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Chapter 6: Enzymes

Part 1: Enzymatic Catalysis. Sections 6.1, 6.2, 6.4 What is an enzyme? What is a catalyst?Mechanisms of enzyme catalysisExamples. What is an enzyme?. Globular proteins acting as the body's catalystsNomenclature:Root aseClassificationOxidoreductasesTransferasesHydrolasesLyasesIsomerase

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Chapter 6: Enzymes

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    1. Chapter 6: Enzymes Dr. Clower Chem 4202

    2. Part 1: Enzymatic Catalysis Sections 6.1, 6.2, 6.4 What is an enzyme? What is a catalyst? Mechanisms of enzyme catalysis Examples

    3. What is an enzyme? Globular proteins acting as the body’s catalysts Nomenclature: Root + ase Classification Oxidoreductases Transferases Hydrolases Lyases Isomerases Ligases/synthetases E.C. Numbers

    4. What is a catalyst? Speed up time for reaction to reach equilibrium Lowers activation energy

    5. Enzymes as Catalysts

    6. Methods of catalysis Provide a reaction surface/suitable environment Bring reactants together Position reactants correctly for reaction Weaken bonds in the reactants

    7. Enzymes as catalysts Provide a reaction surface (the active site) Provide a suitable environment (hydrophobic) Bring reactants together Position reactants correctly for reaction Weaken bonds in the reactants Provide acid / base catalysis Provide nucleophiles

    8. An example reaction: Reduction of pyruvate to lactate Homolactic fermentation LDH = Lactate dehydrogenase (enzyme) NADH = reducing agent & coenzyme Pyruvic acid = substrate

    9. The active site Hydrophobic hollow or groove on the enzyme surface Accepts reactants (substrates and coenzymes) Contains amino acids which bind reactants (substrates and coenzymes) catalyze the reaction

    10. Cofactors/Coenzymes

    11. Enzyme catalysis Binding interactions: strong enough to hold the substrate sufficiently long for the reaction to occur weak enough to allow the product to depart Drug design: designing molecules with stronger binding interactions results in enzyme inhibitors which block the active site

    12. Substrate binding: bonding forces Ionic H-bonding Hydrophobic

    13. Binding of pyruvic acid in LDH

    14. Substrate binding: induced fit Active site is nearly the correct shape for the substrate Binding alters the shape of the enzyme (induced fit) Binding will strain bonds in the substrate

    15. Induced Fit

    16. Induced fit Active site alters shape to maximize intermolecular attractions

    17. Binding of pyruvic acid in LDH

    18. Binding of pyruvic acid in LDH

    19. Enzymes are Stereospecific Asymmetrical Synthesis

    20. Catalysis Mechanisms Acid-Base Covalent Metal Ion Electrostatic Proximity and Orientation Effects Preferential TS Binding

    21. Proton-transfer reaction stabilize TS Example: Enzymes as acids/bases Acid-Base Catalysis

    22. Acid-base Catalysis What happens if both acid and base are present? Solution Enzyme

    23. aka Nucleophilic Catalysis Covalent Catalysis

    25. Metal Ion Catalysis Metalloenzymes Metal-activated enzymes How do metal ions participate in the catalytic process? Orient substrates Redox Facilitate formation of hydroxide ion Electrostatic stabilization

    26. Electrostatic Catalysis Exclusion of water from active site Increases polarity Strong electrostatic interactions Stabilize TS Ion channel

    27. Proximity and Orientation Effects Reactants must come together with the proper spatial relationship for a reaction to occur Enzymes: Bring substrates into contact with catalytic groups and/or each other Bind substrates in proper orientation for reaction Restrict rotational motion of substrates so align reactive portions of molecules

    28. Compare reactions of imidazole with a carbonyl Intermolecular Intramolecular Proximity and Orientation Effects

    29. Preferential TS Binding Enzymes stabilize TS Increase formation of TS Typically bind TS better than either reactant or product Induced fit can distort bond lengths/angles

    30. An example: Lysozyme Bactericidal agent Destroy cell walls Hydrolyze glycosidic linkages in cell wall peptidoglycans Alternating NAG and NAM

    31. Hen Egg White Lysozyme Most widely studied/understood species Single polypeptide 129 AA residues, 5 a-helices, 1 b-sheet, 4 disulfide bonds Roughly ellipsoidal Cleft across one side = active binding site

    32. Substrate D distorts to half-chair to avoid steric strain and maximize H-bonding D-E b1?4 glycosidic bond cleaves

    33. Mechanism

    37. Other examples Hexokinase Enolase Serine proteases (chymotrypsin) See your textbook section 6.4

    38. Coming up next… Enzyme kinetics and inhibition section 6.3 Enzyme regulation section 6.5

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