pH

• Enzymes

  Enzymes have an optimum pH or a pH at which they operate best
• Enzyme denaturation


  Enzymes are denatured at extremes of pH
• Enzyme structure


  Hydrogen and ionic bonds hold the tertiary structure of the protein (ie. the enzyme) together
• Below and above the optimum pH of an enzyme

  Solutions with an excess of H+ ions (acidic solutions) and OH- ions (alkaline solutions) can cause these bonds to break
• Breaking of bonds


  Alters the shape of the active site, which means enzyme-substrate complexes form less easily
• Enzyme-substrate complexes can no longer form


  Complete denaturation of the enzyme has occurred
• Pepsin


  Pepsin is found in the stomach, an acidic environment at pH 2 (due to the presence of hydrochloric acid in the stomach's gastric juice)
• Pepsin's optimum pH


  pH 2
• Buffer solutions


  Buffer solutions each have a specific pH and maintain this specific pH, even if the reaction taking place would otherwise cause the pH of the reaction mixture to change
• Investigating the effect of pH on the rate of an enzyme-catalysed reaction

  1. Add a measured volume of the buffer solution to the reaction mixture

  2. The same volume (of each buffer solution being used) should be added for each pH value that is being investigated
• Investigating the effect of pH on enzyme reaction rates using amylase and starch

  1. Wear goggles and gloves

  2. Place single drops of iodine solution in rows on the tile

  3. Label a test tube with the pH to be tested

  4. Use the syringe to place 2cm3 of amylase in the test tube

  5. Add 1cm3 of buffer solution to the test tube

  6. Use another test tube to add 2cm3 of starch solution to the amylase and buffer solution, start the stopwatch whilst mixing using a pipette

  7. After 10 seconds, use a pipette to place one drop of the mixture on the first drop of iodine

  8. Repeat every 10 seconds until iodine solution remains orange-brown
• The less time the iodine solution takes to remain orange-brown, the quicker all the starch has been digested and so the better the enzyme works at that pH
• A colorimeter can be used to measure the progress of the reaction more accurately; with a solution containing starch being darker and glucose lighter (as a result of the colour-change of iodine)- this will affect the absorbance or transmission of light in a colorimeter