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  • globular proteins that act as biological catalysts
  • speed up metabolic reactions
  • break down large molecules or build up small molecules
  • tertiary globular proteins where the protein chain folds back on itself to a spherical/globular shape
  • hydrophilic R group on outside- soluble
  • each has own sequence of amino acids
  • held in tertiary form by hydrogen bonds, disulphide bridges and ionic bonds
  • complex and precise 3D shape, only active site region is functional
  • react with substrates
  • each enzyme has own special region of active site which is specific to one substrate type
  • enzymes and substrates form enzyme substrate complexes
  • temporary bonds hold substrate in place
  • products are released when process is complete
  • enzyme is unchanged
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Properties & Models

  • enzymes are specific to one type of substrate- active site a particular shape to fit only that substrate
  • enzymes have a high turnover number- very fast and convert many molecules per unit of time
  • lower activation energy- reactions need energy to start, this is activation energy, increaseing the temp is not possible in living organisms. reactants need to collide with sufficient energy. enzymes help by altering the substrate to help metabolic processes take place at lower temps

lock & key

  • each key (substrate) has a specific shape and only fits into a particular lock (enzyme)

induced fit

  • enzymes shape being altered by binding molecule
  • enzymes are flexible
  • enzyme has a general shape whcih alters slightly to accomidate the substrate
  • the change in shape of enzyme places strain on substrate, distorting bonds to lower the activation energy to break it
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Enzyme Controlled Reaction

  • when enzyme and substrate mix initially, there are a large amount of substrate molecules
  • both molecules are in constant motion
  • substrate molecules come into contact with active sites
  • active sites become occupied with substrate, rapid breakdown to products
  • decreasing amount of substrate increased amount of product
  • all substrate used, no further reaction or products can be formed
  • enzymes can live intracellularly or extracellularly 
  • environmental conditions effect functioning
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Temperature & pH Effects

  • rise in temp= rise in kinetic energy= molecules move faster=more successful collisions
  • more collisions of the enzyme and substrate means more enzyme substrate complexes and an increased rate of reaction 
  • rate of reaction doubles for every 10oC increase
  • optimum temp of 40oC 
  • after optimum temperature the vibrations of molecules breaks hydrogen bonds
  • alters tertiary structure- active site changes shape and substrate can no longer fit
  • enzyme is denatured permanently
  • freezing temperatures inactivate the enzymes so they are dormant due to no kinetic energy


  • small changes in pH outside optimum cause reversible changes resulting in inactivation
  • extremes of pH denature an enzyme
  • charges on amino acid side chains of the active site attract hydrogen/hydroxyl ions
  • charge on active site must match substrate
  • too many H+ ions in the active site will alter the charge and repel the substrate
  • at extremes the hydrogen bonding in the 3D shape is affected and the active site changes
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Substrate & Enzyme Concentration

  • rate will vary with changes to substrate concentration
  • with constant enzyme concentration, rate of reaction will increase as substrate concentration increases
  • at low sc enzymes are limited and the active sites are not working at full capacity
  • with increasing the sc there will be a point where every active site is filled and the rate of reaction is at a maximum

enzyme concentration

  • enzymes are reusable, therefore only a small amount of enzymes are needed to catalyse a large number of substrate molecules
  • turnover number= the amount of substrate an enzyme can turn into products in one minute
  • increasing enzyme concentration increases the rate of reaction because of the increased number of active sites
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when enzyme action is slowed down or stopped by another substance, by an inhibitor combining directly or indirectly preventing it from making enzyme substrate complexes


  • molecular shape similar to substrate, occupy active site in its plac
  • compete with substrate for active site
  • malonic acid
  • increasing substrate concentration reduces the effect of competitive inhibitors
  • more substrate= increased chance of enzyme substrate complexes


  • bind to enzyme at a site not the active site
  • alters shape of enzyme so active site can no longer accomidate a substrate
  • not competing for active site , an increase in substrate concentration will not affect it
  • cyanide
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Immobilised Enzymes

  • enzyme molecules fixed, bound or trapped in an inert matrix such as a gel capsule
  • these can then be packed into a column, have substrate added and react as it passes through
  • once set up it can be used repeatedly
  • product need not be purified to separate enzymes 
  • immobilising enzymes creates a microenvironment for reactions to occur at higher temperatures
  • activity is increased so production is increased
  • enzymes can tolerate a wider range of conditions
  • easily recovered for reuse reducing costs
  • several enzymes with different pH and temperature optima can be used together
  • enzymes can be easily added or removed so greater control
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  • work on the principle that enzymes are specific and able to select one type of molecule from a mixture even in low concentrations
  • used in the rapid and accurate detection of minute traces of biolologically important molecules
  • great potential in areas of medical diagnosis and environmental monitoring
  • electrode probe detects changes in substrate/product/temperature/optical properties
  • blood sugar in diabetics
  • electrode probe= specific enzyme immobilised in a membrane placed in blood sample
  • glucose diffuses through membrane,
  • forms enzyme substrate complex,
  • producing small electrical current,
  • picked up by electrode transducer,
  • current is read by a meter producing a reading
  • blood contains a mixture of molecules, electrode placed in sample, glucose diffuses through and reacts, oxygen is taken up, rate of oxygen uptake is proportional to glucose concentration, digital display shows accurate reading
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