2.1.4

• Enzymes


  Globular proteins that act as catalysts to metabolic reactions in living organisms, usually speeding up metabolic reactions so that they occur at a reasonably fast pace even at body temperature
• Enzymes


  • They are required to build all the structures of the body and to control the activity of the body
  • They may be intracellular (working inside cells) or extracellular (working outside cells)
• Enzyme properties


  • The molecule has a three-dimensional shape (tertiary structure)
  • Part of the molecule is an active site that is complementary to the shape of the substrate molecule
  • Each enzyme is specific to the substrate
  • They have a high turnover number
  • They have the ability to reduce the energy required for a reaction to occur
  • Their activity is affected by temperature, pH, enzyme concentration and substrate concentration
  • The enzyme is left unchanged at the end of the reaction
• Enzyme specificity


  The ability to catalyse just one reaction or type of reaction, as only one particular substrate molecule will fit into the active site
• Lock and key hypothesis

  The shape of the active site is caused by the specific sequence of amino acids, producing a specific tertiary structure
• Catalysing the reaction


  Enzymes lower the activation energy required for the reaction to occur
• Induced-fit hypothesis


  The active site of an enzyme molecule does not have a perfectly complementary fit to the shape of the substrate, but when the substrate moves into the active site, it interacts with the active site and alters the shape of the active site to give a perfect fit
• Course of an enzyme-controlled reaction

  1. Substrate enters the enzyme's active site, forming the enzyme-substrate complex (ESC)

  2. This destabilises and strains the bonds in the substrate, forming the enzyme-product complex

  3. The product then leaves the active site and the enzyme is free to take up another substrate molecule
• Different enzymes are used in different parts of complex processes such as digestion as enzymes are specific to particular substrates
• Optimum pH


  The pH at which an enzyme works best, as hydrogen ions that cause acidity affect the interactions between R groups in the tertiary structure, altering the shape of the active site
• At low temperatures (0-45°C)


  Enzyme activity increases as temperature rises
• At higher temperatures


  Increased kinetic energy causes the enzyme to vibrate, breaking hydrophilic and hydrophobic interactions, as well as hydrogen bonds and ionic bonds in the tertiary structure, causing the enzyme to become denatured
• Enzyme concentration


  If there are more enzyme molecules, there are more active sites available, increasing the likelihood of collisions between enzyme and substrate molecules, resulting in a higher rate of reaction
• Substrate concentration


  If the substrate concentration is high, there is a greater chance of successful collisions between the enzyme active sites and substrate molecules, forming enzyme-substrate complexes and resulting in higher product formation
• The effect of pH and temperature on rates is usually because at extremes, some enzyme molecules are denatured and the concentration of active enzyme molecules is reduced
• Factors that can be investigated experimentally to study effects on enzyme activity

  • pH
  • Temperature
  • Enzyme concentration
  • Substrate concentration
• Coenzymes


  Larger organic substances that take part in the reaction, usually transferring other reactants between enzymes
• Cofactors


  Inorganic substances, usually metal ions, that increase the rate of catalysis by binding to the active site of the enzyme
• Competitive inhibitors


  Have a shape similar to the shape of the substrate and complementary to the shape of the active site, fitting into the active site and stopping the substrate molecules from fitting in
• Non-competitive inhibitors


  Fit into a different site (the allosteric site) on the enzyme molecule, causing a change in the shape of the enzyme molecule that affects the active site
• Reversible and non-reversible inhibitors

  Reversible inhibitors occupy the enzyme site only briefly, whereas non-reversible inhibitors bind permanently to the enzyme
• Many metabolic poisons act by inhibiting enzymes, and many medicinal drugs also act as inhibitors of enzymes in the body
• Product inhibition


  Occurs when the product of an enzyme-controlled reaction inhibits the enzyme, to prevent too much product being formed