Enzymes

• Enzyme


  A protein that functions as a biological catalyst - a substance that speeds up a chemical reaction without being changed by the reaction
• Structure of enzymes


  • Contain up to 20 different amino acids linked together to form a chain which then folds into the globular enzyme shape
  • Have active sites which only match specific substrates
• Lock and key model


  Enzymes are folded into complex shapes that allow smaller molecules (substrates) to fit into them. The place where these molecules fit is called the active site.
• Enzymes are highly specific - each type of enzyme can catalyse only one type of reaction (or just a few types of reactions)
• Denaturation


  Changing the shape of an enzyme's active site, for example because of high temperatures or extremes of pH. Denatured enzymes no longer work.
• As temperature increases


  The rate of an enzyme-catalysed reaction increases
• At high temperatures


  The rate of the enzyme-catalysed reaction decreases again because the enzyme becomes denatured and can no longer function as a biological catalyst
• At low temperatures


  The enzymes and substrates have low kinetic energy, resulting in fewer successful collisions between the substrate and the enzyme's active site
• As temperature increases


  The kinetic energy increases, leading to more collisions and enzyme substrate complexes formed per unit time, increasing the rate of reaction
• At the optimum temperature


  The maximum number of enzyme-substrate complexes form per unit time
• If temperature continues to increase past the optimum


  The increased kinetic energy breaks the weak hydrogen bonds holding the enzyme's unique active site shape, so enzyme–substrate complexes can no longer form as the substrates no longer fit into the active site
• Optimum pH


  The pH value at which an enzyme works best
• Different enzymes work best at different pH values
• The optimum pH for an enzyme depends on where it normally works
• Intestinal enzymes have an optimum pH of about 7.5, but stomach enzymes have an optimum pH of about 2