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Enzymes

Enzymes. Chapter 8. Important Group of Proteins. Catalytic power can incr rates of rxn > 10 6 Specific Often regulated to control catalysis Coupling  biological pathway. Catalysis Happens…. Enzymes use many intermolecular forces At enzyme active site From R grps of aa’s

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Enzymes

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  1. Enzymes Chapter 8

  2. Important Group of Proteins • Catalytic power can incr rates of rxn > 106 • Specific • Often regulated to control catalysis • Coupling  biological pathway

  3. Catalysis Happens… • Enzymes use many intermolecular forces • At enzyme active site • From R grps of aa’s • Substrates brought together • Optimal orientation • Making/breaking bonds facilitated • Transition state stabilization • Allows high energy transition state • Enzyme native conformation crucial

  4. Additional Chemical Components • Prosthetic Groups • Cofactors (Table 8-1) • Coenzymes (Table 8-2) • Bound to apoenzyme •  holoenzyme

  5. Table 8-1

  6. Table 8-2

  7. Rxns Occur at Enzyme Active Sites (8-1) • Physical clefts • “Lined” w/ aa funct’l grps • Stabilize transition state S  P • Complex ES forms (reversible)

  8. Fig. 8-1

  9. Energetics • For any (cat’d) rxn involving S: • D G = D H - T D S • D G: • If negative • If = 0 • If positive D G: • Depends on free energy prod’s – free energy reactants • Independent on path of rxn (so catalysis doesn’t alter) • No info on rate of rxn

  10. For S < == > P at Equilibrium • Keq = [P] / [S] • DG = D G’o + RT ln [P] / [S], and • DG = 0, so • DG’o = - RT ln [P] / [S] • DG’o = - RT ln Keq’ • So Keq directly related to D G for rxn (Table 8-4)

  11. Table 8-4

  12. DG’o = diff in free energy between S, P • Enzymes do NOT effect Keq’, D G’o • Enzymes impt when energy must be added for rxn to proceed

  13. S* = Transition State = High Energy Intermediate • Must add energy for S <==> S* • Common rxn intermediate • “Fleeting molecular moment” • Can go to S or P (8-2) • D G*(SP) = Activation Energy • Diff in energy S to S* • Enzymes lower D G*

  14. Fig. 8-2

  15. ES* = Enzyme Substrate Complex • Must add energy for E + S < == > ES* • BUT less energy • So lower rxn pathway • Can go to E + S or E + P (8-3) • Note: E is always regenerated • D G*(cat’d) • Diff in energy S to ES* • So rxn more energetically favorable in presence of catalyst

  16. Fig. 8-3

  17. Enzymes Effect Rxn Rate • Use rate constant (k) to describe rate S < == > P • Velocity (V) of rxn dependent on [S], k • V = k [S] • First order rxn • Can relate k to D G* • Eq’n 8-6 • Relationship between k and DG* is inverse and exponential

  18. Table 8-5

  19. Summary • Enzymes don’t change overall energy difference, equilibrium • Enzymes do lower EA • Enzymes do increase k

  20. Source of Energy from within Enzyme to Facilitate Rxn S <==> P • Most impt: ES complex • Existence proven experimentally, theoretically • Enzyme active site • Aa residues directly participate (catalytic grps) • Only small part of total volume • Catalytic grps may be far apart in primary structure • Folding is important!

  21. Fig. 8-4

  22. Substrate Binding to Enzyme Active Site • Multiple weak interactions • What are these? • Must have proper orientation between atoms • Substrate, active site have complementary shapes • Commonly crevice is nonpolar • Polar residues at site commonly participate • Water excluded unless it participates • So: microenvironment w/ aa funct’l grps that have particular properties essential for catalysis of rxn

  23. Binding Specificity • DNA evolution  protein w/ optimal aa sequence  optimal E/S interactions  lowering energy nec for rxn • So, depends on precisely arranged atoms in active site

  24. Two theories of E/S “match” • Lock & key (Fisher, 1894) (8-4) • If precise match to S, why  S* or P? • Complementarity to S* • Enz active site complementary to transition state • So weak interactions encourage S*, then stabilize it • Best energetically when S* fits best into enz active site • Must expend energy for rxn to take place • BUT overall many weak interactions lower net activation energy • E/S “match” also confers specificity

  25. Fig. 8-5

  26. Fig. 8-5 - cont’d

  27. Other Factors that Reduce Activation Energy • Besides multiple weak atom-atom interactions • Physical, thermodynamic factors influence energy, rate of catalyzed rxn • Entropy reduction (8-7) • S held in proper orientation • Don’t rely on random, productive collisions

  28. Fig. 8-7

  29. Desolvation • H-bonds between S and solvent decreased • Incr’s productive collisions • Induced fit • Enzyme conformation changes when S binds • Brings impt funct’l grps to proper sites • Now has enhanced catalytic abilities

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