Enzymes

• What are enzymes?

  - Enzymes lowers the activation energy of the reaction it catalyses thus speeding up the rate of reaction

  - Enzymes are biological catalysts
• Lock and key model (outdated)


  - Active site is a fixed shape / doesn't change shape; it is complementary to one substrate

  - After a successful collision, an enzyme-substrate complex forms, leading to a reaction
• Induced-fit model


  1. before reaction, enzyme active site not completely complementary to substrate

  2. Active site shape changes as substrate binds and enzyme-substrate complex forms

  3. This distorts bonds in substrate leading to a reaction
• Specifity of enzymes


  - Enzymes have a specific shaped tertiary structure and active site

  - Sequence of amino acids (primary structure) determines tertiary structure

  - Active site is complementary to specific substrate

  - Only this substrate can bind to the active site, inducing fit and forming an enzyme-substrate complex
• Factors affecting rate of reaction: Enzyme concentration


  More enzymes -> more available active sites

  More successful E-S collisions and E-S complexes

  At a certain point, rate of reaction plateaus as substrate concentration = limiting factor
• Factors affecting rate of reaction: Substrate concentration


  More successful E-S collisions and E-S complexes

  At a certain point, rate of reaction plateaus as all active sites are saturated
• Factors affecting rate of reaction: Temperature


  Increasing temp. up to optimum -> rate of reaction increases

  - Increase in kinetic energy

  - More successful E-S collisions and E-S complexes

  Increasing temp. above optimum -> rate of reaction decreases

  - Enzymes denature; tertiary structure and active site change shape (hydrogen/ionic bonds break)

  - fewer E-S collisions and E-S complexes
• Factors affecting rate of reaction: pH


  pH above/below optimum pH -> rate of reaction decreases

  - Enzymes denature; tertiary structure and active site changes shape (hydrogen/ionic bonds break)

  - Complementary substrate can no longer bind to active site

  - Fewer E-S collisions and E-S complexes
• Factors affecting rate of reaction: Concentration of competitive and non-competitive inhibitors


  Competitive inhibitors decrease rate of reaction

  - Similar shape to substrate

  - Competes for/binds to/blocks active site so substrates can't bind

  - Fewer E-S complexes

  - Increasing substrate conc. reduces effect of inhibitor

  Non-Competitive inhibitors decrease rate of reaction

  - Binds to site way from the active site (allosteric site)

  - Enzyme tertiary structure/active site changes shape so substrate can't bind to active site

  - Fewer E-S complexes

  - Increasing substrate conc. has no effect on rate of reaction as causes permanent change to active sites