Enzymes

• What Are Enzymes?

  • Enzymes are:
  • Catalysts that speed up the rate of a chemical reaction without being changed or used up in the reaction
  • Proteins
  • Biological catalysts (biological because they are made in living cells, catalysts because they speed up the rate of chemical reactions without being changed)
  • Necessary to all living organisms as they maintain reaction speeds of all metabolic reactions (all the reactions that keep an organism alive) at a rate that can sustain life
• • Enzymes are specific to one particular substrate (molecule/s that get broken down or joined together in the reaction) as the enzyme is a complementary shape to the substrate
  • The product is made from the substrate(s) and is released

  
• Enzyme specificity: lock and key model of enzyme activity

  A) substrate

  B) active site

  C) enzyme-substrate complex

  D) products

  
• • 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
  • This is because the enzyme is a protein and has a specific 3-D shape
  • This is known as the lock and key hypothesis
  • 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

  
• How enzymes work

  1. Enzymes and substrates randomly move about in solution
  2. When an enzyme and its complementary substrate randomly collide - with the substrate fitting into the active site of the enzyme - an enzyme-substrate complex forms, and the reaction occurs.
  3. 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.
• • Enzymes are proteins and have a specific shape, held in place by bonds
  • This is extremely important around the active site area 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
  • Heating to high temperatures beyond the optimum will break the enzyme's bonds and it will lose its shape - this denatures the enzyme
  • Substrates cannot fit into denatured enzymes as the shape of their active site has been lost
  • Denaturation is irreversible
• Effect of temperature on enzyme activity

  A) active site

  B) amino acids

  C) unique shape

  D) amino acid chain

  E) distorted

  F) fits

  G) high temperatures

  H) forces

  
• • Increasing the temperature from 0⁰C to the optimum increases the activity of enzymes as the more 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
• Graph showing the effect of temperature on the rate of enzyme activity

  
• • 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 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

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

  A) denatured

  B) activity

  C) optimum

  D) gradual loss

  E) distorted