Edexcel IGCSE Biology 复习笔记 2.3.3 Enzymes

Enzymes

• Enzymes are proteins that act as biological catalysts to speed up the rate of a chemical reaction without being changed or used up in the reaction
• They are biological because they are made in living cells
• Enzymes are necessary to all living organisms as they maintain reaction speeds of all metabolic reactions at a rate that can sustain life
  • For example, if we did not produce digestive enzymes, it would take around 2 - 3 weeks to digest one meal; with enzymes, it takes around 4 hours
  • Often the products of one reaction are the reactants for another (and so on)

   

The mechanism of enzyme action

• Enzymes are specific to one particular substrate(s) as the active site of the enzyme, where the substrate attaches, is a complementary shape to the substrate
• When the substrate moves into the enzyme’s active site they become known as the enzyme-substrate complex
• After the reaction has occurred, the products leave the enzyme’s active site as they no longer fit it and it is free to take up another substrate
  • Step One: Enzymes and substrates randomly move about in solution
  • Step Two: When an enzyme and its complementary substrate randomly collide an enzyme-substrate complex forms, and the reaction occurs
  • Step Three: A product (or products) forms from the substrate(s) which are then released from the active site. The enzyme is unchanged and will go on to catalyse further reactions

   

How enzymes work

• Enzymes are proteins and have a specific shape, determined by the amino acids that make the enzyme and held in place by bonds
• This is extremely important around the active site as the specific shape is what ensures the substrate will fit into the active site and enable the reaction to proceed
• Enzymes work fastest at their ‘optimum temperature’
  • In the human body, the optimum temperature is 37⁰C

   

• Heating to high temperatures (beyond the optimum) will break the bonds that hold the enzyme together and it will lose its shape
  • This is known as denaturation

   

• Substrates cannot fit into denatured enzymes as the shape of their active site has been lost
• Denaturation is irreversible - once enzymes are denatured they cannot regain their proper shape and activity will stop

Effect of temperature on enzyme activity

• Increasing the temperature towards the optimum increases the activity of enzymes as the more kinetic energy the molecules have the faster they move and the number of collisions with the substrate molecules increases, leading to a faster rate of reaction
• This means that low temperatures do not denature enzymes, they just make them work more slowly due to a lack of kinetic energy

Graph showing the effect of temperature on the rate of enzyme activity

• The optimum pH for most enzymes is 7
  • Some enzymes that are produced in acidic conditions, such as the stomach, have a lower optimum pH (pH 2)
  • Some that are produced in alkaline conditions, such as the duodenum, have a higher optimum pH (pH 8 or 9)

   

• If the pH is too high or too low, the bonds that hold the amino acid chain together to make up the protein can be disrupted/destroyed
• This will change the shape of the active site, so the substrate can no longer fit into it, reducing the rate of activity
• Moving too far away from the optimum pH will cause the enzyme to denature and activity will stop

Effect of pH on enzyme activity

Graph showing the effect of pH on the rate of activity for an enzyme from the duodenum