1.4.2 Many proteins are enzymes

• How do enzymes act as biological catalysts?

  ● Each enzyme lowers activation energy of reaction it catalyses

  ● To speed up rate of reaction
• Describe the induced-fit model of enzyme action

  1. Substrate binds to (not completely complementary) active site of enzyme
  2. Causing active site to change shape (slightly) so it is complementary to substrate
  3. So enzyme-substrate complex forms
  4. Causing bonds in substrate to bend / distort, lowering activation energy
• Describe how models of enzyme action have changed over time

  ● Initially lock and key model (now outdated)

  ○ Active site a fixed shape, complementary to one substrate

  ● Now induced-fit model
• Explain the specificity of enzymes

  ● Specific tertiary structure determines shape of active site

  ○ Dependent on sequence of amino acids (primary structure)

  ● Active site is complementary to a specific substrate

  ● Only this substrate can bind to active site, inducing fit and forming an enzyme-substrate complex
• Describe and explain the effect of enzyme concentration on

  the rate of enzyme-controlled reactions

  ● As enzyme conc. increases, rate of reaction increases

  ○ Enzyme conc. = limiting factor (excess substrate)

  ○ More enzymes so more available active sites

  ○ So more enzyme-substrate (E-S) complexes form

  ● At a certain point, rate of reaction stops increasing / levels off

  ○ Substrate conc. = limiting factor (all substrates in use)

  
• Describe and explain the effect of substrate concentration on

  the rate of enzyme-controlled reactions

  ● As substrate conc. increases, rate of reaction increases

  ○ Substrate conc. = limiting factor (too few enzyme molecules to occupy all active sites)

  ○ More E-S complexes form

  ● At a certain point, rate of reaction stops increasing / levels off

  ○ Enzyme conc. = limiting factor

  ○ As all active sites saturated / occupied (at a given time)

  
• Describe and explain the effect of temperature on the rate of enzyme-controlled reactions

  ● As temp. increases up to optimum, rate of reaction increases

  ○ More kinetic energy

  ○ So more E-S complexes form

  ● As temp. increases above optimum, rate of reaction decreases

  ○ Enzymes denature - tertiary structure and active site change shape

  ○ As hydrogen / ionic bonds break

  ○ So active site no longer complementary

  ○ So fewer E-S complexes form

  
• Describe and explain the effect of pH on the rate of enzyme-controlled reactions

  ● As pH increases / decreases above / below an optimum, rate of reaction decreases

  ○ Enzymes denature - tertiary structure and active site change shape

  ○ As hydrogen / ionic bonds break

  ○ So active site no longer complementary

  ○ So fewer E-S complexes form

  
• Describe and explain the effect of concentration of competitive inhibitors on the rate of enzyme-controlled reactions

  ● As concentration of competitive inhibitor increases, rate of reaction decreases

  ○ Similar shape to substrate

  ○ Competes for / binds to / blocks active site

  ○ So substrates can’t bind and fewer E-S complexes form

  ● Increasing substrate conc. reduces effect of inhibitors (dependent on relative concentrations of substrate and inhibitor)
• Describe and explain the effect of concentration of non-competitive inhibitors on the rate of enzyme-controlled reactions

  ● As concentration of non-competitive inhibitor increases, rate of reaction decreases

  ○ Binds to site other than the active site (allosteric site)

  ○ Changes enzyme tertiary structure / active site shape

  ○ So active site no longer complementary to substrate

  ○ So substrates can’t bind so fewer E-S complexes form

  ● Increasing substrate conc. has no effect on rate of reaction as change to active site is permanent

  
• RP1: Give examples of variables that could affect the rate of an enzyme-controlled reaction

  ● Enzyme concentration / volume

  ● Substrate concentration / volume

  ● Temperature of solution

  ● pH of solution

  ● Inhibitor concentration
• RP1: Describe how temperature can be controlled.


  ● Use a thermostatically controlled water bath

  ● Monitor using a thermometer at regular intervals and add hot / cold water if temperature fluctuates
• RP1: Describe how pH can be controlled.


  ● Use a buffer solution

  ● Monitor using a pH meter at regular intervals
• RP1: Why were the enzyme & substrate solutions left in the water bath for 10 mins before mixing?

  ● So solutions equilibrate / reach the temperature of the water bath
• RP1: Describe a control experiment.

  ● Use denatured enzymes (eg. by boiling)

  ● Everything else same as experiment, eg. same conc. / volume of substrate (at start) and enzyme, same type / volume of buffer solution, same temperature
• RP1: Measure the rate of an enzyme-controlled reaction

  1. Measure time taken for reaction to reach a set point (concentration / volume / mass / colour of substrate or product)

  2. Measure concentration / volume / mass / colour of substrate or product at regular intervals (or using a continuous data logger) throughout reaction

  3. Plot on a graph with time on the x axis and whatever is being measured on the y axis

  4. Draw a tangent at t = 0 (or any other time for rate at a particular point)

  5. Initial rate of reaction = change in y / change in x
• RP1: Rate of reaction


  1 / time; example units = s^-1
• RP1: Initial rate of reaction


  change in y / change in x; example units = cm^3 s^-1
• RP1: Suggest a safety risk and explain how to reduce this risk.

  ● Handling enzymes may cause an allergic reaction

  ● Avoid contact with skin by wearing gloves and eye protection
• RP1: Explain why using a colorimeter to measure colour change is better than comparison to colour standards.

  ● Not subjective

  ● More accurate
• RP1: Explain a procedure that could be used to stop each reaction.


  ● Boil / add strong acid / alkali → denature enzyme

  ● Put in ice → lower kinetic energy so no E-S complexes form

  ● Add high concentration of inhibitor → no E-S complexes form
• RP1: Describe how processed data can be presented as a graph


  ● Independent variable on x axis, rate of reaction on y axis, including units

  ● Linear number sequence on axis, appropriate scale (graph should cover at least half of grid)

  ● Plot coordinates accurately as crosses

  ● Join point to point with straight lines if cannot be certain of intermediate values OR draw a smooth curve but do not extrapolate
• RP1: Explain why the rate of reaction decreases over time throughout each experiment


  ● Initial rate is highest as substrate concentration not limiting / many E-S complexes form

  ● Reaction slows as substrate used up and often stops as there is no substrate left