chapter6 RKW - Lecture Connections 6 | Enzymes 2009 W H...

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Lecture Connections 6 | Enzymes © 2009 W. H. Freeman and Company
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CHAPTER 6 Enzymes Physiological significance of enzymes Origin of catalytic power of enzymes Chemical mechanisms of catalysis Mechanisms of chymotrypsin and lysozyme Description of enzyme kinetics and inhibition Mechanism of chymotrypsin Key topics about enzyme function :
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What are Enzymes? Enzymes are catalytically active biological macromolecules Most enzymes are globular proteins, however some RNA (ribozymes, and ribosomal RNA) also catalyze reactions Study of enzymatic processes is the oldest field of biochemistry, dating back to late 1700s Study of enzymes has dominated biochemistry in the past and continues to do so
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Why Biocatalysis? Higher reaction rates Greater reaction specificity Milder reaction conditions Capacity for regulation COO OH O COO COO O COO NH 2 OOC COO O OH OH COO NH 2 COO - - - - - - - - Chorismate mutase Metabolites have many potential pathways of decomposition Enzymes make the desired one most favorable Substrate and Product
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Enzymatic Substrate Selectivity No binding OOC NH 3 H OOC NH 3 H H NH H OH OH H O H CH 3 OOC NH 3 H OH - - - + + + Binding but no reaction Example: Phenylalanine hydroxylase
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Enzyme-Substrate Complex Enzymes act by binding substrates the non-covalent enzyme substrate complex is known as the Michaelis complex ] [ ] ][ [ S K S E k v m cat + = allows thinking in terms of chemical interactions allows development of kinetic equations
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Enzymes as Catalyts
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The Fandango Catalyst
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Rate Acceleration The enzyme lowers the activation barrier compared to the uncatalyzed aqueous reaction In theory, the enzyme may also facilitate the tunneling through the barrier. This may be important for electrons.
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Transition State Theory Slow reactions face significant activation barriers that must be surmounted during the reaction k = k B T h exp -∆ G RT transition state theory is applicable for catalysis rate constants and free energies can be related K is the rate constant, k b is the Boltzmann constant
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How to Lower G ? Enzymes organizes reactive groups into proximity Uncatalyzed bimolecular reactions: two free reactants single restricted transition state conversion is entropically unfavorable Uncatalyzed unimolecular reactions: flexible reactant rigid transition state conversion is entropically unfavorable for flexible reactants Catalyzed reactions: Enzyme uses the binding energy of substrates to organize the reactants to a fairly rigid ES complex Entropy cost is paid during binding Rigid reactant complex transition state conversion is entropically OK
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Rate constants For gas phase or solution kinetic reactions: collisions bringing reactants together collisions with sufficient energy to react some are unproductive collisions orientation of collision unproductive because of orientation k = (collisions)(collision w/energy to react)(orientation)
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