Enzymes Summary

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Enzyme activity

  • The chemical reactions within an organism represent its metabolism. Each metabolic reaction involves a substrate being converted to a product and is catalysed by an enzyme.
  • Catalysts speed up the rate of reaction by lowering the activation energy required for the reaction to occur.
  • Enzymes are globular proteins with a specific tertiary shape, part of which forms an active site.
  • A substrate binds with the active site to form an enzyme-substrate complex. Products are formed at the active site. The products are released from the enzyme molecules, which are unaltered.
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Enzyme Action

There are two models of enzyme action: the lock-and-key hypothesis and the induced-fit hypothesis.

  • The lock-and-key hypothesis explains enzyme specifity, due to the complementary shape of the substrate and the enzyme's active site
  • The induced-fit hypothesis suggests that binding of the substrate induces a change in enzyme structure which, through putting the substrate molecule under tension, explains why activation energy is lowered in catabolic reactions.
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Rate of reaction

  • Factors that increase the rate at which substrate and enzyme molecules might collide will increase the rate of reaction and include temperature, substrate concentration and enzyme concentration.
  • Factors that affect the tertiary structure of the enzyme will have an adverse affect on enzyme action by preventing binding of the substrate and include high temperatures and pH changes away from the optimum.
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Cofactors / Inhibitors / Immobilisation

  • Cofactors are non-protein substances that are necessary for the actions of some enzymes. Cofactors can be ions or organic molecules (coenzymes).
  • Inhibitors are substances that reduce the activity of enzymes:                                                                 -Competitive inhibitors mimic the substrate and compete with it for the enzyme's active site; the extent of the inhibition depends on the relative proportion of substrate and inhibitor molecules.                                                                                                                             -Non-competitive inhibitors tend to stop enzyme activity and act in different ways
  • Enzymes can be immobilised in a variety of ways and provide many advantages for their commercial use.
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