2.4:ENZYMES

• enzymes are biological catalysts made up of globular proteins
• the active site has a specific shape due to the folding and bonding of the tertiary structure of the protein
• due to the specific shape of an enzymes active site the substrate has to be complementary in shape to the active site to bind to enzyme
• enzymes can catalyse intracellular and extracellular cells
• enzymes catalyse reactions by lowering the activation energy
• activation energy refers to the minimum amount of energy required to start a reaction
• when enzymes attach to the substrate they lower the activation energy required resulting in a faster reaction
• the lock and key hypothesis suggest the enzyme is like a lock and the substrate is like a key that fits into the lock due to the enzymes tertiary structure resulting in a complementary shape
• lock and key model also suggest that when substrates attach to enzymes they form enzymes substrate complexes and the charged groups with the active site were thought to have distort the substrate therefore lowering activation energy
• induced fit model suggests that as the substrate binds it causes slight changes within the enzyme which results in active site to mould around the substrate to become complementary
• the induced fit model suggest that when the enzyme substrate complexes are formed it puts strain on the bonds therefore lowering activation energy
• factors affecting enzyme controlled reactions

  temperature

  pH

  substrate concentration

  enzyme concentration
• how does temperature affect enzymes?

  • at low temperatures there is insufficient kinetic energy for successful collisions to occur
  • high temperatures cause enzymes to denature resulting in active site changing shape and less enzyme substrate complexes being formed
  • High temperatures break bond in tertiary structure and cause 3d shape to change
• the Q10 temperature coefficient is a measure of rate of change of an enzyme controlled reaction as a result of increasing temperature by 10 degrees
• Q10 = rate of reaction at temperature (Initial+10)/ rate of reaction at a temp of x(initial)
• how does pH affect enzymes?

  • too high or too low a pH will interfere with the charges in the amnio acids in the active site
  • too high or too low a pH causes hydrogen and ionic bonds to break altering the tertiary 3D structure changing shape of active site and enzyme denatures
• enzymes have different optimal pH values they work at
• how does substrate concentration affect enzymes controlled reaction?

  • if there is a low concentration of substrates reaction will lower as fewer enzyme substrate complexes are being formed
  • increasing substrate concentration will increase rate of reaction as it is more likely for successful collision to occur forming more enzyme substrate complexes
  • at high substrate concentration rate of reaction will plateau (level off) because all enzyme active sites are in use
  • substrate concentration does NOT cause denaturation of enzyme
• how does enzyme concentration affect enzyme controlled reaction?

  • at low enzyme concentration there will be a low rate of reaction as there are fewer enzyme active sites for substrates to bind to
  • increasing enzyme concentration will increase rate of reaction as there is more enzyme substrate complexes being formed
  • at high enzyme concentration, unless more substrate is added, rate of reaction ill plateau as there is insufficient amount of substrate to bind to large amount of enzyme
• competitive inhibitors are the same shape as the substrate, so they are complementary and can bind to the active site
• competitive inhibitors form enzyme inhibitor complexes and prevent the substrate from binding to the enzyme and lowers rate of reaction
• most competitive inhibitors are reversible and can be removed
• if a high concentration of substrate was added they would collide and with the inhibitors knocking the inhibitor out of the enzyme allowing substrates to bind to active site and increase rate of reaction
• at high concentration of substrates competitive inhibitors have no impact
• non competitive inhibitors bind to the enzyme at the allosteric site
• once non competitive inhibitor binds to allosteric site it causes the tertiary structure of enzyme to change shape changing shape of active site therefore substrate can no longer bind regardless
• non competitive inhibitors cause fewer enzyme substrate complexes to form lowering rate of reaction
• end product inhibitors are products produced by the enzyme itself and are reversible
• end product inhibitors enable reactions to be controlled
• during end product inhibition if there is a lot of product present it will bind to the enzyme and inhibit it causing the reaction to slow or stop
• end product inhibition prevent resources being wasted
• cofactors and coenzymes are non protein molecules that bind to the active site to make it complementary to the substrate
• coenzymes are organic molecules
• cofactors are inorganic molecules ( don't contain carbon )
• prosthetic groups is a type of cofactor that permanently attach to an enzyme by covalent or non covalent forces
• holoenzyme is when the prosthetic group is attached to the apoenzyme
• apoenzyme is when the prosthetic group has been removed from the enzyme
• precursor activation is when enzymes in an inactive form require to be activated by cofactor this prevent enzyme from causing damage from within cells and ensures they are only used when needed
• enzyme activated by binding of cofactor causes a change in shape to tertiary structure of enzyme so active site becomes complementary to substrate
• the precursor protein is known as apoenzyme