Enzymes

Enzymes

Action of Enzymes
1. What are enzymes?
  1. Enzymes are biological catalysts that speed up chemical reactions by reducing the activation energy.
  2. They are globular proteins that have an active site which has a specific shape that is determined by the tertiary structure.
  3. The active site is complementary to the substrate, if it's not, it won't bind and the reaction won't be catalysed.
2. How an enzyme works
  1. Enzymes reduce the activation energy of a reaction, meaning the reaction will happen at a lower temperature, speeding up the rate of reaction.
    Annotations:
    - Activation Energy = the minimum amount of energy needed to be supplied for a reaction to occur.
  2. When a substrate binds to an enzyme's active site, an enzyme-substrate complex is formed.
    1. The formation of these is what lowers the activation energy.
  3. If two substrate molecules need to be joined, they are held by the enzyme which reduces repulsion so they can join more easily.
  4. If the enzyme is catalysing a breakdown reaction, fitting into the active sites strains the bonds in the substrate which means it breaks up more easily.
Enzyme Models
1. Lock and Key
  1. The substrate fits into the enzyme in the same way a key fits into a lock.
2. 'Induced Fit'
  1. This model helps to explain why enzymes are so specific.
  2. The substrate doesn't only have to be the right shape, it has to make the active site change shape slightly.
Factors Affecting Enzyme Activity
1. Temperature
  1. Increases the kinetic energy, making the molecules in the enzyme vibrate more.
  2. The increased vibration breaks some of the the bonds which changes the shape of the active site, denaturing the enzyme.
  3. The changed active site means the substrate will no longer fit.
2. pH
  1. If pH is too high, the increased H+ ions will bond with the negative R groups in the enzymes active site, meaning the substrate won't fit in.
    1. It can also break the bonds that hold the enzymes tertiary structure in place, denaturing the enzyme.
  2. Enzyme Concentration
    1. The more enzymes there are, the more likely a substrate molecule is to successfully collide.
    2. If the amount of substrate is limited, there comes a point when there's more than enough enzyme but not enough substrate so more enzyme has not effect.
3. Substrate Concentration
  1. The more substrate there is, the more likely there is to be a successful collision.
  2. At a saturation point, there are so many substrate molecules that the enzymes active sites are all full so adding more substrate has no effect.
Measuring Enzyme Activity
1. Measuring how fast the product appears.
  1. e.g. collect the gas produced and measure how fast it's given off.
2. Measure the disappearance of the substrate.
  1. e.g. regularly sample the solution and measure the time it takes for the substrate to disappear.
3. Describing an Experiment
  1. Describe method & apparatus
  2. Say what's being measured
  3. Describe how the independent variable is being varied
  4. Describe what variables are being kept constant.
  5. State that the experiment needs to be repeated at least 3 times.
  6. State that a control is needed
Cofactors/Coenzymes
1. Inorganic
  1. They help the enzyme and substrate bind together. They don't participate in the reaction so aren't used up or changed in any way.
2. Organic
  1. These are coenzymes. They participate in the reaction so are changed by it and end up as a 'second substrate'.
  2. They act as carriers, moving chemical groups between different enzymes. They are continually recycled during the process.
Inhibitors
1. Competitive
  1. They have a similar shape to that of the substrate molecule and they compete with the substrate to bind to the active site but no reaction takes place.
  2. They block the active site so no substrate can get in. If there is a high concentration of inhibitor then it will take up nearly all the active sites and hardly any of the substrate will fit in.
2. Non-Competitve
  1. They have a complementary shape to the secondary site. This causes the active site to change shape so the substrate molecules can no longer fit. They don't 'compete' with the substrate molecules to bind to the active site because they are a different shape.
  2. Increasing the concentration of substrate won't make any difference, the enzyme activity will still be inhibited.
3. Metabolic Poisons
  1. Cyanide = irreversible inhibitor of cytochrome c oxidase.
  2. Malonate = inhibits succinate dehydrogenase.
  3. Arsenic = inhibits the action of pyruvate dehydrogenase.
4. Drugs
  1. Some antivirals inhibit the enzyme reverse transcriptase
  2. Some antibiotics inhibit transpeptidase

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