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Enzymes

Enzymes. Chapter 6. Important Group of Proteins. Catalytic power can incr rates of rxn > 10 17 Specific Often regulated to control catalysis Coupling  biological pathway. Catalysis Happens…. Enzymes use many intermolecular forces At enzyme active site

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Enzymes

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  1. Enzymes Chapter 6

  2. Important Group of Proteins • Catalytic power can incr rates of rxn > 1017 • Specific • Often regulated to control catalysis • Coupling  biological pathway

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

  4. Additional Chemical Components • Prosthetic Groups • Cofactors (Table 6-1) • Coenzymes (Table 6-2) • Bound to apoenzyme (apoprotein) •  Holoenzyme

  5. Rxns Occur at Enzyme Active Sites • Physical clefts • “Lined” w/ atoms that make up aa R grps • Stabilize transition state S  P • Complex ES forms (reversible)

  6. DG Calc’d for Any Rxn S  P • DG = Diff in free energy between S, P • REMEMBER: D G = D H _ T D S • What are these terms??

  7. Energetics • 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 of path of rxn • Catalysis doesn’t alter • No info on rate of rxn

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

  9. 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 • Note: E is always regenerated • D G*(cat’d) • Diff in energy S to ES* • So rxn more energetically favorable in presence of catalyst

  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

  11. 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

  12. 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 6-6 • Relationship between k and DG* is inverse and exponential

  13. Summary • Enzymes don’t change overall energy difference (S  P), equilibrium (Keq) • Enzymes do lower EA • Enzymes do increase rate (k)

  14. Source of Energy from within Enz to Facilitate Rxn S  P • Most impt: ES complex • ES proven experimentally, theoretically • Enzyme active site • Aa’s directly participate (catalytic grps) • Only small part of total volume • Catalytic grps may be far apart in primary structure • Folding impt!

  15. Substr Binding to Enz Active Site • Multiple weak interactions • What are these? •  Binding energy (DGB) • Stabilizes ES* • Must have proper orientation between atoms • Substrate, active site have complementary shapes

  16. Commonly crevice nonpolar • If polar aa’s, often participate • Water excluded unless participates in rxn • So: microenvironment w/ aa funct’l grps that have partic prop’s essential for catalysis of rxn

  17. 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

  18. Two Theories of E/S “Match” • Lock & key (Fisher, 1894) • 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

  19. 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 act’n energy • E/S “match” also confers specificity

  20. Other Factors that Reduce Act’n Energy • Besides multiple, weak, atom-atom interactions • Physical, thermodynamic factors influence energy, rate of catalyzed rxn • Entropy reduction • S held in proper orientation • Random, productive collisions not nec

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

  22. http://www.blobs.org/science/enzyme/imgs/active2.gif

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