ch06 - 3/3/2011 Enzymes catalyze reactions 1 3/3/2011...

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3/3/2011 1 Enzymes catalyze reactions
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3/3/2011 2 Catalysis = enhancing rate of specific chemical reaction that would otherwise occur very slowly Catalysts do not change equilibrium point, but simply increase rate at which equilibrium is reached The catalyst is not used up or permanently altered Enzymes are highly specialized proteins with several important features: 1. Extraordinary catalytic power typically accelerate reactions by factors of at least a million. e.g. hydration of CO 2 is catalyzed by carbonic anhydrase: CO 2 + H 2 O H 2 CO 3 Each enzyme molecule can hydrate 10 5 molecules CO 2 per sec. = 10 7 times faster than uncatalyzed. 2. High degree of specificity - can tell difference between L and D isomers 3. Catalyze reactions without formation of byproducts 4. Work under mild conditions of temp and pH - nonenzymatic hydrolysis of proteins typically requires 6 M HCl at 110 ° C; Trypsin acts at neutral pH and 37 ° C.
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3/3/2011 3 Binding energy is used to facilitate transition state stabilization: 1. Entropy reduction (proximity and orientation) enzyme holds substrate(s) so that susceptible bond is in close proximity to catalytic groups. 2. Positions appropriate acid-base groups; some enzymes also use metal ions 3. Desolvation of substrate Enzyme-substrate replaces most of the water-substrate interactions. 4. Substrate distortion remember, enzyme binds transition state tighter than ground state. Binding energy also provides specificity again due to multiple interactions.
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3/3/2011 4 Reducing entropy increases rates in model systems. Enol-keto tautomerization . Uncatalyzed Acid catalyzed Base catalyzed
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3/3/2011 5
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3/3/2011 6 pKa of acid groups can be altered depending on the environment. Non-polar environment destabilizes charged forms. Carboxylic acid ionization would be disfavored, so pKa would increase. Enzymes Mechanisms We know specific binding reduces substrate entropy Positions acid/base groups Desolvates substrates Some enzymes also use covalent catalytic intermediates to “break reaction into pieces”
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7 3. Covalent catalysis Chymotrypsin hydrolyzed peptide bonds in which carbonyl is donated by aromatic side chain (Trp, Tyr, Phe). Trypsin is homolog cleaving cationic side chains (Lys, Arg) Elastase cuts small side chains, like Ala Subtilisin is bacterial serine protease Serine proteases are VERY common in living systems for digestion and cell regulation Serine protease (Cht) active site Ser-195 formed covalent acyl intermediate; known from irreversible inhibitors like DFP His-57 was also identified as active site residue by chemical modification (chloro methyl ketones) Asp-102 was identified by examining x-ray crystal structure and by evolutionary conservation in all serine proteases Led to model of His-57, Asp-102, Ser-195 = catalytic triad . This would increase acidity of Ser 195 from pKa ~ 14 to ~
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This note was uploaded on 05/01/2011 for the course CH 53890 taught by Professor Raymonds during the Spring '09 term at University of Texas at Austin.

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ch06 - 3/3/2011 Enzymes catalyze reactions 1 3/3/2011...

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