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Enzymes

Enzymes. (If you don’t have the energy, we can help!). Enzymes. Biological catalysts Made primarily from proteins Bind to substrate Not used up Speed up reaction. Catalysis. Uncatalyzed. Catalyzed. Transition. G. E A. R. G. R. P. P. time. time. Catalysts.

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Enzymes

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  1. Enzymes (If you don’t have the energy, we can help!)

  2. Enzymes • Biological catalysts • Made primarily from proteins • Bind to substrate • Not used up • Speed up reaction

  3. Catalysis Uncatalyzed Catalyzed Transition G EA R G R P P time time

  4. Catalysts • Increase rate of reaction • Provide activation energy • Energy is conserved • Activation energy has to come from somewhere

  5. Sources of Activation energy • Binding energy • Solvent released from active site when substrate binds increases ΔSsurr • Induced fit • Enzyme forces substrate into unstable transition state • Catalytic antibodies • Binding substrate brings reactive groups together

  6. Catalysis by an enzyme • E + S > ES > EP > E + P

  7. Enzyme Example Serine Proteases • http://www.bio.cmu.edu/courses/03231/Protease/SerPro.htm • Active site and substrate • Note side chain interactions • Substrate diffuses into active site

  8. Serine proteases • Substrate binds to active site • Chemical groups interact • Force substrate into unstable intermediate

  9. Serine Proteases • Peptide bond is cleaved • Serine in active site is bound to carboxyl side of peptide

  10. Serine proteases • Half of protein diffuses out • Enzyme used up • Has to be regenerated

  11. Serine proteases • Water diffuses into active site • Juxtaposes chemical groups • Similar reaction to first

  12. Serine proteases • New unstable intermediate generated

  13. Serine Proteases • Carboxyl end of peptide diffuses out • Enzyme regenerated • Ready for another round

  14. Serine protease summary • Enzyme stabilized by various side chain interactions • Substrate binds to enzyme. • Fits pocket • Forms unstable intermediate • Chemical groups on enzyme do reaction • Chemistry happens and product diffuses out • Enzyme regenerated

  15. Kinetics • Study of reaction rates • Why? • Used to determine mechanisms • Michaelis Menton kinetics V = rate of reaction • Vmax = maximum reaction rate [S] • Substrate concentration • Km = substrate concentration where rate is half maximal

  16. Michaelis-Menton Plot

  17. I E S I S E Enzyme inhibition, Competitive • Both substrate and inhibitor bind to active site • Compete • Inhibitor blocks substrate from binding

  18. Michaelis- Menton Plot for competitive inhibition V uninhib Vmax inhibited Vmax/2 [S] Km Kmi

  19. Michaelis- Menton Plot for competitive inhibition • Vmax is not changed • Km increased • Since substrate has to outcompete the inhibitor for the active site, it takes more substrate to get to the same rate.

  20. Noncompetitive Inhibition S I I • Inhibitor binds to an allosteric site on the enzyme • Changes active site so substrate doesn’t bind

  21. Michaelis- Menton Plot for noncompetitive inhibition V uninhib Vmax uninhib Vmax inhib Vmax/2 [S] Km

  22. Michaelis- Menton Plot for noncompetitive inhibition • Inhibition lowers Vmax • Km unchanged • Since inhibitor doesn’t bind to active site, changing amount of substrate will have no effect

  23. Michaelis Menton Kinetics Summary • Competitive inhibition • Inhibitor competes with substrate for active site • Vmax unchanged • Km increased • Noncompetitive inhibition • Inhibitor binds to allosteric site changing active site • Vmax lowered • Km unchanged

  24. Allosteric Activation S S A A • Active site will not bind substrate • Allosteric activator binds and changes shape of active site • Now substrate binds

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