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How is the structure of an enzyme related to its f

  • They are globular proteins.
  • Have an active site. An active site is a depression on the surface of the molecule incorporated into an enzyme's three-dimensional structure that has a specific shape because of the way the whole molecule is folded.
  • Catalyse reactions. Catalysts speed up the rate of a reaction without themselves being chemically changed at the end of a reaction.
  • They lower the activation energy needed for a chemical reaction to take place. To convert substrate(s) into product(s), bonds must change within the mocelule(s). Breaking chemical bonds requires energy whilst energy is released when bonds form. The energy needed to break bonds and start the reaction is the activation energy.
  • Do not alter the end-product or the nature of a reaction.
  • Remain unchanged at the end of a reaction, able to bind with another substrate molecule.
  • Show great specificity. Enzymes are proteins and have a very specific shape as a result of their tertiary and quaternary structures, and this means they only catalyze specific reactions.
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How do enzymes work?

Substrate + enzyme = enzyme/substrate complex = enzyme + product.

Lock-and-key theory:

  • Either a single molecule with a complementary shape or more than one molecule that together have a complementary shape, can fit into the active site because of it's very specific shape.
  • These substrate molecules form temporary bonds with the amino acids of the active site to produce an enzyme-substrate complex.
  • In anabolic reactions, the reacting substances are brought closer together, making it easier for bonds to form between them.
  • In catabolic reactions, the active site affects the bonds in the substrate, making it easier for them to break.
  • When the reaction has taken place, the products are released as they can no longer fit the active site.
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How do enzymes work?

Induced-fit theory:

  • Evidence suggests that the active site is not rigid but rather flexible while maintaining a very distinctive shape and arrangement.
  • When a substrate molecule enters the active site, the enzyme molecule changes shape slightly, fitting more closely around the substrate.
  • Once the products have left the complex, the enzyme reverts to its inactive, relaxed form.
  • However, only a specific substrate will induce the change in shape of an enzyme's active site.
  • How do enzymes reduce the activation energy?
  • The attraction of oppositely charged groups on the surface of the substrates and the enzyme molecules may distort the shape of the substrates and assist in the breaking of bonds or formation of new bonds.
  • In some cases, the active site may contain amino acids with acidic side chains; the acidic environment created within the active site may provide conditions favorable for the reaction.
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Factors affecting enzymes and their rate of reacti

  • Enzymes are affected by the number of substrate molecules present. Increasing the number of substrate molecules in an enzyme-controlled reaction will increase the rate of the reaction up until a certain point where the enzyme is said to be saturated – all the active sites are occupied- and a further increase in substrate concentration will fail to increase the rate of the reaction. Substrate molecule won't be able to enter an active site until one becomes free again. Now the reaction has become enzyme-limiting.
  • A substrate-limiting reaction is one where increasing the substrate concentration produces more successful enzyme-substrate collisions so the rate increases.
  • If enzyme concentration was to constantly increase, and given the substrate is present in excess, the rate of the reaction is to assume a linear pattern of increase in the rate of the reaction.
  • Temperature affects the rate of an enzyme-catalyzed reaction. At temperature above 40 C most proteins, including enzymes, start to lose their tertiary and quaternary structures – they unravel or denature.
  • pH has a major effect on enzyme activity. Changes in pH affect the formation of hydrogen bonds and sulfur bridges that hold together the three-dimensional structure of the protein. 
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Extra Points


  • Those reactions that build up new chemicals are known as anabolic reactions.
  • Those that break substances down are catabolic reactions.
  • Most of the metabolic reactions occur as a sequence of reactions known as a metabolic chain or pathway.

Temperature and pH:

  • Different enzymes work in different ranges of pH, because of the particular arrangement of weak bonds which hold their shape together.
  • The optimum pH for an enzyme-controlled reaction is not always that of its surroundings, this is a way through which the cell can control the effects of their intracellular enzymes, increasing or decreasing their activity by minute changes in the pH level.
  • Changes in temperature and pH affect the efficiency of an enzyme because they affect the intramolecular bonds within the protein which are responsible for the shape of the molecule.
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