Factors that Affect Enzyme Action (College Board AP Biology)

• Disruption of the structure of an enzyme may result in a change of function or efficiency of an enzymatic system.
• In certain environments (e.g. incorrect temperatures or pH levels), the rate at which an enzyme catalyzes a reaction drops sharply, as the enzyme begins to denature:
  • Bonds (eg. hydrogen bonds) holding the enzyme molecule in its precise shape start to break
  • This causes the tertiary structure of the protein (ie. the enzyme) to change
  • This permanently damages the active site, preventing the substrate from binding
  • Denaturation has occurred if the substrate can no longer bind

Denaturation of an Enzyme Diagram

If environmental conditions are outside of the optimum, the shape of the enzyme may be disrupted leading to denaturation

Temperature & Enzyme Action

• Enzymes have a specific optimum temperature – the temperature at which they catalyse a reaction at the maximum rate
• Lower temperatures either prevent reactions from proceeding or slow them down this is because:
  • Molecules move relatively slowly at lower temperatures
  • Therefore there is a lower frequency of successful collisions that occur between substrate molecules and the active site of the enzyme
  • So there are less frequent enzyme-substrate complexes formed
  • Substrates and enzymes collide with less energy, making it less likely for bonds to be formed or broken (stopping the reaction from occurring)
• Higher temperatures speed up reactions this is because:
  • Molecules move more quickly at higher temperatures
  • Which results in higher frequency of successful collisions between substrate molecules and the active sites of enzymes
  • So there are more frequent enzyme-substrate complexes formed
  • Substrates and enzymes collide with more energy, making it more likely for bonds to be formed or broken (allowing the reaction to occur)
• However, as temperatures continue to increase, the rate at which an enzyme catalyses a reaction drops sharply, as the enzyme begins to denature:
  • Bonds (e.g. hydrogen bonds) holding the enzyme molecule in its precise shape start to break
  • This causes the tertiary structure of the protein (i.e. the enzyme) to change
  • This permanently damages the active site, preventing the substrate from binding
  • Denaturation has occurred if the substrate can no longer bind
  • Very few human enzymes can function at temperatures above 50°C
    • This is because humans maintain a body temperature of about 37°C, therefore even temperatures exceeding 40°C will cause the denaturation of enzymes

Effect of Temperature on Enzymes Diagram

The effect of temperature on the rate of an enzyme-catalysed reaction. Enzyme activity will have an optimum temperature specific for each enzyme. 

pH & Enzyme Action

• All enzymes have an optimum pH which is a pH at which they operate best
• Enzymes are denatured at extremes of pH
  • Hydrogen and ionic bonds hold the tertiary structure of the protein (i.e. the enzyme) together
  • Below and above the optimum pH of an enzyme, solutions with an excess of H⁺ ions (acidic solutions) and OH^- ions (alkaline solutions) can cause these bonds to break
  • This alters the shape of the active site, which means enzyme-substrate complexes form less easily
  • Eventually, enzyme-substrate complexes can no longer form at all
  • At this point, complete denaturation of the enzyme has occurred
• Where an enzyme functions, can be an indicator of its optimal environment:
  • E.g. pepsin is found in the stomach, an acidic environment at pH 2 (due to the presence of hydrochloric acid in the stomach’s gastric juice)
  • Pepsin’s optimum pH, not surprisingly, is pH 2

Effect of pH on Enzymes Diagram

The effect of pH on the rate of an enzyme-catalysed reaction for three different enzymes (each with a different optimum pH)

• When investigating the effect of pH on the rate of an enzyme-catalysed reaction, you can use buffer solutions to measure the rate of reaction at different pH values:
  • Buffer solutions each have a specific pH
  • Buffer solutions maintain this specific pH, even if the reaction taking place would otherwise cause the pH of the reaction mixture to change
  • A measured volume of the buffer solution is added to the reaction mixture
  • This same volume (of each buffer solution being used) should be added for each pH value that is being investigated

Enzyme Concentration & Enzyme Action

• Enzyme concentration affects the rate of reaction
• The higher the enzyme concentration in a reaction mixture, the greater the number of active sites available and the greater the likelihood of enzyme-substrate complex formation
• As long as there is sufficient substrate available, the initial rate of reaction increases linearly with enzyme concentration
• If the amount of substrate is limited, at a certain point any further increase in enzyme concentration will not increase the reaction rate as the amount of substrate becomes a limiting factor

Enzyme Concentration Diagram

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

Substrate Concentration & Enzyme Action

• The greater the substrate concentration, the higher the rate of reaction:
  • As the number of substrate molecules increase, the likelihood of enzyme-substrate complex formation increases
  • If the enzyme concentration remains fixed but the amount of substrate is increased, past a certain point all available active sites eventually become saturated and any further increase in substrate concentration will not increase the reaction rate
  • When the active sites of the enzymes are all full, any substrate molecules that are added have nowhere to bind in order to form an enzyme-substrate complex and so the reaction rate will not increase any further until active sites become free again
• For this reason, in the graph below there is a linear increase in reaction rate as substrate is added, which then plateaus when all active sites become occupied

Substrate Concentration Diagram

The effect of substrate concentration on the rate of an enzyme-catalysed reaction