2.1 Essential ideas

2.1.5 Enzymes

  • All enzymes are proteins catalysts – catalysts are substances that increase the rate of chemical reactions without changing shape or chemical composition.

The active site and enzyme activity

  • Enzymes are large globular proteins with (comparatively) small active sites.

active siteFigure 2.1.5a – The active site binds to specific substrates. Here the enzyme (catalase) is shown with the substrate (hydrogen peroxide). 

  • The active site of each enzyme binds to only one or two types of molecule, known as substrates. There is enzyme-substrate specificity.

enzyme activityFigure 2.1.5b – Simplified model of enzyme activity for the decomposition of hydrogen peroxide by catalase (H2O2 à H2O + O2)

  • The association between an enzyme’s active site and the substrate(s) results in a chemical reaction.
  • Substrates are converted into products, which are released from the active site.
  • The enzyme is unaltered by the reaction and is reused in further reactions.

Factors that affect enzyme activity rates

  • Catalysis occurs when a substrate and the enzyme randomly collide and become associated. In order to do so, the substrate must be oriented towards the active site of the enzyme.
  • Temperature, pH, and substrate concentration are factors that affect the chance of a collision between a substrate and the active site of an enzyme

1. Temperature 

  • Print Figure 2.1.5c. Annotate the regions a–d to explain the effect of increasing temperature on enzyme activity. Name the point marked X on the graph.

temperature rate of reactionFigure 2.1.5c – Effect of temperature on the rate of an enzyme catalysed reaction

  • The kinetic molecular theory suggests that as the temperature of a substance increases, the movement of its particles increases. This increases the rate of reaction because it increases the chance of collisions.
  • Denaturation lowers the rate of reaction because the shape of the active site is permanently changed and substrates cannot bind successfully even if there are lots of collisions. As the temperature increases, a greater proportion of the enzymes become denatured.
  • At the optimum temperature, the rate of reaction peaks because the effect of enzyme denaturation begins to overcome the effect of increasing molecular movement.

2. pH 

  • The pH scale is an index of acidity and alkalinity. If there are more hydrogen ions, H+, in solution, the pH is lower and the solution is more acidic.

pH scaleFigure 2.1.5d – pH is a logarithmic scale of hydrogen ion concentration ranging from 0 (very acidic) to 14 (very alkaline).

  • Each enzyme has an optimum pH at which its activity is highest.
  • When the conditions move away from the pH optimum, the structure of the active site is affected by the change in hydrogen ion concentration. This leads to fewer successful associations between enzyme and substrate.

pH denaturationFigure 2.1.5e – A change in pH can denature enzymes.

  • Most enzymes are active within a narrow range of pH, after which they become denatured and non-functional.

3. Substrate concentration

substrate concentrationFigure 2.1.5f – Effect of substrate concentration on enzyme activity

  • As the concentration of substrate increases, the rate of reaction increases because there will be more successful collisions with enzyme molecules.
  • However, the rate of change becomes progressively slower as the substrate concentration increases. Once the substrate is bound, the active site is unavailable for a new collision until products are formed and released.
  • A maximum rate of reaction is reached when all the active sites are saturated with substrate. Increasing the substrate concentration past the saturation point does not further increase the rate of reaction.

Industrial uses of immobilised enzyme

  • Commercially useful enzymes can be isolated and immobilised, so that they may be used to harvest products from a steady stream of substrates.
  • There are many ways to immobilise enzymes through physical or chemical processes. All of the methods involve concentrating the enzyme in a small space, so that the efficiency of production is very high.

methods of immobilisationFigure 2.1.5g – Methods of enzyme immobilisation

  • The products of enzymatic reactions are used in medicines, diagnostic tests, food additives, textiles and detergents.

enzymeFigure 2.1.5h – Enzyme specificity
Enzymes control the metabolism of a cell

Key concepts

  • Enzymes have an active site that binds to specific substrates.
  • Enzyme catalysis involves the collision of substrates with the active site.
  • The rate of activity of enzymes is affected by temperature, pH and substrate concentration.
  • Enzymes can be denatured.
  • Immobilised enzymes are widely used in industry.

Try it!

  • Catalase is an enzyme found in almost all living things – see Figure 2.1.5a. Balance the chemical equation for the decomposition of hydrogen peroxide (the catalase reaction): H2O2 à H2O + O2
  • Sketch graphs showing the effect of pH on the enzyme activity of the enzymes in the following table:

Enzyme

pH optimum

Salivary amylase

6.8

Urease

7.4

Pepsin

1.5

Pancreatic lipase

8.0

Exam tip

  • You should be able to sketch graphs to show the expected effects of temperature, pH and substrate concentration on the activity of enzymes, and explain the shape of the graphs.

Discussion

  • How does a change in pH lead to enzyme denaturation? Think about the types of intermolecular interactions that can occur.

Course links

  • HL students can learn more about enzymes in 8.1.1.
  • HL: Enzymes are chemically immobilised for use in home pregnancy kits. See 11.2.1.