As Biology - Enzymes

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Biological catalysts

  • Catalyse metabolic reactions e.g. digestion and respiration
  • Even your phenotype ( physical appearance ) is determined by enzmyes that catalyse the reactions that cause growth and development
  • Enzyme action can be intracellular - within cells, or extracellular - outside cells (e.g. in places like the blood and digestive system)
  • Enzymes are globular proteins
  • Enzymes have an active site, which has a specific shape. The active site is the part of the enzyme where the substrate molecules ( the substance that enzymes interact with ) bind to.
  • Enzymes are highly specific due to their tertiary structure

Lower the activation energy of a reaction

  • In a chemical reaction a certain amount of energy needs to be supplied to the chemicals before the reaction will start.
  • This is called activation energy - often provided as heat.
  • Enzymes lower the amount of activation energy needed, often making reactions happen at lower temperatures than they would without an enzyme - this speeds up the rate of reaction.
  • When a substrate fits into the enzyme's active site it forms an enzyme-substrate complex - it's this that lowers the activation energy
  • Here are 2 reasons why:
    1) If two substrate molecules need to be joined, being attached to the 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 bonds in the substrate, so the substrate molecules break up more easily.

Lock and key model

  • Enzymes only work with substrates that fit their active site
  • The substrate fits into the enzyme in the same way a key fits into a lock
  • Further developments showed that although the enzyme and substrate do have to fit together originally, in addition the enzyme-substrate complex changed shape slightly to complete the fit.
  • This locks the substrate even more tightly to the enzyme.
  • Scientists modified the old lock and key model and came up with the 'induced fit' model.
  • 'Induced fit' model helps to explain why enzymes are so specific and only bond to one particular substrate.
  • Substrate not only has to be the right shape to fit the active site, it also has to make the active site change shape in the right way as well.
  • This is a prime example of how a widely accepted theory can change when new evidence comes along. 

Properties in relation to tertiary structure

  • Very specific - usually only catalyse one reaction


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