M2:S4 Enzymes

• Enzymes


  Biological catalysts that speed up chemical reactions without being used up in the reaction itself
• Enzymes

  • They catalyse metabolic reactions at a cellular level (e.g. respiration) and for the organism as a whole (e.g. digestion in mammals)
  • They can affect structures in an organism (e.g. enzymes are involved in the production of collagen) as well as functions (like respiration)
  • Enzyme action can be intracellular-within cells, or extracellular-outside cells
• Intracellular enzyme example

  • Catalase - catalyses the breakdown of hydrogen peroxide to harmless oxygen and water
• Extracellular enzyme examples

  • Amylase - catalyses the hydrolysis of starch into maltose in the mouth
  • Trypsin - catalyses the hydrolysis of peptide bonds, turning big polypeptides into smaller ones
• Enzyme structure

  • Enzymes are globular proteins
  • Enzymes have an active site with a specific shape, determined by the enzyme's tertiary structure
• Enzyme-substrate interaction

  • The substrate has to fit into the active site (its shape has to be complementary)
  • Enzymes usually only work with one specific substrate
• Activation energy

  The amount of energy needed to start a chemical reaction
• Enzymes

  Reduce the activation energy needed for a reaction, often allowing it to occur at a lower temperature
• Enzyme-substrate complex

  The complex formed when a substrate binds to an enzyme's active site
• Lock and key model

  Early model of enzyme action where the substrate fits into the enzyme's active site like a key fits into a lock
• Induced fit model

  Improved model where the enzyme's active site changes shape slightly to better fit the substrate when it binds
• The induced fit model explains why enzymes are so specific and only bond to one particular substrate
• Temperature has a big influence on enzyme activity
• As temperature increases

  The rate of an enzyme-controlled reaction increases
• Enzyme denaturation

  At high temperatures, the enzyme's bonds are broken and its active site changes shape, so it no longer functions as a catalyst
• Temperature coefficient (Q10)

  Shows how much the rate of a reaction changes when the temperature is raised by 10°C
• Most enzyme-controlled reactions have a Q10 value of around 2
• pH also affects enzyme activity
• Optimum pH

  The pH value at which an enzyme works best
• Above and below the optimum pH

  Acid and alkali ions can disrupt the enzyme's structure, causing it to denature
• Enzyme concentration

  The higher the enzyme concentration, the faster the reaction rate (up to a point)
• Substrate concentration

  The higher the substrate concentration, the faster the reaction rate (up to a point)
• Substrate concentration decreases over time during a reaction, so the initial rate of reaction is the highest
• Measuring the rate of an enzyme-controlled reaction

  1. Measure the appearance of the product (e.g. oxygen produced by catalase)

  2. Measure the disappearance of the substrate (e.g. time for starch to disappear with amylase)
• Investigating the effect of temperature on catalase activity

  1. Set up boiling tubes with hydrogen peroxide and buffer at different temperatures

  2. Add catalase and measure the volume of oxygen produced in 60 seconds

  3. Repeat 3 times at each temperature to find the mean
• Buffer solutions


  Have different pHs
• Measuring the volume of oxygen produced

  1. Set up apparatus with delivery tube and upside down measuring cylinder

  2. Put boiling tubes in water bath at different temperatures

  3. Add catalase to each boiling tube

  4. Record oxygen produced in first 60 seconds

  5. Repeat experiment 3 times at each temperature
• A negative control reaction without catalase should also be carried out at each temperature
• Calculating the mean rate of reaction

  1. Divide the volume of oxygen produced by the time taken (60 s)

  2. Unit is cm³/second or cm³ s¹
• It's easy to alter this experiment to investigate the effect of a different variable on catalase activity
• Enzymes are pretty fussy - they'll only work best when they are nice and comfortable
• Enzymes


  Biological catalysts that speed up chemical reactions without being used up in the reaction itself
• Enzymes


  • They catalyse metabolic reactions at a cellular level (e.g. respiration) and for the organism as a whole (e.g. digestion in mammals)
  • They can affect structures in an organism (e.g. enzymes are involved in the production of collagen) as well as functions (like respiration)
  • Enzyme action can be intracellular-within cells, or extracellular-outside cells
• Intracellular enzyme example


  • Catalase - catalyses the breakdown of hydrogen peroxide to harmless oxygen and water
• Extracellular enzyme examples


  • Amylase - catalyses the hydrolysis of starch into maltose in the mouth
  • Trypsin - catalyses the hydrolysis of peptide bonds, turning big polypeptides into smaller ones
• Enzyme structure


  • Enzymes are globular proteins
  • Enzymes have an active site with a specific shape, determined by the enzyme's tertiary structure
• Enzyme-substrate interaction


  • The substrate has to fit into the active site (its shape has to be complementary)
  • If the substrate shape doesn't match the active site, the reaction won't be catalysed
• Enzyme-substrate complex


  When a substance binds to an enzyme's active site, an enzyme-substrate complex is formed
• Induced fit model


  The enzyme-substrate complex changes shape slightly to complete the fit, locking the substrate even more tightly to the enzyme
• Enzymes are specific and only bond to one particular substrate