Factors Affecting Enzyme Activity (College Board AP® Biology): Study Guide

Denaturation of enzymes

• Disruption of the structure of an enzyme may result in a change of function or efficiency of an enzymatic system

• Enzymes have optimum environmental conditions in which they work the fastest

• Temperature and pH outside of the optimal range cause changes in enzyme structure and the enzyme starts to denature

  • Denaturation occurs when the bonds (e.g. hydrogen bonds) that hold the enzyme molecule in its precise shape, start to break

  • This causes the tertiary structure of the protein to change

  • This permanently damages the active site, preventing the substrate from binding

  • The rate at which an enzyme catalyzes a reaction drops sharply, because the substrate can no longer bind and therefore cannot catalyze the reaction

  • In some cases, denaturation may be reversible and the enzyme can regain activity once again

Effect of enzyme & substrate concentration on enzyme action

Enzyme concentration

• The greater the substrate concentration, the higher the rate of reaction

  • This is because there are more active sites available and so a greater chance 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

Substrate concentration

• The greater the substrate concentration, the higher the rate of reaction

  • As substrate concentration increases, enzyme-substrate complex formation becomes more likely

  • However, if enzyme concentration remains constant, a saturation point is reached where all active sites are occupied

  • Beyond this, adding more substrate won’t increase the reaction rate until active sites become available 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

Effect of pH & temperature on enzyme action

Temperature

• 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 a 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

pH

• All enzymes have an optimum pH which is the pH at which they function best

• Enzymes are denatured at extremes of pH

  • Hydrogen and ionic bonds holding the tertiary structure of the protein together are broken by a pH outside of the optimum

  • 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 and complete denaturation of the enzyme has occurred

• The surrounding pH of 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

Calculating pH

• If the hydrogen ion (H⁺) concentration of a solution is known, the pH can be calculated using the equation:

pH = -log₁₀ [H⁺]

• You can find the ‘log’ function on your calculator (‘log’ is the same as ‘log₁₀’ so don’t worry if your calculator doesn’t say ‘log₁₀’)