5-Enzyme mechamism handout3

4 nucleophilic acyl substitution nas base catalysis

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Unformatted text preview: bilization Acid/base catalysis Specificity 3. Chymotrypsin: (Example 2) Reaction cycle Covalent catalysis Acid/base catalysis Transition state stabilization Chymotrypsin: Covalent catalysis (E-S bond) sitsonserine ES complex onenzyme,O- isnucleophile,attackscarbonyl, 2 formsbond O-Cbond,betweencovalentbondform 3 enzymeholdssubstrate,stabilizes anythinginbetween,making Product covalentbonds*** #1 tetrahedralintermediate 4 1 7 EP complex acylintermediate tetrahedralintermediate 5 6 Nucleophilic acyl substitution (NAS): Tetrahedral intermediate (from Chem 233) Overview 1. Introduction: Thermodynamics vs. kinetics Catalysis Enzyme catalysis 2. RNase: (Example 1) Function, structure, reaction Mechanism without catalysis Transition state stabilization Acid/base catalysis Specificity 3. Chymotrypsin: (Example 2) Reaction cycle Covalent catalysis Acid/base catalysis Transition state stabilization Chymotrypsin: Acid/base catalysis 2 -> 3: His-57 first acts as a base to abstract the proton from Ser-195, thus generating a stronger nucleophile. 2 ES complex OHcomingfromtheenzyme, notfromthesubstrateasinRNAse Tetrahedral intermediate acidcomesin,betterleavinggroup 3 byprotonating.acidisthe conjugateacidofthebaseofsame Product histidine. #1 Sameprinciplethough basemakesstrongernuclephile 3 -> 4: Protonated His-57 then acts as an acid to donate a proton to the amide N, thus generating a good leaving group. 4 Nucleophilic acyl substitution (NAS): Base c...
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