Mechanism of Enzyme Action

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Mechanism of Enzyme Action

  • Molecules in a solution move and collide randomly. For a reaction, molecules need to collide in the correct orientation
  • When high temperatures and pressures are applied --> the speed of the molecules increase, this means the no. of successful collisions and rate of reaction will increase 

Each enzyme catalyses 1 biochemical reaction --> enzymes are specific 

  • Activation energy --> energy supplied for a reaction to start. Sometimes it is so large that it prevents the reaction happening under normal conditions 
  • Enzymes help the molecules collide successfully, this means the activation energy required is reduced 
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Lock and Key Hypothesis

  • the area within the tertiary structure of an enzyme (active site) has a shape complementary to the shape of a specific substrate molecule 
  • only a specific substrate will 'fit' the active site of an enzyme to form an enzyme-substrate complex 


  • The active site and substrate are held in such a way that the right atom groups are close enough to react. The R-groups within active site interact with the substrate- puts strain on the temporary bonds --> this helps the rate of reaction 
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Induced Fit Hypothesis

  • The active site of an enzyme actually changes shape slightly as the substrate enters- induced fit hypothesis
  • It is a modified version of the lock and key hypothesis 


  • The intial reaction between the enzyme and substrate is weak
  • The weak interactions rapidly induce changes in the enzymes tertiary structure that strengthens binding- strain on the substrate molecule
  • This can weaken a particular bond/bonds in the substrate = activation energy lowered for the reaction
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Intracellular Enzymes

Enzymes that act within cells. They play an essential role in both the structure and function of cells and whole organisms 


-synthesis of polymers from monomers e.g. making polysaccharides from glucose requires enzymes

  • Hydrogen peroxide is a toxic product of many metabolic pathways
  • The enzyme catalse ensures hydrogen peroxide is broken down into oxygen and water quickly, preventing accumulation
  • Found in both plant and animal tissues 
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Extracellular Enzymes

Enzymes that are released from cells to break down large nutrient molecules into smaller molecules- digestion (so, they work outside the cell that made them)

  • Both single-celled and multicellular organisms rely on extracellular enzymes to make use of polymers for nutrition 

Single-celled Organisms:

  • e.g. bacteria and yeast release enzymes into their immediate environment 
  • They break down larger molecules e.g. proteins and the smaller molecules produced e.g. amino acids and glucose are then absorbed by the cells

Multicellular Organisms:

  • many eat food to gain nutrients- large molecules have to be digested so smaller enzymes can be absorbed into the bloodstream 
  • They're transported around the body to be used as substrates in cellular reactions
  • e.g. amylase and tipsin 
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