ENZYMES

• Enzymes

  Organic (carbon-based) catalysts which increase the rate of chemical reactions
• Enzymes

  • Reusable / not consumed by the reaction
  • Highly specific (every enzyme has a 3D active site, one enzyme catalyses one reaction)
  • Most enzyme-catalysed reactions are reversible
  • Most enzymes are proteins
  • Names typically end with suffix '-ase'
  • Very sensitive to changes in temperature and pH
• Most enzymes are intercellular, a few act in extracellular locations (lysozyme in tears / amylase in saliva)
• Enzyme Specificity

  The most important component of an enzyme is the active site, which is a complex 3D shape that interacts with a specific substrate to catalyse a specific reaction
• Enzyme-Substrate Interaction

  1. Substrate binds to active site

  2. Active site undergoes conformational change to accommodate substrate

  3. Enzyme-substrate complex formed
• Lock and Key Model

  Enzyme acts as 'lock' and substrate as 'key', active site is rigid and fixed
• Induced Model

  Active site does not have a rigid shape, when substrate enters it induces a change in shape which enables optimum fit, empty active site returns to relaxed state
• Anabolic Reactions

  Substrates are built from smaller molecules, energy is required (endergonic)
• Catabolic Reactions

  Substrates (complex molecules) are broken down, energy is released (exergonic)
• Biochemical pathways maybe coupled (ie. a reaction that releases energy maybe coupled with a reaction that requires energy)
• Activation Energy


  Every chemical reaction requires energy to activate (start), if the reactants do not have sufficient energy, no reaction will occur
• Enzymes


  Lower the activation energy of chemical reactions
• Every chemical reaction requires an input of energy to start regardless of whether the reaction is anabolic or catabolic
• Factors affecting enzyme activity

  • Temperature
  • pH
  • Substrate concentration
  • Enzyme concentration
• When temperature increases


  Enzymes and substrates move faster and collide with each other more often, allowing reactions to occur faster
• Optimum temperature


  Each enzyme has a temperature at which it operates most effectively (in humans, this is approx. 37 oC)
• If temperature rises significantly above optimum

  Thermal denaturation can occur, active site of enzyme will be permanently altered (denaturation is irreversible / enzyme is inactivated)
• If temperature falls below optimum


  Enzyme and substrate will lack the energy to interact, when temperature rises, enzyme will reactivate and operate as normal
• Optimum pH


  The optimum pH at which enzymes function varies (some require acidic, others neutral or basic environment), most biological fluids have a pH of 6 - 8
• If an enzyme is exposed to a pH level which is above or below optimum

  It will denature and no longer function
• Substrate Concentration


  The concentration of enzyme compared to substrate affects the rate of reaction (enzyme concn higher than substrate, reaction will occur rapidly, enzyme concn lower than substrate, reaction will occur more slowly)
• Enzyme Inhibitors


  Molecules that bind to an enzyme and prevent it from performing its function
• Competitive Inhibition


  Inhibitor binds directly to active site of enzyme, prevents substrate from interacting with enzyme
• Non-competitive Inhibition


  Inhibitor binds to enzyme (at allosteric site) causing the active site to change shape, prevents substrate from interacting with enzyme
• Reversible Inhibition


  Enzyme is not permanently inhibited or damaged, bonds formed between enzyme and inhibitor are weak hydrogen bonds, therefore easily broken, can act competitively or non-competitively
• Irreversible Inhibition


  Enzyme is permanently inhibited and can no longer take part in reactions, bonds formed between enzyme and inhibitor are strong covalent bonds, usually only act competitively, irreversible inhibitors of enzymes = poisons (cyanide / nerve gas)
• End-product Inhibition (Feedback Inhibition)


  A way of regulating final product concentration, final product (isoleucine) is inhibitor of 'upstream' enzyme (threonine), when amount of inhibiting product is high, pathway slows down, when amount of inhibiting product is low, pathway speeds up
• Cofactors


  Additional components needed by some enzymes to enable them to catalyse a reaction
• Coenzymes


  Organic, non-protein molecules that are a subset of cofactors, needed for reactions in biochemical pathways such as cellular respiration and photosynthesis, during reaction they bind to active site of enzyme and donate energy or molecules, can be recycled
• Loaded Coenzyme


  A coenzyme that can release stored chemical energy by donating chemical groups (ATP, NADH and NADPH)
• Unloaded Coenzyme


  A loaded coenzyme that binds to an enzyme and releases energy (ADP, NAD+, and NADP+)