How to Determine the Activation Energy From an Enzyme Assay

Enzymes are organic catalysts that dramatically increase the rate of chemical reactions. Without enzyme assistance, most metabolic reactions would simply be too slow for living organisms. Enzymes lower activation energy, the amount of energy required to initialize a reaction. Reaction rate is determined by assay in which the substrate, molecules that react, or the products are measured for a given temperature over time. Determining the reaction rate at different temperatures supplies data necessary to calculate activation energy (Ea). You can use the Arrhenius equation ln k = -Ea/RT + ln A to determine activation energy.

Things You'll Need

• Enzyme assay data
• Graph paper
• Calculator with natural log capability
• Thermometer

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Instructions

• 1

  Formulate data from the enzyme assay in tabular form. A plot of the data would show that rate increases exponentially as the temperature increases to supply more heat energy for the reaction. The measurable unit, a decrease in substrate or an increase in product will be recorded as (moles/liter)/time (s).

• 2

  Calculate each temperature (T) in degrees Kelvin. Enzyme assay data is generally measured using the Celsius scale. For you to use the Arrhenius equation, the value of T is in degrees Kelvin. The conversion formula is degrees C + 273 = degrees K.

• 3

  Determine the natural log (ln) of the reaction rate for each experimental temperature. You can perform this function using a calculator supplied with natural log capabilities by typing the number and then pressing the ln key. Calculators with natural log as a secondary function may require pressing 2nd function key, ln and the number followed by the equals sign key.

• 4

  Plot natural log values for reaction rate (y axis) versus the 1/T in Kelvin degrees (x axis). Change in reaction rate with respect to temperature is exponential. Natural log will convert reaction rate values into a linear plot.

• 5

  Determine the slope intercept equation for the line where y=mx+b. The equation is equivalent when written in the form ln k= -(Ea/R)T + ln A, the Arrhenius equation. The slope for the line represents the negative quotient for activation energy (Ea) over the gas constant (R). The given value for the gas constant is equal to 8.314472 Joules Moles#--1 Kelvin#--1 where # represent an exponent.

• 6

  Calculate Activation energy by multiplying the slope (m) by the gas constant (R) to find the value Ea. Note that the value for the slope represents a negative value for Ea and activation energy should be recorded as absolute value or positive value to determine Ea.