How to Calculate Enzyme Rate of Reaction from Absorbance

Calculation Results:

Change in Absorbance (ΔA): 0

Rate (Molarity/min): 0 M/min

Rate (µmol/L/min): 0 µM/min

function calculateEnzymeRate() { var A1 = parseFloat(document.getElementById("initialAbsorbance").value); var A2 = parseFloat(document.getElementById("finalAbsorbance").value); var time = parseFloat(document.getElementById("timeInterval").value); var epsilon = parseFloat(document.getElementById("extinctionCoeff").value); var length = parseFloat(document.getElementById("pathLength").value); if (isNaN(A1) || isNaN(A2) || isNaN(time) || isNaN(epsilon) || isNaN(length) || time <= 0 || epsilon <= 0 || length c = A / (epsilon * l) // Rate = deltaC / deltaTime var rateM = (deltaA / (epsilon * length)) / time; var rateuM = rateM * 1000000; document.getElementById("deltaAbs").innerText = deltaA.toFixed(4); document.getElementById("rateMolar").innerText = rateM.toExponential(4); document.getElementById("rateMicromolar").innerText = rateuM.toFixed(4); document.getElementById("resultsArea").style.display = "block"; }

How to Calculate Enzyme Rate of Reaction from Absorbance

In biochemistry, the rate of an enzyme-catalyzed reaction is often measured using a spectrophotometer. This method relies on the Beer-Lambert Law, which correlates the absorbance of light by a substance to its concentration in a solution.

The Beer-Lambert Law Formula

The fundamental equation used for these calculations is:

A = ε · c · l

• A: Absorbance (no units).
• ε (Epsilon): Molar extinction coefficient (M⁻¹ cm⁻¹). It represents how strongly a substance absorbs light at a specific wavelength.
• c: Concentration of the absorbing species (Molar, M).
• l: Path length of the cuvette (usually 1 cm).

Step-by-Step Calculation Logic

To find the rate of reaction (velocity), we measure how the concentration changes over a specific period. The process follows these steps:

1. Determine the Change in Absorbance (ΔA): Subtract the initial absorbance from the final absorbance (ΔA = |A₂ – A₁|).
2. Calculate Concentration Change (Δc): Using the Beer-Lambert law, Δc = ΔA / (ε · l).
3. Divide by Time (t): The rate is the change in concentration divided by the time interval (Rate = Δc / Δt).

Example Calculation

Suppose you are measuring the oxidation of NADH to NAD⁺. The extinction coefficient (ε) for NADH at 340 nm is 6,220 M⁻¹ cm⁻¹.

• Initial Absorbance (t=0): 0.800
• Final Absorbance (t=2 min): 0.500
• ΔA: 0.800 – 0.500 = 0.300
• Δc: 0.300 / (6,220 · 1) = 0.00004823 M
• Rate: 0.00004823 M / 2 min = 0.00002411 M/min
• Result: 24.11 µM/min

Key Factors to Consider

For accurate results, ensure the following conditions are met:

• Linearity: Ensure the reaction is in its initial linear phase. If the absorbance change plateaus, the enzyme may be saturated or the substrate exhausted.
• Temperature Control: Enzyme activity is highly temperature-dependent; measurements should be taken in a temperature-controlled cuvette holder.
• Blanking: Always "blank" the spectrophotometer with a buffer solution that contains everything except the absorbing species being measured.
• Path Length: Standard cuvettes have a path length of 1 cm, but micro-cuvettes or plates may differ.