Describe and explain what happens to enzyme activity as the pH is decreased below the optimum pH.

Describe: As the pH decreases below the optimum, enzyme activity also decreases. Explain: As pH decreases, the concentration of hydrogen ions increases (the environment becomes more acidic). Enzymes are proteins and their tertiary structure (overall shape) is held in place by hydrogen bonds and ionic bonds. These bonds form between oppositely charged groups on the amino acids that make up the enzyme protein molecule. Hydrogen ions have a positive charge and therefore are attracted to the negatively charged groups on amino acids, interfering with the hydrogen bonds and ionic bonds which hold the tertiary structure (and the active site) in place. At extremely low pH values, this interference causes the protein to unfold, the shape of the active site is no longer complementary to the substrate molecule and the reaction can no longer be catalysed by the enzyme. The enzyme has been denatured.