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Chapter 8 (continued) Enzymes

Chapter 8 (continued) Enzymes. You should read the entire chapter. For the midterm, you will no t be tested on the following topics : kinetic details of different types of enzyme inhibition (pp. 266-268)

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Chapter 8 (continued) Enzymes

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  1. Chapter 8 (continued) Enzymes You should read the entire chapter. For the midterm, you will not be tested on the following topics : kinetic details of different types of enzyme inhibition (pp. 266-268) kinetic details of allosteric enzymes (pp. 280, middle of page, to 281, middle of page). Details of regulation by phosphorylation (pp. 282, near bottom of page, to 286, near bottom of page). Hexokinase and Enolase--but these will be included later, when we study glycolysis Chymotrypsin

  2. Enzyme Kinetics:Michaelis-Menton Equation Vmax[S] Vo = ____________ KM + [S] KM = [S] when Vo = Vmax _____ 2

  3. 1 KM 1 ______ = _______ +______ Vo Vmax[S] Vmax Double-Reciprocal orLineweaver-Burk Plot

  4. k1 k-1 k2 E + SESE + P Vmax [S] Vo =____________ KM + [S] Vmax = k2[Etotal] This is true only for this specific (rather simple) mechanism!

  5. kcat[Etotal][S] _________________ Km + [S] = Vmax[S] _________ Km + [S] Vo = kcator Turnover Numbers Vmax = kcat[Etotal] More general:

  6. Catalytic efficiency,kcat/KmMeasures how the enzyme performs when S is lowThe upper limit for kcat/Kmis the diffusion limit - the rate at which E and S diffuse together kcat[Etotal][S] _________________ Km + [S] kcat[Etotal][S] _________________ Km Vo = = when S is low

  7. b. Ping-pong or double-displacement reaction mechanisms Many enzymes catalyze reactions with two or more substrates a. Mechanisms involving ternary complex

  8. Ping-pong or Double-Displacement Reaction Mechanisms Ternary Complex Reaction Mechanism Steady-State Kinetics Can Help Determine Mechanism

  9. Inhibitor (I) binds only to E, not to ES Inhibitor (I) binds only to ES, not to E. This is a hypothetical case that has never been documented for a real enzyme, but which makes a useful contrast to competitive inhibition Enzyme Inhibition Inhibitor (I) binds either to E and ES or to ES alone.

  10. CompetitiveUncompetitiveMixed InhibitionInhibitionInhibition Kmchanges while Vmax does not Km and Vmax both change Km and Vmax both change

  11. 1 KM 1 ______ = _______ +______ Vo Vmax[S] Vmax Double-Reciprocal orLineweaver-Burk Plot

  12. Irreversible Inhibition (Irreversible) diisopropylfluorophosphate chymotrypsin

  13. Enzyme Activity is Affected by pH

  14. Structure of Chymotrypsin

  15. Pre-steady state evidence for an acyl enzyme intermediate

  16. 1. Allosteric enzymes 2. Regulation by covalent modification Regulatory Enzymes important in controlling flux through metabolic pathways

  17. Example of an allosteric regulatory enzyme Two views of aspartate transcarbamoylase • 12 subunits: 6 are catalyticandsix are regulatory • Regulatory subunits are shown in red and yellow

  18. Regulation by Feedback Inhibition Conversion of L-threonine to L-isoleucine catalyzed by a sequence five enzymes, E1-E5 L-isoleucine is an inhibitory allosteric modulator of E1

  19. 2. Regulation by covalent modification Most common

  20. 2. Regulation by Covalent Modification: Proteolytic cleavage of zymogen

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