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Enzim internet

  1. 1. Organic Chemistry Chapter 24 4 Edition th Paula Yurkanis Bruice Catalysis Irene Lee Case Western Reserve University Cleveland, OH
  2. 2. CatalystA catalyst is a substance that increases the rate of areaction without itself being consumed or changedA catalyst increases the rate of the reaction by loweringthe ∆G‡ of the reactionA catalyst can decrease ∆G‡ of the reaction by one ofthree different ways
  3. 3. The Catalyst Converts the Reactant to a Less Stable Species
  4. 4. The Catalyst Stabilizes the Transition State
  5. 5. The Catalyst Changes the Mechanism of the Reaction
  6. 6. A catalyst can provide a more favorable pathway foran organic reaction by: • increasing the susceptibility of an electrophile to nucleophilic attack • increasing the reactivity of a nucleophile • increasing the leaving ability of a group by converting it to a weaker base
  7. 7. Nucleophilic CatalysisBoth the formation of the acyl imidazole and itssubsequent hydrolysis are both faster than esterhydrolysis
  8. 8. An Acid-Catalyzed ReactionA proton is donated to the reaction
  9. 9. The acid increases the rates of both slow steps of thereaction
  10. 10. In specific-acid catalysis, the proton is fully transferredbefore the slow step of the reactionIn general-acid catalysis, the proton is transferred duringthe slow step of the reaction
  11. 11. Compare Specific-Acid Catalysis with General- Acid Catalysis
  12. 12. A specific-acid must be a strong acid A general-acid can be a weaker acid
  13. 13. Base CatalysisA base catalyst increases the rate of the reaction byremoving a proton from the reaction specific-base catalyzed dehydration
  14. 14. The rate of the reaction is accelerated by stabilizationof the transition stateIn specific-base catalysis, the proton is completelyremoved before the slow step of the reaction
  15. 15. general-base catalysisIn general-base catalysis, the proton is removed duringthe slow step of the reaction
  16. 16. Metal-Ion CatalysisA. The metal ion increases the susceptibility of electron attackB. The metal ion makes the leaving group a weaker baseC. The metal ion increases the nucleophilicity of water
  17. 17. An Example of a Metal-Ion-Catalyzed Reaction
  18. 18. Metal-Ion-Catalyzed Decarboxylation
  19. 19. Metal-Ion-Catalyzed Ester HydrolysisThe metal-bound hydroxide is a better nucleophile thanwaterThe metal ion also decreases the basicity of the leavinggroup
  20. 20. The relative rates are also called the effective molarityThe effective molarity is the advantage given to areactionThe relative rate of reactant D is higher than the relativerate of B because the groups in D are less apt to adoptan unfavorable conformation for the reaction
  21. 21. Putting a reacting group and a catalyst in the samemolecule increases the rate of the reactionIntramolecular catalysis is also known as anchimericassistance
  22. 22. The trans isomer reacts much faster than the cis isomer
  23. 23. The rate of phenyl acetate hydrolysis is enhanced byan intramolecular general base catalysis
  24. 24. In the presence of nitro groups, the ortho-carboxylsubstituent acts as an intramolecular nucleophiliccatalyst
  25. 25. An Intramolecular Metal-Ion Catalysis
  26. 26. Most Biological Catalysts Are EnzymesThe reactants are called substratesThe substrate specifically fits and binds to the active site
  27. 27. Hexokinase undergoes a conformational change uponbinding to a substrate red: before substrate-binding green: after substrate-binding
  28. 28. Important Features that Contribute to the Catalytic Ability of Enzymes• Reacting groups are brought together at the active site in the proper orientation for reaction• Some of the amino acids in the enzyme serve as catalytic groups; many enzymes have metal ions as catalysts• Groups on the enzyme can stabilize the transition state of the reaction
  29. 29. Carboxypeptidase A catalyzes the hydrolysis of theC-terminal peptide
  30. 30. The binding pocket at the active site of serine proteasesdictates substrate specificity
  31. 31. The Proposed Reaction Mechanism of a Serine Protease
  32. 32. Lysozyme Is an Enzyme thatDestroys Bacterial Cell Walls
  33. 33. The amino acids at the active site of lysozyme areinvolved in binding the substrate
  34. 34. The Proposed Reaction Mechanism for Lysozyme
  35. 35. The pH-rate profile of an enzyme is a function of thepKa values of the catalytic groups in the enzyme a group is a group is catalytically catalytically active in its basic active in its acidic form form
  36. 36. Glucose-6-phosphate Isomerase