enzymes and metabolism

• Enzymes


  Globular proteins that act as catalysts in metabolic reactions
• Catalysts


  Substances that speed up the rate of a reaction without themselves being permanently changed
• Metabolism


  The complex network of interdependent and interacting chemical reactions occurring in living organisms
• Without enzymes, metabolic reactions occur slowly, if at all, at body temperature. Enzymes act as catalysts, increasing the rate of reaction for metabolic reactions in cells
• Enzyme specificity


  Many different enzymes are required by living organisms, and control over metabolism can be exerted through these enzymes
• Metabolism


  1. Enzyme-catalysed chemical reactions occurring in a cell or living organism

  2. Each metabolic reaction is catalysed by a specific enzyme

  3. Each enzyme is specific, and catalyses one specific chemical reaction

  4. Every metabolic pathway requires many different enzymes to catalyse each reaction

  5. The cell can control metabolism through the use of enzymes
• Examples of anabolism


  • Formation of macromolecules from monomers by condensation reactions including protein synthesis, glycogen formation and photosynthesis
• Examples of catabolism


  • Hydrolysis of macromolecules into monomers in digestion and oxidation of substrates in respiration
• Anabolism


  The synthesis of complex molecules from simpler molecules, including the formation of macromolecules from monomers by condensation reactions
• Catabolism


  The breakdown of complex molecules into simpler molecules, including the hydrolysis of macromolecules into monomers
• Active site


  The region of an enzyme where substrates bind and are catalysed to products
• Active site of enzymes

  • Composed of a few amino acids, but the three-dimensional shape of the enzyme ensures that the active site can catalyse the reaction of the substrate
• Substrate


  Reactants in enzyme-catalysed reactions
• Induced fit model of enzyme activity

  1. The substrate approaches and enters the active site of the enzyme

  2. The substrate induces the active site of the enzyme to change shape so that there is an optimal fit between the substrate and active site

  3. An enzyme-substrate complex is formed, with the substrate fitting into the active site

  4. Catalysis occurs as the active site provides optimum conditions for the conversion of the substrate into the product

  5. The product has different chemical properties than the substrate, and is released from the active site
• Collision theory


  Chemical reactions happen when particles collide, and the moving particles must have sufficient energy and the correct orientation as they collide for the reaction to occur
• Substrate colliding with the active site

  An enzyme can only catalyse a reaction when the substrate collides with the active site of the enzyme
• Increasing temperature


  As temperature increases, particles move faster. Faster moving substrates are more likely to collide with an active site with sufficient energy for a reaction to occur
• Increasing substrate concentration


  An increase in substrate concentration increases the chance of a substrate particle colliding with an active site, and this increases the rate of reaction
• Immobilization of substrates


  Large substrates, such as proteins and starch, can be immobilized to allow for repeated enzymatic hydrolysis of a macromolecule into monomers
• Immobilization of enzymes in membranes

  Immobilizing an enzyme keeps it in close proximity to the substrates that it catalyses. The membrane also helps to maintain the stability of the enzyme
• Enzyme-substrate specificity


  The shape and chemistry of an enzyme's active site allows one specific type of substrate to be catalysed
• Denaturation of enzymes


  All proteins, including enzymes, have a specific shape which determines their biological function
• Students should be able to explain these relationships
• Enzyme-Substrate Specificity


  • The shape and chemistry of an enzyme's active site allows one specific type of substrate to be catalysed
  • Enzymes are specific to one substrate, as only one substrate can enter the active site
• Denaturation


  A permanent change in shape of proteins, that results in the loss of the proteins' biological functions (i.e. a change in their nature)
• Temperature and Enzyme Activity

  • At low temperatures the kinetic energy of particles is low. The number of collisions between substrates and active sites is low, resulting in a low rate of reaction
  • As temperature increases, the substrate particles gain energy and move faster. As temperature increases, the rate of reaction increases until the enzymes' optimum temperature
  • Above the optimum temperature, the enzyme becomes denatured. The substrate is no longer able to bind with the active site. The rate of reaction drops quickly, as the enzymes no longer catalyse the reaction
• Each enzyme has a specific optimum temperature. In humans, the optimum temperature for most enzymes is around 37°C
• pH and Enzyme Activity

  • Changes in pH affect the shape of enzymes
  • All enzymes have an optimum pH, at which they work best
  • An increase or a decrease in pH, from the optimum, results in a decrease in enzyme activity, as the active site is no longer as efficient at binding to the substrate
  • A large change of pH will denature the enzyme. The enzyme will stop working, as the substrate is no longer able to bind to the active site
  • Different enzymes have different optimum pHs
• Substrate Concentration and Enzyme Activity

  • When there is no substrate present, there is no enzyme activity
  • As the substrate concentration increases, the rate of reaction increases, due to increased collisions between substrate particles and the enzymes' active sites
  • Reaction rate continues to increase as substrate concentration increases, until all of the enzymes' active sites are saturated
  • The rate of reaction plateaus, as all enzymes are working at their optimum rate
• Activation energy


  The minimum amount of energy required for a chemical reaction to occur
• Enzymes lower the activation energy for chemical reactions, allowing metabolism to occur at body temperature
• Enzymes do not affect the change in energy between the substrates and reactants. They provide an alternative pathway where less activation energy is required
• The lock and key model has been replaced with the induced fit model
• What are examples of structure–function relationships in biological macromolecules?


  What biological processes depend on differences or changes in concentration?