All you need to know for the enzyme module in the AQA AS Biology course.

Enzymes Structure, How they work, Factors that effect their work and Inhibitors

Natalie Beard :)

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Enzyme Structure

Enzymes= Biological Catalysts

  • They lower the activiation energy and enable the reaction to take place at a reasonable temperature
  • Specfic to one reaction only
  • Can hydroysise or condense (join or break molecules)
  • Not used up during reaction
  • Globular proteins
  • Shape determined by tertiary strcuture, held together by hydrogen bonds and ionic forces.
  • Enzymes have an active site, designed to roughly be the same shape as the substrate molecule that they catalyse.
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How enzymes work

Enzymes only work in solution. Below are the two therories used to explain how enzymes function:

  • Lock and Key: The basic explanation. The enzyme has an active site with a specfic shape so a specfic substrate molecule fits into it, it is complementary. When these two meet it is called an enzyme-substrate complex. The active site changes shape to break the substrate bonds, lowering activation energy, as the products created don't fit thr active site, they leaving, leaving the enzyme free ro react again.
  • Induced Fit: This is similar, the discovery that compeitiors for the active site could fit even if slightly larger, hence the substrate and active site are both flexible, leading to the induced fit model. When the two form a complex, the substrate induces the active site to change shape. The active site now fits exactly round the substrate. Once the reaction is over, the active site returns to its original shape.
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Factors affecting the rate of enzyme reactions


Enzymes work the fastest they can at their optimum temperature. Below this at around 0 degrees the enzyme works, but incredibly slowly. As temperature increases, the substrate and enzyme molecules gain more kinetic energy and so move faster resulting in more collisons and more products and so the rate of the reaction increases. Above the optimum temperature, the enzymes begin to denature, meaning the hydrogen bonds in the tertiary strcuture begin to break, this causes the active site to lose its shape and so the substrate will no longer fit and no more product will be produced.


As with temperature, enzymes also have an optimum PH. If the PH strays too far from the optimum (this margin can be very narrow) this changes the charge on the active site as a incorrect PH disrupts the charges on the hydrogen and ionic bonds within the molecule, changing the shape of the active site, so no enzyme-substrate complexes can be formed, resulting in no product.

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Enzyme and Substrate Concentration

Enzyme Concentration

At low enzyme concentration, there is more competition among substrates for the active sites and the rate of reaction is low. As the enzyme concentration increases, the rate of reaction does too as there are more active sites for the reaction to take place. Eventually though, increasing this will have no effect at all as then the substrate conventration will become the limiting factor.

Substrate Concentration

When the substrate concentration is low, as is the reaction rate. Tghis is because there are many active sites available but not in use. Once more is added, the reaction rate will increase, if too many are added, it will have no effect as the active sites would become saturated so no more enzyme-substrate complexes would be created.

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Competitive Inhibitors have a similar shape to the substrate molecule and so compete with this molecule for the active site. They will bind with the active site, but obviously no reaction will happen. These inhibitors can be combated by adding more substrate molecules so then there is a higher chance of these finding the active sites rather than the inhibitors.


Radically different in shape when compared to the substrate, so this inhibitor cannot bind to the active site. Instead it binds to another part of the enzyme and changes the whole shape of the enzyme, including the active site. After this, substrate molecules can no longer bind to these inactive enzymes and so the rate of reaction drops. These therefore reduce the enzyme concentration. Some are reverrsible and some aren't.

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Louise 'Johnlock' Holmes


Really great flash cards thanks, very, very helpful :D

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