Mechanisms of enzyme action

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Mechanisms of enzyme action

  1. 1. Mechanisms of Enzyme Action
  2. 2.  Activation Energy (AE) – The energy require to reach transition state from ground state.  AE barrier must be exceeded for rxn to proceed.  Lower AE barrier, the more stable the transition state (TS)  The higher [TS], the move likely the rxn will proceed.
  3. 3. S  Ts  P
  4. 4.  Transition state = unstable high-energy intermediate  Rate of rxn depends on the frequency at which reactants collide and form the TS  Reactants must be in the correct orientation and collide with sufficient energy to form TS  Bonds are in the process of being formed and broken in TS  Short lived (10 –14 to 10 -13 secs)
  5. 5. • Intermediates are stable. • In rxns w/ intermediates, 2 TS’s are involved. • The slowest step (rate determining) has the highest AE barrier. • Formation of intermediate is the slowest step.
  6. 6.  Enzyme binding of substrates decrease activation energy by increasing the initial ground state (brings reactants into correct orientation, decrease entropy)  Need to stabilize TS to lower activation energy barrier.
  7. 7. ES complex must not be too stable Raising the energy of ES will increase the catalyzed rate •This is accomplished by loss of entropy due to formation of ES and destabilization of ES by •strain •distortion •desolvation
  8. 8. • Equilibrium between ES <-> TS, enzyme drives equilibrium towards TS • Enzyme binds more tightly to TS than substrate
  9. 9. Transition state analog
  10. 10.  Substitutions rxns  Bond cleavage rxns  Redox rxns  Acid base catalysis  Covalent catalysis
  11. 11. Nucleophillic Substitution– Nucleophillic = e- rich Electrophillic = e- poor
  12. 12. Direct Substitution Transition state
  13. 13.  Loose e - = oxidation (LEO)  Gain e - = reduction (GER)  Central to energy production  If something oxidized something must be reduced (reducing agent donates e - to oxidizing agent)
  14. 14.  Oxidations = removal of hydrogen or addition of oxygen or removal of e -  In biological systems reducing agent is usually a co-factor (NADH of NADPH)
  15. 15.  Heterolytic vs homolytic cleavage  Carbanion formation (retains both e - ) R 3 -C-H  R 3 -C: - + H +  Carbocation formation (lose both e - ) R 3 -C-H  R 3 -C + + H: -  Hydride ion Free radical formation (lose single e - ) R 1 -O-O-R 2  R 1 -O* + *O-R 2
  16. 16.  Accelerates rxn by catalytic transfer of a proton  Involves AA residues that can accept a proton  Can remove proton from –OH, -NH, -CH,
  17. 17. Carbanion intermediate
  18. 18.  20% of all enzymes employ covalent catalysis A-X + B + E <-> BX + E + A  A group from a substrate binds covalently to enzyme (A-X + E <-> A + X-E)  The intermediate enzyme substrate complex (AX) then donates the group (X) to a second substrate (B) (B + X-E <-> B-X + E)
  19. 19. Protein Kinases ATP + E + Protein <-> ADP + E + Protein-P 1) A-P-P-P(ATP) + E-OH <-> A-P-P (ADP) + EO-PO 4 - 2) E-O-PO 4 - + Protein-OH <-> E + Protein-OPO 4 -
  20. 20. The Serine Proteases Trypsin, chymotrypsin, elastase, thrombin, subtilisin, plasmin, TPA • All involve a serine in catalysis - thus the name • Ser is part of a "catalytic triad" of Ser, His, Asp (show over head)
  21. 21.  Serine proteases are homologous, but locations of the three crucial residues differ somewhat  Substrate specificity determined by binding pocket
  22. 22. Chymotrpsin Trypsin Elastase

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