ENZYMES

• Enzymes


  Proteins that act as catalysts for biochemical reactions
• Enzymes


  • Undergo all the reactions of proteins including denaturation
  • They are a "true catalyst" - they remain unchanged after the reaction

  
• General classes of enzymes

  • Simple enzyme (protein only)
  • Conjugated enzyme (nonprotein part + protein part)

  
• Apoenzyme


  Protein part of a conjugated enzyme
• Cofactor


  Nonprotein part of a conjugated enzyme
• Holoenzyme


  Biochemically active conjugated enzyme

  
• Types of cofactors


  • Small organic molecules (co-enzymes or co-substrates)
  • Inorganic molecules (non-metallic ion cofactor Cl-)
  • Derived from dietary vitamins
  • Derived from dietary minerals
• Substrate


  Reactant in an enzyme-catalyzed reaction, the substance upon which the enzyme "acts" and is converted to products
• Nomenclature of enzymes


  • Unsystematic/Nonsystematic name (suffix -ase identifies it as an enzyme, exceptions have suffix -in)
  • Systematic name (e.g. ATP: creatine phosphotransferase (EC 2.7.3.2))

  
• Six major classes of enzymes

  • Oxidoreductases
  • Transferases
  • Hydrolases
  • Lyases
  • Isomerases
  • Ligases

  
• Oxidoreductases


  • Catalyze oxidation-reduction reactions
  • Oxidation occurs when hydrogen atoms decrease and oxygen atoms increase, reduction is the opposite
• Transferases


  Catalyze the transfer of a functional group from one molecule to another
• Hydrolases


  Catalyze hydrolysis reactions, splitting or separation
• Lyases


  Catalyze the addition or removal of groups to/from substrates without hydrolysis

  
• Isomerases


  Catalyze the interconversion of isomers, the rearrangement of atoms within a molecule
• Ligases


  Catalyze the joining or formation of new bonds, not separation
• Models of enzyme action

  • Active site - where substrate binds
  • Enzyme-substrate complex - intermediate reaction species formed when substrate binds to active site
  • Enzyme remains unchanged after catalysis, can catalyze again

  
• Enzymes as catalysts


  • Can speed up reactions greatly
  • Are not permanently altered or consumed by the reaction
  • Most are proteins, some are RNA
  • Have complex, specific structures unlike metal catalysts
  • Work under mild conditions of temperature and pressure

  
• Fermentation in yeast


  First use of enzymes as a reference
• Enzymes are the first recognition that components are separable from living cells
• Most catalysts enzymes are protein, though some are made of RNA
• Active site


  Each enzyme has an active site where catalysis takes place
• Catalysis


  The increase in rate of a chemical reaction due to an added substance known as a catalyst
• Enzyme catalysts


  • Have complex, specific structures
  • The catalysis may be associated with conformational changes
  • Work under mild conditions (T and P)
  • Act only on a specific substance (substrate)
  • Some will act on the L-enantiomer, but not the D
  • Some are more "Promiscuous" than others

  
• Knowing "WHEN" and "WHY" enzymes work is as important as "HOW"
• Classes of Enzymes


  • Oxidoreductases
  • Transferases
  • Hydrolases
  • Lyases
  • Ligases
  • Isomerases

  
• Reaction velocity


  How fast the reaction is catalyzed by the enzyme, expressed as micromol of product formed per minute
• Factors that affect reaction velocity

  • Temperature
  • pH
  • Substrate concentration
  • Enzyme concentration
  • Presence of inhibitors
  • Presence of cofactors
• As temperature increases


  The reaction rate increases
• At the optimum temperature

  The reaction rate is fastest
• If temperature increases further

  The reaction rate decreases due to enzyme denaturation
• Optimum temperature for human enzymes

  37 C (body temperature)
• Increased temperature (high fever)

  Leads to decreased enzyme activity
• If enzymes are denatured


  Some biochemical reactions will shut down
• Optimal pH for most enzymes

  7.0 - 7.5
• Pepsin


  Optimum pH = 2.0
• Trypsin


  Optimum pH = 8.0
• As pH increases for Trypsin

  The reaction rate increases
• If pH continues to increase for enzymes

  The reaction rate decreases due to denaturation
• Increasing substrate concentration


  Increases enzyme activity (with constant enzyme concentration)