2.4.4 Enzyme Activity: Temperature

• Enzymes have a specific optimum temperature
  • This is the temperature at which they catalyse a reaction at the maximum rate

• Lower temperatures either prevent reactions from proceeding or slow them down because:
  • Molecules move relatively slowly as they have less kinetic energy 
  • Less kinetic energy results in a lower frequency of successful collisions between substrate molecules and the active sites of the enzymes which leads to less frequent enzyme-substrate complex formation
  • Substrates and enzymes also collide with less energy, making it less likely for bonds to be formed or broken (stopping the reaction from occurring)

• Higher temperatures cause reactions to speed up because:
  • Molecules move more quickly as they have more kinetic energy
  • Increased kinetic energy results in a higher frequency of successful collisions between substrate molecules and the active sites of the enzymes which leads to more frequent enzyme-substrate complex formation
  • Substrates and enzymes also collide with more energy, making it more likely for bonds to be formed or broken (allowing the reaction to occur)

Denaturation

• If temperatures continue to increase past a certain point, the rate at which an enzyme catalyses a reaction drops sharply, as the enzyme begins to denature:
  • The increased kinetic energy and vibration of the enzyme molecules puts a strain on them, eventually causing the weaker hydrogen and ionic bonds that hold the enzyme molecule in its precise shape to start to break
  • The breaking of bonds causes the tertiary structure of the protein (i.e. the enzyme) to change
  • The active site is permanently damaged and its shape is no longer complementary to the substrate, preventing the substrate from binding
  • Denaturation has occurred if the substrate can no longer bind

At high temperatures enzymes are denatured - the active site changes shape and is no longer complementary to the substrate. This change is irreversible.

The effect of temperature on the rate of an enzyme-catalysed reaction

• The optimum temperature of an enzyme and the temperature at which an enzyme is denatured varies according to the habitat to which an organism is adapted
  • Most enzymes present in living organisms denature at temperatures above 60 °C

• Very few human enzymes can function at temperatures above 50 °C
  • Humans maintain a body temperature of about 37 °C and even temperatures exceeding 40 °C can cause the denaturation of some enzymes

• Some bacteria that live in thermal springs have enzymes that can withstand temperatures in excess of 80 °C
  • These enzymes are thermostable

Temperature coefficient

• The temperature coefficient for a biological reaction is the ratio between the rates of that reaction at two different temperatures
  • For most enzyme-catalysed reactions the rate of the reaction doubles for every 10 °C increase in temperature
  • The temperature coefficient (Q) for a reaction that follows this pattern is: Q₁₀ = 2

• The temperature coefficient can be calculated using the following equation:

Temperature coefficient = (rate of reaction at (x + 10) °C) ÷ (rate of reaction at x °C)

Step One: Using the graph, note the intercepts on the vertical axis at 20 °C and 30 °C

At 20 °C the rate of reaction is 10 arbitrary units and at 30 °C the rate of reaction is 20 arbitrary units

Step Two: Write out the equation and substitute in the known values

Temperature coefficient = (rate of reaction at (x + 10) °C) ÷ (rate of reaction at x °C)

Q₁₀ = rate of reaction at 30 °C ÷ rate of reaction at 20 °C

Q₁₀ = 20 ÷ 10

Step Three: Calculate the temperature coefficient

Q₁₀ = 2

(There is no unit for Q₁₀ as it is a ratio)