1.4.2 Enzymes

• enzymes are complex chemicals that control reactions in living cells
• enzymes are biological catalysts that can be used over and over again
• enzymes speed up the rate of reactions by up to 10^12 times
• most enzymes end with -ase
• enzymes are globular proteins (tertiary structure)
• the reaction takes place in the active site
• enzymes are specific, they only bind to one substrate and control one reaction
• the substrate and the active site are complementary in shape
• when enzymes combine with the substrate they form a enzyme substrate complex
• many enzymes need other chemicals to function- coenzymes
• enzyme function can be slowed or stopped by inhibitors
• enzyme +substrate = enzyme-substrate complex = enzyme +products
• lock and key hypothesis

  • rigid model where the shape of the active site is the inverse shape of the substrate. the active site is complementary to the substrate
  • sub rate fits into this precise shape forming the ESC
• Induced fit hypothesis

  • active site is not rigid shape
  • substrate is attracted to the active site
  • causes the active site to change shape and it moulds around the substrate to become fully complementary
  • ESC is formed and causes bonds to form/break
  • products have a different shape so are no longer complementary to the active site
  • so the products detach
• how are both hypotheses similar

  • substrate fits to active site, ESC formed
• how are both hypotheses different

  • IF - active site changes shape slightly to become fully complementary to the substrate
  • LK- active site stays the same/ is rigid and is already complimentary to substrate
• activation energy- the energy required to break bonds
• enzymes lower the activation energy
• factors affecting the rate of reaction

  • temperature
  • pH
  • enzyme concentration
  • substrate concentration
• inhibitors

  • competitive
  • non competitive
• competitive inhibitors

  • similar shape to substrate
  • bind to active site of enzyme
  • reduces number of ESC
  • increasing substrate concentration can overcome the effects of the inhibitor
• non competitive inhibitor

  • binds to enzyme at allosteric site
  • changes it’s shape, changes show of active site
  • substrate no longer fits
  • reduces/ no ESC
  • increasing substrate concentration can’t overcome the effects of the inhibitor
• Temperature

  Low temp = low rate

  enzymes have low kinetic energy so few successful collisions, inactive and low ESC

  
  

  Optimum temp=high rate

  enzymes have high kinetic energy, lots of successful collisions, high ESC

  
  

  High temp= low rate

  heat breaks bonds and tertiary structure, active site changes shape- substrate no longer fits, no successful collisions, low ESC

  
• pH

  low pH== low rate

  acidity breaks bonds and tertiary structure, active site denatures, no collisions, low ESC

  optimum pH= high rate

  many successful collisions, many ESC

  high pH= low rate

  alkalinity breaks bonds and tertiary structure, active site denatures, no collisions, low ESC

  
• Enzyme concentration

  low enzyme concentration= low rate

  few active sites, low successful collisions, few ESC formed

  enzyme concentration increases= rate increases

  more active sites, higher chance of successful collisions, more ESC

  Rate levels off, substrate mols are all able to find an active site

  
• Substrate concentrations

  low substrate concentrations= low rate

  few active sites, less successful collisions, few ESC formed

  substrate concentrations increase= rate increases

  more active sites, higher chance of successful collisions, more ESC

  rate levels off, substate mols ae all able to find an active site