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Enzyme Kinetics

Enzyme Kinetics. Law Depends on Mechanism. Remember our previous example If first mechanistic step is rate determining, rate law can be written from first step . O 3 + 2 NO 2  O 2 + N 2 O 5. O 3 + NO 2  NO 3 + O 2 slow NO 3 + NO 2  N 2 O 5 fast.

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Enzyme Kinetics

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  1. Enzyme Kinetics

  2. Law Depends on Mechanism • Remember our previous example • If first mechanistic step is rate determining, rate law can be written from first step O3 + 2 NO2 O2 + N2O5 • O3 + NO2 NO3 + O2 slow • NO3 + NO2 N2O5 fast Rate = k[O3][NO2]

  3. More Complex Mechanism • What if the second step is rate limiting? • What if RDS is unknown? • Using the Steady State Approximation, we can write a general rate law and determine which RDS fits the experimental evidence • Your book presents steady state and other techniques, but steady state assumption works well for all situations generally encountered

  4. Breakdown of Ozone k1 O3 O2 + O O + O3 2O2 Rate = k2[O][O3] but O is an intermediate. How do we determine [O]? k-1 k2

  5. Steady State Approximation • Assumption: concentration of intermediates stays about constant during reaction • Low, constant [Intermediate] • Rate of appearance = rate of disappearance • For this mechanism: O3 O2 + O O + O3 2O2 k1[O3] = k-1[O2][O] + k2[O][O3] k1 k-1 k2

  6. General Rate Law • To determine overall rate law, solve for [O] k1[O3] = k-1[O2][O] + k2[O][O3] • Insert into rate law from second step Rate = k2[O][O3] [O] Rate

  7. Application of General Rate Law • This rate law looks complicated, but it really isn’t • How does it simplify if the first step is RDS? • How does it simplify if the second step is RDS? Proposed mechanism k1 Rate k-1 k2

  8. Same Approach to Enzyme Kinetics • Mechanism: E + S ES  E + P Enzyme/Substrate complex is intermediate k2 k1 k-1

  9. Enzyme Kinetics Steady State: k1[E][S] = k-1[ES] + k2[ES] k1[ET][S] - k1[ES][S] = k-1[ES] + k2[ES] Total enzyme (ET) = free enzyme (E) – complexed enzyme (ES) Solve for [ES] and plug into rate law [ES] Rate This again looks complex, but it teaches us simple lessons about the extreme cases of catalysis

  10. Kinetics of Catalysis • What is the rate law when the [S] is very low? (The catalyst has a hard time finding substrate) • What is the rate law when the [S] is very high? (enzyme is saturated) Rate

  11. Same Applies to any Binding Catalyst

  12. Orotidine Decarboxylase • Key enzyme in production of nucleotides for DNA • T1/2 = 14 ms • But what makes it a great enzyme?

  13. The Speed of the UncatalyzedRxn

  14. Mechanism and RDS

  15. Stabilizing the Transition State

  16. Transition State Analogues

  17. Mechanism of Catalysis

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