Lecture 1

• Effect of temperature on enzymes:


  • Rate generally increases until denaturation (protein unfolds)
  • Rate drops after denaturation
  • Optimum temperature suits environment (e.g, 37.C in humans)
  • Protein loses 3D structure & active site shape so no longer functional
  • Thermal denaturation usually irreversible
• Effect of pH on enzymes:

  • At high or low pH, enzymes denature & unfold
  • optimum pH suited to enzyme's environment
• Pepsin pH:


  1-2 (stomach)
• Chymotrypsin pH:


  around 8 (small intestine)
• What do inhibitors do?


  Slow rate down by binding to enzyme
• What do activators do?


  Increase rate by binding to enzyme
• Function of inhibitors & activators:


  Switch on/off regulatory enzymes that catalyse key steps in metabolic pathways (don't want all enzyme-catalysed reactions happening at same time-enzymes regulated according to cell's needs)
• What are allosteric regulatory enzymes?


  Can adopt 2 or more shapes

  Change shape in response to binding of small molecules which can stabilise a more or less active shape
• Which state do allosteric activators stabilise?


  Active state (makes enzyme more active)
• Which state do allosteric inhibitors stabilise?


  Inactive state (decreases enzyme activity)
• What are exergonic reactions?


  • Reactants at start are higher energy than products (change in energy negative)
  • Energetically going downhill
  • Reactions occur spontaneously (thermodynamically favourable)
• What are endergonic reactions?


  • energetically going uphill
  • Reactants have lower energy that products at start of reaction
  • Won't occur unless energy provided (not spontaneous)
  • (change in energy is positive)
• What is Aspartate transcarbamoylase?


  Regulatory enzyme- inhibited by binding of cytosine triphosphate (CTP) at regulatory sites (active site changes shape as inactive state stabilised)

  CTP is an end-product of pathway enzyme is part of (feedback inhibition mechanism used here)
• How are inhibitors important in medicine?


  • Drugs can act as enzyme inhibitors
  • 2 main types: reversible & irreversible inhibitors
• What do reversible inhibitors do?


  • Bind & release the enzyme

  Form non-covalent bonds with the enzyme

  Enzyme can be used again once inhibitor is removed
• What do irreversible inhibitors do?


  • Permanently alter activity

  Form covalent bonds with the enzyme

  Enzyme permanently inactivated-cell must make new enzyme molecules to reverse the effects
• What is Indinavir?


  • Reversible inhibitor of a HIV protease enzyme
  • Anti-AIDS drug
  • Binds non-covalently in active site of protease (structure is similar to that of a protein so can fit in active site)
• What is Penicillin?


  Irreversible inhibitor

  First antibiotic to be discovered

  Forms covalent bond with enzyme that makes bacterial cell walls (glycopeptide transpeptidase) (stops bacteria replicating)

  C-N bond reactive so forms covalent bond with enzyme (OH on serine)
• Effect of enzyme concentration on enzyme activity:


  Increasing enzyme concentration increases reaction rate

  (directly proportional)
• Effect of substrate concentration on enzyme activity:


  • Hyperbolic curve
  • Increasing substrate increases reaction rate initially until max velocity (VMax) reached & enzyme becomes saturated