ENZYMES (Topic 3)

• Enzymes are ​globular proteins​ that increase the ​rate of reaction​ ​by lowering the ​activation energy​ ​of the reaction they catalyse
• The ​active site​ is the area of the enzyme where the reaction with the ​substrate​ takes place
• Each enzyme has a ​specific ​shape that must be complementary​ to the substrate, meaning that only one type of substrate fits into the active site of each enzyme (enzyme specificity)
• When the enzyme and substrate form a complex​, the structure of the enzyme is altered so that the active site of the enzyme fits around the substrate. This is called the ​induced fit model​
• Enzymes can be ​intracellular​ (function inside cells), for example, DNA polymerase. They can also be ​extracellular​, such as the enzymes used in digestion
• Lock and Key Theory:

  (Proposed by Fischer in 1894)

  • Active site and substrate have​ complementary shapes before binding
  • The enzyme binds with the substrate forming an enzyme-substrate complex
  • Products are released from the active site and enzymes can be reused
  • Only one substrate can fit each active site
• Induced Fit Theory:

  (Proposed by Koshland in 1958)

  • The enzyme has an active site
  • The enzyme is moulded around the substrate as it enters to ​become complementary forming the enzyme an enzyme-substrate complex
  • Bonds form between oppositely charged groups on substrate and R groups to induce a better fit. This puts a strain on the substrate molecule so reactions occur more easily.
• Factors affecting the rate of enzyme-controlled reactions:

  1. Enzyme concentration
  2. Substrate concentration
  3. Temperature
  4. pH
  5. The concentration of competitive reversible inhibitors
  6. The concentration of non-competitive reversible inhibitors
• Enzyme​ ​concentration


  The rate of reaction increases as enzyme concentration increases as there are more active sites for substrates to bind to, however, increasing the enzyme concentration beyond a certain point does not affect the rate of reaction as there are more active sites than substrates so substrate concentration becomes the limiting factor.
• Substrate concentration​


  As the concentration of substrate increases, the rate of reaction increases as more enzyme-substrate complexes are formed. However, beyond a certain point, the rate of reaction no longer increases as enzyme concentration becomes the limiting factor.
• Temperature​


  The rate of reaction increases up to the optimum temperature as kinetic energy increases. The rate of reaction decreases beyond the optimum temperature. At very high temperatures, bonds in the enzyme's tertiary structure break, changing the shape of the active site so reactions cannot occur. This is called denaturation.
• pH


  As the pH moves away from the enzyme's optimum, the rate of reaction decreases. The pH is a measure of the concentration of hydrogen ions. Each enzyme has an optimum pH: the wrong pH alters the charges on the amino acids which make up the active site, breaking the bonds in the enzyme's tertiary structure and leading to denaturation. Thus, when the enzyme is not at its optimum pH, the substrate can no longer become attached to the active site and the enzyme-substrate complex cannot form.
• The concentration of competitive reversible inhibitors

  As the concentration of competitive reversible inhibitors increases, the rate of reaction decreases as the active sites are temporarily blocked by inhibitors so substrates cannot bind to them.
• Concentration of non-competitive reversible inhibitors

  As concentration on non-competitive reversible inhibitors increases, the rate of reaction decreases as the shape of the enzyme (not the active site) is altered by the inhibitors.
• Michaelis-Menten Equation


  The equation can be used to calculate the ​maximum rate of reaction (Vmax)​ ​by relating the ​velocity of enzyme reactions (V)​ ​to the concentration of a substrate [S].​ Vmax represents the maximum rate of reaction achieved by the system at maximum substrate concentration.
• Immobilising enzymes


  Attaching enzymes to an insoluble, inert material e.g. calcium alginate which forms a gel capsule around them, holding them in place during the reaction
• Enzymes in solution

  • Can only be used once
  • Very difficult and time-consuming to separate them from the product
• Immobilised enzymes


  Can be reused as they can be easily separated from the products
• Immobilising enzymes


  1. Attaching enzymes to an insoluble, inert material

  2. Forming a gel capsule around them
• Immobilised enzymes


  • Enable the reaction to flow continuously
  • Much cheaper than using enzymes in solution as they can be reused
• Immobilised enzymes are used in industry