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Enzymes

Enzymes. Large molecules made of various amino acids Act as catalysts to speed up reactions w/out being destroyed Highly specific Lowers energy of activation level. Enzymes lower the energy of activation for a reaction. Enzyme Kinetics. E + S  ES complex  E + P

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Enzymes

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  1. Enzymes • Large molecules made of various amino acids • Act as catalysts to speed up reactions w/out being destroyed • Highly specific • Lowers energy of activation level

  2. Enzymes lower the energy of activation for a reaction

  3. Enzyme Kinetics E + S  ES complex  E + P • Effect of substrate concentration • 10 test tubes of fixed [E] • Add gradations of [S] • Measure rate of reactions

  4. Vmax occurs when enzyme active sites are saturated with substrate • Km (Michaelis-Menten constant) reflects affinity of enzyme for its substrate • smaller the Km, the greater the affinity an enzyme has for its substrate

  5. Enzyme Kinetics • [S] generally < than its Km • Only uses fraction of enzyme catalytic ability • Enzyme is able to respond to changes in [S] • Isozymes (isoenzymes) are variations of same enzyme • Four isozymes of hexokinase • Three have low Km and fourth has a high Km

  6. Hexokinase can phosphorylate glucose even with a low blood [glucose]; a high Km prevents liver from taking up blood glucose when [glucose] is low

  7. Regulation of Enzymes • Enzymes concentration • Will increase Vmax but not Km • Vmax proportional to [E] • Competitive inhibition

  8. Regulation of Enzyme Kinetics • Competitive inhibition • have similar geometric shape • Compete with enzyme for substrate • Can be overcome by  [S] • Will not affect Vmax but will  Km

  9. Regulation of Enzyme Kinetics • Allosteric regulation (noncompetive inhibitor/stimulator) • Allosteric enzymes don’t follow Michaelis-Menton kinetics; rather, most follow a sigmoidal model • Does not bind to active site on E • Changes shape of E which either  of  ability to bind with S • Will change Vmax but not Km

  10. Regulation of Enzyme Kinetics • Phosphorylation • cAMP activates protein kinase • activates catabolic enzymes • inactivates anabolic (synthetic) enzymes • Effect of temperature, pH

  11. Metabolic Pathway E1E2E3 A ------> B ------> C ------> D initial substrate final substrate First step is usually irreversible and controlled by an allosteric enzyme

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