Enzymes

• What are enzymes made from?

  Protein - they have primary, secondary and tertiary structures.
• What codes for making an enzyme?

  A gene - codes for the polypeptide that makes up the enzyme.
• What is the function of enzymes and how is this carried out?

  To catalyse a wide range of intracellular and extracellular reactions by lowering their activation energies.
• What does it mean for an enzyme to be specific?

  Enzymes are specific to only one substrate as they have complementary-shaped active sites to only that molecule. This is due to each having a unique tertiary structure formed from the differing locations and presences of specific R groups on amino acids in the original polypeptide chain that determined where bonds and folding formed.
• What is the lock and key hypothesis?


  An enzyme with a complementary-shaped active site to a specific substrate (also with a complementary shape) will bind to form an enzyme-substrate complex, after colliding. The products of the reaction are then released and the enzyme remains unchanged.
• What is the induced fit hypothesis?


  The active site of an enzyme doesn't have a complementary shape to its substrate however the binding of the substrate to the enzyme causes a change in the tertiary structure of the enzyme and changes the shape of the active site. This makes the active site now complementary to the substrate, so an enzyme-substrate complex forms and products are released from the reaction. Following this, the enzyme returns to its original shape.
• How do competitive inhibitors work?


  They bind to the active site of an enzyme due to having a similar shape to its substrate. As a result fewer enzyme-substrate complexes form and fewer products are released.

  To reduce the effect of a competitive inhibitor, increase the substrate concentration to 'dilute' it.
• How do non-competitive inhibitors work?


  They bind to a site other than the active site (an allosteric site), altering the tertiary structure of the enzyme. This changes the shape of the active site so that it is no longer complementary to the substrate and so the substrate cannot bind. As a result, fewer enzyme-substrate complexes form and so fewer products are released.

  The effect of non-competitive inhibitors cannot be reduced by increasing substrate concentration.
• How does enzyme concentration affect the rate of an enzyme-controlled reaction?

  As enzyme concentration increases, rate of reaction increases as more enzyme-substrate complexes can form at one time.
• How does substrate concentration affect the rate of an enzyme-controlled reaction?

  As substrate concentration increases, rate of reaction increases as collisions are more frequent, so more enzyme-substrate complexes form.

  This however is limited by the number of enzymes as they will eventually reach a point of saturation, where all active sites are in use at one time. Therefore increasing substrate concentration will have no effect after this point.
• How does concentration of competitive and non-competitive inhibitors affect the rate of an enzyme-controlled reaction?

  As concentrations of both increase, rate of reaction decreases as fewer enzyme-substrate complexes are able to form.

  The rate of reaction is typically reduced more however by non-competitive inhibitors.
• How does pH affect the rate of an enzyme-controlled reaction?

  pH that is too low means that conditions are too acidic and so there is a high concentration of H+ ions

  pH that is too high means that conditions are too alkaline and so there is a high concentration of OH- ions

  These ions can alter ionic bonds and hydrogen bonds in the tertiary structure of the enzyme, altering it (it denatures). As a result its active site changes so that it is no longer complementary to the substrate, so fewer enzyme-substrate complexes form and rate of reaction decreases.

  Therefore enzymes have optimum pHs.
• How does temperature affect the rate of an enzyme-controlled reaction?

  As temperature increases, rate of reaction increases as enzymes and substrates have more kinetic energy, so collisions are more frequent and more enzyme-substrate complexes are formed.

  At too high temperatures however, high kinetic energy levels break hydrogen bonds within the tertiary structure of the enzyme, altering it. As a result it becomes denatured and the active site is no longer complementary to the substrate, so fewer enzyme-substrate complexes can form.

  Therefore enzymes have optimum temperatures.