Enzymes

• what do enzymes do?

  • they are BIOLOGICAL CATALYSTS: they lower the activation energy of the reaction they catalyse, increasing the rate of reaction without being used up
  • e.g. in respiration (cellular level) and digestion (organism as a whole)
  • Enzyme action can be intracellular (within cells) or extracellular (outside cells)
• Which biological molecules are ENZYMES made of

  • they are proteins
  • so they are made of polypeptide chains made of amino acids
• ENZYMES: active sites

  • Part with a specific shape which is where the substrate molecules bind to the enzyme to form an enzyme-substrate complex
  • the substrate is complementary/specific to the active site
  • The active site shape is determined by the enzyme’s tertiary structure
• What is activation energy

  The minimum amount of energy required to start a chemical reaction
• How does an enzyme speed up the rate of reaction

  • enzymes lower the amount of activation energy required to start a reaction
  • forming enzyme-substrate complexes is what lowers the activation energy
  • making reactions happen at lower temperatures - they provide an alternate reaction pathway that requires less energy
• How does forming E-S complexes lower activation energy

  • CATABOLIC
  • if substrate molecules need to be joined
  • being attached to the enzyme holds them close together
  • this reduces repulsion between them and aligns them
  • so they can bond more easily/using less energy
  • ANABOLIC
  • If substrate molecules need to be broken down
  • binding to the active site puts a strain on bonds in the substrate
  • so the molecules break up more easily/using less energy
• lock and key model

  • enzyme's active site is perfectly complementary/specific to the substrate
  • therefore when the enzyme and substrate collide and bind, the enzyme's active site does not have to change shape at all
• induced fit model

  • enzymes are not rigid - its parts can move
  • active sites are NOT already complementary/specific to the substrate
  • the enzyme and the substrate will collide
  • THEN, the active site will change shape slightly to become complementary/specific to the substrate
  • this then puts strain/stress on the substrate, and lowers the activation energy
• how are enzyme properties related to their tertiary structure

  • active site is determined by enzyme's tertiary structure
  • different enzymes have different tertiary structures and so a different shaped active site
  • the shape of an active site is specific, so only 1 specific/complementary substrate will fit to an active site to form E-S complexes
  • therefore, an enzyme can usually only catalyse one reaction
• if the substrate shape doesn't match the active site...

  ...an enzyme-substrate complex won't be formed and the reaction won't be catalysed
• what happens if tertiary structure of enzyme is ALTERED

  • the shape of the active site will change
  • the substrate won't be specific/complementary to and won't fit into the active site
  • NO ENZYME-SUBSTRATE COMPLEXES WILL BE FORMED
  • the reaction won't be catalysed
• how can TERTIARY STRUCTURE/ACTIVE SITE SHAPE of an enzyme be ALTERED

  1. change in pH far from the optimum - denatures enzyme
  2. temperature too HIGH - denatures enzyme
  3. mutation in gene that codes for enzyme - change in primary and therefore tertiary structure
• factors affecting enzyme activity

  • TEMPERATURE
  • pH
  • ENZYME CONCENTRATION
  • SUBSTRATE CONCENTRATION
• factor affecting enzyme activity: TEMPERATURE

  • increasing temperature increases rate of reaction
  • because increased kinetic energy causing the particles to move faster
  • they have more frequent successful collisions and form more enzyme-substrate complexes
  • HOWEVER, if the temperature goes above the optimum, the enzyme can become DENATURED
  • due to bonds breaking/weakening
  • this changes the shape of the active site
  • enzyme and substrate are no longer complementary/specific and cannot bind
  • the enzyme no longer functions as a catalyst
• factors affecting enzyme activity: pH

  • any pH that is too far away from the optimum results in the enzyme becoming DENATURED
  • this is because the H+ and OH- ions can mess up hydrogen and ionic bonding in the enzyme's tertiary structure
  • the bonds can break/weaken, causing the active site to change shape
  • the enzyme and substrate no longer fit together
  • the enzyme has lost its function as a catalyst
• factor affecting rate of reaction: ENZYME CONCENTRATION

  • The more enzymes there are, the more likely a substrate and enzyme are to collide and form an E-S complex
  • THEREFORE, increasing the concentration of enzyme increases the rate of reaction
  • HOWEVER, if the amount of substrate is fixed, there will be a point where all the substrate molecules present have an enzyme to collide with
  • so adding more enzyme has no further effect on the rate of reaction
  • something else (e.g. temperature) becomes the limiting factor
• factors affecting enzyme activity: SUBSTRATE CONCENTRATION

  • increasing substrate concentration will increase the rate of reaction
  • collision between enzyme and substrate is more likely
  • increasing the number of E-S complexes
  • HOWEVER
  • this is only true up until a saturation point
  • where: adding more substrate does not change the rate of reaction because all the enzymes are being used
  • the concentration of substrate decreases over time
  • so the rate of reaction at the start of the reaction is the highest
• competitive inhibition

  • competitive inhibitor molecules have a similar shape to the substrate molecules
  • They compete with the substrate molecules to bind to the active site
  • but no reaction takes place
  • they block the active site, so no substrate molecules can fit, stopping E-S complexes from being formed
  • overall, the competitive inhibitor SLOWS down the rate of reaction, but eventually, the reaction is still catalysed
• non-competitive inhibition

  • Non-competitive inhibitor molecules bind to the enzyme away from its active site
  • This causes the active site to change shape
  • so the substrate molecules can no longer bind to it
  • They don't 'compete' with the substrate molecules to bind to the active site because they are a different shape
  • Increasing substrate concentration won't make a difference to the rate of reaction
  • enzyme activity will still be inhibited
• ENZYMES: rate of reaction for COMPETITIVE vs NON-COMPETITIVE inhibition

  • COMPETITIVE inhibition only SLOWS down the rate of reaction, the substrate is still catalysed
  • because eventually the inhibitor will move away from the active site
  • NON-COMPETITIVE inhibition STOPS enzyme activity completely
  • because the active site changes shape so it can no longer bind to the substrate
• ENZYMES: competitive inhibition vs non-competitive inhibition vs no inhibitors

  • :)

  A) competitive

  B) non-competitive