5.21 The Effect of Temperature on Enzyme Reactions

• Changing air temperature can have a significant impact on the metabolism of living organisms due to the effect of temperature on enzyme activity
• 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
  • 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 
• Higher temperatures cause reactions to speed up
  • 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 

Denaturation

• If temperatures continue to increase past a certain point, the rate at which an enzyme catalyses a reaction drops sharply as the enzymes begin to denature
  • The increased kinetic energy and vibration of an enzyme puts a strain on its bonds, 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 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 can denature

The rate of an enzyme catalysed reaction is affected by temperature. Note that 35 C is not the optimum temperature for all enzyme-controlled reactions.

Calculating the temperature coefficient

• The temperature coefficient, represented by Q₁₀, calculates the increase in rate of reaction when the temperature is increased by 10 C
• Q₁₀ can be calculated using the following equation

Q₁₀ = rate at higher temperature rate at lower temperature

• A Q₁₀ value of 2 indicates that the reaction rate doubles with an increase in temperature of 10 C, while a value of 3 indicates that it trebles with every 10 C increase

Step 1: Write out the relevant equation

Q₁₀ = rate at higher temperature  rate at lower temperature

Step 2: Substitute numbers into the equation

Q₁₀ = 6.8 3.5

Step 3: Complete calculation

Q₁₀ = 1.94

This value is close to 2, indicating that the rate of reaction has almost doubled

Enzyme activity and living organisms

• Changes to enzyme activity that result from changing global temperatures can affect living organisms
• Some chemical reactions take place faster at higher temperatures 
  • Photosynthesis is essential for converting carbon dioxide into carbohydrates, the process which produces food for producers and other organisms higher up the food chain; it relies on the function of proteins in the electron transport chain and that of enzymes such as rubisco
    • E.g. blue-green algae, also known as cyanobacteria, photosynthesise at a higher rate in warmer water due to increased enzyme activity; this increases the formation of potentially harmful algal blooms
• Some chemical reactions are slowed down at higher temperatures 
  • At high temperatures plants carry out a reaction called photorespiration at a faster rate; this reaction uses the enzyme rubisco and so slows down photosynthesis
    • This can reduce crop yields as temperatures rise
  • Some fish eggs have been shown to develop more slowly at higher temperatures
    • Many species' successful egg development is dependent on temperature, with impacts such as
      • Extreme temperature fluctuations can reduce hatching rates in some invertebrates
• The sex of the young inside the egg of some species is determined by temperature, so increasing temperatures can affect the sex ratios in a species
  • E.g. in alligators
• Species may have to change their distribution in response to changing temperatures in order to survive
  • Species may migrate to higher altitudes or further from the equator to find cooler temperatures