km) for activity measurements | Use 5-10× Km if known; determine if unknown |
| Cofactors | Many enzymes require metal ions or coenzymes | Add optimal concentrations (e.g., Mg²⁺ for kinases) |
| Enzyme concentration | Rate should be proportional to [E] | Test several dilutions to ensure linearity |
| Time course | Measure initial rate (linear portion) | Preliminary experiment to determine linear range |
Oxidizes guaiacol to tetraguaiacol (brown, A₄₇₀). Important in stress responses, lignin synthesis .
Measure disappearance of H₂O₂ at 240 nm. Key antioxidant enzyme .
Inhibits reduction of nitroblue tetrazolium by superoxide. One unit = 50% inhibition .
Measures browning at 420 nm. Important in post-harvest quality .
Measures nitrite production. Key enzyme in nitrogen assimilation .
Measures reducing sugars (DNSA method) from sucrose .
Principle: Peroxidase catalyzes: H₂O₂ + guaiacol → tetraguaiacol (brown, A₄₇₀)
Reagents:
Procedure:
Calculation:
Enzyme activity (U/mL) = (ΔA/min × Vtotal) / (ε × d × Venzyme)
where ε = 26.6 mM⁻¹cm⁻¹ (for tetraguaiacol)
Example: If ΔA/min = 0.15, Vtotal = 3.1 mL, Venzyme = 0.1 mL, d = 1 cm:
Activity = (0.15 × 3.1) / (26.6 × 1 × 0.1) = 0.175 U/mL
To characterize an enzyme, we measure activity at different substrate concentrations and fit to the Michaelis-Menten equation:
Where:
A linear transformation: 1/v = (Km/Vmax)(1/[S]) + 1/Vmax
A researcher measures peroxidase activity at different H₂O₂ concentrations (keeping guaiacol saturating). The Lineweaver-Burk plot gives equation y = 0.25x + 0.5. Calculate Km and Vmax.
Solution:
Enzyme inhibitors are studied by measuring activity with and without inhibitor at different substrate concentrations.
| Inhibition type | Effect on Vmax | Effect on Km | Lineweaver-Burk pattern |
|---|---|---|---|
| Competitive | No change | Increases | Lines intersect on y-axis |
| Non-competitive | Decreases | No change | Lines intersect on x-axis |
| Uncompetitive | Decreases | Decreases | Parallel lines |
| Problem | Possible cause | Solution |
|---|---|---|
| No activity | Enzyme denatured, missing cofactor, wrong pH, substrate not added | Check all assay components; test positive control if available |
| Non-linear progress curve | Substrate depletion, product inhibition, enzyme instability | Use less enzyme; measure initial rate only |
| Activity not proportional to enzyme | Substrate not saturating, enzyme aggregating, assay nonlinear | Increase substrate; try different enzyme dilutions |
| High background | Interfering substances in extract | Include sample blank (without substrate); dialyze or desalt extract |
| Poor reproducibility | Pipetting errors, temperature fluctuations, timing issues | Practice technique; use master mixes; control temperature |
1. Unit calculation: In a peroxidase assay, ΔA₄₇₀/min = 0.12. Total assay volume = 3.0 mL, enzyme volume = 50 μL. The extinction coefficient (ε) for tetraguaiacol is 26.6 mM⁻¹cm⁻¹. Calculate enzyme activity in U/mL.
Activity = (ΔA/min × Vtotal) / (ε × d × Venzyme)
= (0.12 × 3.0) / (26.6 × 1 × 0.05)
= 0.36 / 1.33 = 0.27 U/mL
2. Specific activity: The same enzyme extract has protein concentration 2.5 mg/mL. What is the specific activity?
Specific activity = Activity (U/mL) / Protein (mg/mL)
= 0.27 U/mL / 2.5 mg/mL = 0.108 U/mg protein
3. Km determination: A Lineweaver-Burk plot gives equation y = 0.15x + 0.025 (units: 1/v in min/ΔA, 1/[S] in mM⁻¹). Calculate Vmax and Km.
1/Vmax = 0.025 → Vmax = 40 ΔA/min
Slope = Km/Vmax = 0.15 → Km = 0.15 × 40 = 6.0 mM
Ethiopian researchers studying drought tolerance in teff and wheat often measure antioxidant enzymes (SOD, CAT, POD, APX). Higher activity of these enzymes correlates with better stress tolerance and can be used to select tolerant varieties .
Polyphenol oxidase (PPO) activity is measured in fruits and vegetables to predict browning potential and shelf life. This helps in selecting varieties for processing and export .
Soil enzyme activities (urease, phosphatase, dehydrogenase) are measured as indicators of soil health and microbial activity in Ethiopian agricultural soils .
Researchers at an Ethiopian university compared antioxidant enzyme activities in two teff varieties under drought stress. The tolerant variety showed 40% higher SOD activity and 30% higher CAT activity than the sensitive variety. These assays help identify biochemical markers for breeding .
| Term | Definition |
|---|---|
| Unit (U) | 1 μmol product formed per minute |
| Specific activity | U per mg protein; measure of purity |
| Continuous assay | Measure reaction progress in real time |
| Initial rate | Linear portion of progress curve; proportional to enzyme |
| Km | [S] at half Vmax; indicates affinity |
| Vmax | Maximum velocity at saturating substrate |
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