Enzymes

• What are enzymes ?

  • Enzymes are biological catalysts - speed up reactions
  • vast majority of enzymes are globular proteins - soluble in water, generally more compact and round shape - have functional roles

  
• How do enzymes catalyse a reaction quicker?

  • they lower activation energy -
  • in a chemical reaction, a certain amount of energy needs to be supplied to the chemicals before the reaction will start - this is called the activation energy ( often provided as heat)
  • enzymes lower the the amount of activation energy needed, often making reactions happen at a lower temperature than they could without an enzyme - speeding up the rate of reaction
  • more substrate molecules have enough energy to cross the activation energy barrier and react
• What the induced fit model of enzyme action?

  • on the surface - the tertiary structure of the enzyme folds into a 3
• What is the induced fit model of enzyme action ?

  • on the surface - the tertiary structure of the enzyme folds into a 3D shape called the active site
  • The tertiary structure of the active site is complimentary to the structure of the specific substrate molecule
  • temporary bonds form between the substrate molecule and the amino acids of the surface of the active site -> help lower the activation energy of the reaction, increasing reaction rate
• What is the active site ?

  part of the enzyme where the substrate molecule attaches to form the enzyme substate complex
• What is the difference between the Lock and key model & Induced fit model ?

  Lock and key model = tertiary structure of active site does not change when substrate binds (not correct)

  Induced fit model = tertiary structure of enzyme changes as substrate approaches so active site molds around substrate
• 2 models of enzyme action ?

  1. Lock and key model
  2. Induced fit model
• activation energy graph:

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• what happens to molecules which are not the substrate in the induced fit model?

  • molecules which are not the substrate cannot form the correct bonds to the correct amino acids in the active site
  • because of this the tertiary structure of the enzyme does not change
  • the shape of the active site does not adjust to fit the molecule
  • enzymes are complimentary to their substrate
• What is an enzyme-substrate complex?

  when a substrate fits into the enzymes complimentary active site it forms an enzyme-substrate complex - its this that lowers the activation energy
• how does forming enzyme-substrate complexes lower activation energy and increase the rate of reaction?

  1. if 2 substrate molecules need to be joined, being attached the to enzyme holds them close together - reducing any repulsion between the molecules so they can bond more easily
  2. if the enzyme is catalysing a breakdown reaction, fitting into the active site puts a strain on the bonds in the substrate - so the substrate molecule breaks up more easily
• rate of an enzyme controlled reaction depends on ...

  the frequency of successful collisions between substrate molecules and active sites on enzymes (forming enzyme-substrate complexes)
• What are the two types of enzyme inhibitors:

  1. Competitive inhibitor
  2. non-competitive inhibitors
• What are competitive inhibitors / how do they work?

  • competitive inhibitor molecules have a similar shape to that of the substrate molecules
  • they compete with the substrate molecules to bind to the active site - but no reaction takes place
  • instead they block the active site , so no substrate molecule can fit in it

  
• How does the relative concentrations of inhibitor and substrate affect how much the enzyme is inhibited (for competitive inhibitors)?

  • if there is a high concentration of the inhibitor, it'll take up nearly all of the active sites and hardly any of the substrate will get to the enzyme (few successful collisions)
  • If there's a higher concentration of substrate, it means they are more likely to collide with enzymes and form enzyme-substrate complexes before the inhibitors - so increasing the concentration of substrate will increase the rate of reaction (up to a point)
• rate of reaction with inhibitors graph :

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• What are non-competitive inhibitors / how do they work?

  • Non-competitive inhibitors bind to the enzyme at an alternative/allosteric site away from its active site, which alters the shape of the active site and therefore prevents the substrate from binding to it - active site is no longer complimentary to the substrate
  • they don't 'compete' with substrate molecules to bind to the active site as they are a different shape

  A) allosteric site

  
• How does the relative concentrations of substrate and inhibitors affect how much the enzyme is inhibited? (non-competitive inhibitors)

  • For non-competitive inhibitors, increasing the substrate concentration cannot increase the rate of reaction once more, as the shape of the active site of the enzyme remains changed and enzyme-substrate complexes are still unable to form

  
• List 4 Factors affecting enzyme activity:

  1. Temperature
  2. pH
  3. Enzyme concentration
  4. Substrate concentration
• How does temperature affect enzyme activity (increases)?

  • the rate of an enzyme-controlled reaction increases when the temperature is increased. More heat -> more kinetic energy so molecules move faster
  • Higher frequency successful collisions between substrate molecules and active site of enzyme
  • More frequent enzyme-substrate complex formation
  • The energy of the collisions also increases - meaning each collision is more likely to result in a reaction
• How does temperature affect enzyme activity (too high -> decreases)?

  • if the temperature gets too high then the reaction stops
  • the rise in temperature makes the enzymes molecules vibrate more
  • if the temperature goes above a certain level, this vibration breaks some of the bonds that hold the enzyme in shape
  • the tertiary shape of the enzyme and therefore its active site changes shape and the enzyme and substrate no longer fit together
  • at this point the enzyme is denatured - it no longer functions as a catalyst
• Temperature effects on enzyme activity graph:

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• how does temperature affect enzyme activity (too low -> decreases) ?

  • Lower temperatures either prevent reactions from proceeding or slow them down:
  • Molecules move relatively slow
  • Lower frequency of successful collisions between substrate molecules and active site of enzyme
  • Less frequent enzyme-substrate complex formation
  • Substrate and enzyme collide with less energy, making it less likely for bonds to be formed or broken (stopping the reaction from occurring)
• How does pH affect enzyme activity?

  • enzymes have an optimum pH at which they operate best
  • Enzymes are denatured at extremes of pH
  • Hydrogen and ionic bonds hold the tertiary structure of the enzyme together
  • Below and above the optimum pH of an enzyme, solutions with an excess of H+ ions (acidic solutions) and OH- ions (alkaline solutions) can cause these bonds to break
  • This alters the shape of the active site, which means enzyme-substrate complexes form less easily
  • Eventually, enzyme-substrate complexes can no longer form at all
  • At this point, denaturation of the enzyme has occurred
• Where an enzyme functions can be an indicator of its optimal environment....

  most human enzymes work best at pH 7 (neutral) - there are exceptions

  • E.g. pepsin is found in the stomach, an acidic environment at pH 2 (due to the presence of hydrochloric acid in the stomach’s gastric juice)
  • Pepsin’s optimum pH is pH 2
• pH effect on enzyme activity graph

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• • When investigating the effect of pH on the rate of an enzyme-catalysed reaction, you can use buffer solutions to measure the rate of reaction at different pH values, why?
  • Buffer solutions each have a specific pH
  • Buffer solutions maintain this specific pH, even if the reaction taking place would otherwise cause the pH of the reaction mixture to change
  • A measured volume of the buffer solution is added to the reaction mixture
  • This same volume (of each buffer solution being used) should be added for each pH value that is being investigated
• How does enzyme concentration affect the rate of reaction?

  1. The more enzyme molecules there are in a solution , the more likely a substrate molecule is to collide with one and form a enzyme-substrate complex (more successful collisions) and therefore increasing the concentration of the enzyme increases the rate of reaction
  2. but if the amount of substrate is limited (limiting factor), there comes a point when there's more than enough enzyme molecules to deal with all the available substrate , so adding more enzyme has no further effect
• How does substrate concentration affect the rate of reaction?

  • the higher the substrate concentration , the faster the reaction - more substrate molecules means a collision between a substrate and enzyme molecule is more likely and so more active sites will be used.
  • This is only true up to a 'saturation' point though - after that there are so many substrate molecules that the enzymes have as much as they can cope with (all active sites are full) and adding more makes no difference.
• How does substrate concentration affect the rate of reaction p2?

  • substrate concentration decreases with time during a reaction (unless more substrate is added to the reaction mixture)if no other variables are changed, the rate of reaction will decrease overtime too
  • This makes the initial rate of reaction (the reaction rate at the start) the highest rate of reaction
• graph presenting how the concentration of enzyme affects the rate of reaction...

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• graph presenting how the concentration of substrate affects the rate of reaction....

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