1.5

• Enzymes

  • Biological catalysts which increase the rate of a chemical reaction by lowering the activation energy of the reactions they catalyse, including both anabolic and catabolic and intracellular and extracellular reactions
• How is initial rate of reaction found from a graph?

  Can be measured by calculating the gradient of a concentration-time graph at t=0
• How does enzyme concentration affect rate of reaction?

  The rate of reaction increases as enzyme concentration increases as there are more active sites for substrates to bind to. However, increasing the enzyme concentration beyond a certain point has no effect on the rate of reaction as there are more active sites than substrates so substrate concentration becomes the limiting factor.
• How does substrate concentrationaffect rate of reaction?

  As concentration of substrate increases, rate of reaction also increases as more enzyme-substrate complexes are formed. However, beyond a certain point, the rate of reaction no longer increases as enzyme concentration becomes the limiting factor.
• Temperature

  The rate increases as kinetic energy increases, up to the optimum temperature (the temperature each enzyme works best at). Rate of reaction decreases beyond the optimum temperature because enzymes become denatured as hydrogen bonds are broken within the protein.
• Inhibitors

  Substances which stop the enzyme from binding to its substrate, therefore controlling the progress of a reaction. Inhibition may be reversible or irreversible.
• Categories of inhibition

  • Competitive inhibition
  • Non-competitive inhibition
• Competitive inhibition

  An inhibitor molecule competes with the substrate for binding to the active site of the enzyme, therefore preventing the substrate from binding. It can be reversed by increasing the substrate concentration.
• Non-competitive inhibition

  An inhibitor doesn't bind to the active site but binds to a different part of the enzyme (the allosteric site) and changes the shape of the enzyme. This decreases the reaction rate as the active site doesn't fit the substrate and the substrate cannot bind to the enzyme. It cannot be reversed by increasing substrate concentration.
• Describe the structure of enzymes

  Globular proteins - their specific tertiary structure determines shape of active site, complementary to specific substrate.
• What are the 5 factors that affect the rate of enzyme-controlled reactions?

  Enzyme concentration

  Substrate concentration

  Concentration of inhibitors

  pH

  Temperature
• What happens if an enzyme is denatured?

  Active site is no longer complementary to substrate (will no longer fit)
• How does pH affect rate of reaction?

  Enzymes have a narrow optimum pH range and outside this range denaturation occurs.
• How do competitive inhibitors work?

  They bind to active site since they have similar shape to substrate, this temporarily prevents enzyme-substrate complexes from forming until released
• What does increasing substrate concentration do to competitive inhibitors effect?

  Increasing substrate concentration decreases their effect
• How do non-competitive inhibitors work?

  They bind at allosteric binding site, triggering conformational change of active site and prevents the enzyme from functioning
• What impact does increasing substrate concentration have on non-competitive inhibitors effect?

  It has no impact on their effect
• What is an end-product inhabitation?

  One of the products of a reaction which acts as a competitive or non-competitive inhibitor for an enzyme, preventing further formation of products
• Why is it important/advantageous to calculate initial rate?

  Because it represents maximum rate of reaction before concentration of reactants decreases and end-product inhibition
• What is the concept of substrate specificity?

  The ability of an enzyme to catalyse only a specific reaction or set of reactions which have substrates complementary to the active site of the enzyme
• What is the concept of induced-fit hypothesis?

  A model of enzyme action that describes how once a specific substrate binds to the active site, the enzyme useegoes subtle conformational changes to fit the substrate better
• What is the concepts of the lock and key hypothesis?

  A model of enzyme action that describes how the enzym will only fit a substrate that has a correct complementary shape to the active site