Enzymes

• How do enzymes act as biological catalysts

  ●Each enzyme lowers activation energy of reaction it catalyses

  ●To speed up rate of reaction
• Describe the induced-fit model of enzyme action

  Substrate binds to (not completely complementary) active site of enzyme

  Causing active site to change shape (slightly) so it is complementary to substrate

  So enzyme-substrate complex forms

  Causing bonds in substrate to bend / distort, lowering activation energy
• Describe how models of enzyme action have changed over time

  ●       Initially lock and key model (now outdated) where the active site was a fixed shape, complementary to one substrate

  ●       Now induced-fit model
• Explain the specifity of enzymes


  Specific tertiary structure determines shape of active site - Dependent on sequence of amino acids (primary structure)

  Active site is complementary to a specific substrate

  Only this substrate can bind to active site, inducing fit and forming an enzyme-substrate complex
• Enzymes catalyse a wide range of intracellular and extracellular reactions that determine structures and functions from cellular to whole-organism level
• Effect of enzyme concentration on the rate of enzyme-controlled reactions

  1. As enzyme concentration increases, rate of reaction increases because enzyme concentration is the limiting factor so more available enzyme active sites so more enzyme-substrate complexes form.

  2. At a certain point, rate of reaction stops increasing and levels off because the substrate concentration is the limiting factor and all the substrates are in use
• Effect of substrate concentration on the rate of enzyme-controlled reactions

  1. As substrate conc. increases, rate of reaction increases because substrate concentration is the limiting factor so More enzyme-substrate complexes form2. At a certain point, rate of reaction stops increasing / levels off
• Enzyme concentration as a limiting factor


  • All active sites saturated / occupied (at a given time)
• Effect of temperature on the rate of enzyme-controlled reactions


  1. As temp. increases up to optimum, rate of reaction increases

  2. As temp. increases above optimum, rate of reaction decreases
• Effect of temperature on enzymes


  • More kinetic energy
  • Enzymes denature - tertiary structure and active site change shape
  • As hydrogen / ionic bonds break
  • So active site no longer complementary
  • So fewer E-S complexes form
• Effect of concentration of competitive inhibitors on the rate of enzyme-controlled reactions


  1. As concentration of competitive inhibitor increases, rate of reaction decreases

  2. Similar shape to substrate

  3. Competes for / binds to / blocks active site

  4. So substrates can't bind and fewer E-S complexes form
• Effect of pH on the rate of enzyme-controlled reactions


  As pH increases / decreases above / below an optimum, rate of reaction decreases
• Effect of pH on enzymes


  • Enzymes denature - tertiary structure and active site change shape
  • As hydrogen / ionic bonds break
  • So active site no longer complementary
  • So fewer E-S complexes form
• Increasing substrate concentration


  Reduces effect of inhibitors (dependent on relative concentrations of substrate and inhibitor)
• Effect of concentration of non-competitive inhibitors on the rate of enzyme-controlled reactions


  1. As concentration of non-competitive inhibitor increases, rate of reaction decreases

  2. Binds to site other than the active site (allosteric site)

  3. Changes enzyme tertiary structure / active site shape

  4. So active site no longer complementary to substrate

  5. So substrates can't bind so fewer E-S complexes form
• Increasing substrate concentration has no effect on rate of reaction as change to active site is permanent