Metabolic Processes

1. Metabolic Processes Enzymes, Energy and Chemical Reactions

2. Cellular Energy Processing • Metabolism: the sum of all chemical reactions • Anabolism: assembly, polymerization, etc. • requires energy • Catabolism: disassembly, depolymerization • releases energy • somereactions couple anabolism with catabolism • catabolism drives all anabolism • all reactions depend on enzyme catalysts

3. Energy can be stored or used for workFigure 6.1

4. Cellular Energy Processing • cellular processes change chemical structures & transport materials • change and movement require energy exchanges • energy exchanges have to follow the law(s)

5. Cellular Energy Processing • First Law of Thermodynamics • during any event, Initial Energy = FinalEnergy

6. …neither created nor destroyedFigure 6.2

7. Cellular Energy Processing • First Law of Thermodynamics • during any event, Initial Energy = FinalEnergy • Second Law of Thermodynamics • during any event, some energy is unavailable to do work

8. …some is unusable; disorder increasesFigure 6.2

9. Cellular Energy Processing • cells obtain energy from outside sources

10. …an external source is requiredFigure 6.2

11. Total energy =Figure 6.2

12. Cellular Energy Processing • total energy = usable energy + unusable energy, or • enthalpy = free energy + (entropy · absolute temperature) • H=G +TS, so, G=H-TS (three unmeasurable variables) • DG=DH-TDS (change in free energy at constant temperature)

13. DG > 0; energy requiredFigure 6.3

14. Cellular Energy Processing • DG=DH-TDS describes energy changes in chemical reactions • positiveDG describes an energy-requiring reaction; anabolism; decrease in entropy • negativeDG describes an energy-yielding reaction; catabolism; increase in entropy

15. DG < 0; energy releasedFigure 6.3

16. Cellular Energy Processing • spontaneity (≠ rate) • a spontaneous reaction goes more than half way to completion without an energy input; it is exergonic; DG < 0 • a nonspontaneous reaction goes less than half way to completion without an energy input; it is endergonic; DG > 0 • if A=>B is exergonic, B=>A is endergonic

17. Cellular Energy Processing • reactions are reversible • A <=> B • add more A, increase => rate • add more B, increase <= rate • equilibrium occurs when rates are equal • the closer to completion equilibrium occurs, the more free energy is released

18. reversible reaction at equilibriumFigure 6.4

19. ATP: the cell’s chief energy currencyFigure 6.5

20. cellular respiration supplies ATP for anabolismFigure 6.6

21. ATP hydrolysis coupled to glutamine synthesisFigure 6.7

22. cellular energy transfer • Adenosine TriPhosphate (ATP) is the predominant energy currency in the cell • ATP hydrolysis is exergonic (DG = -7.3 kcal/mol) • ATP + H2O => ADP + Pi • ATP synthesis is endergonic • ATP shuttles energy from exergonic reactions to endergonic reactions • each ATP is recycled ~10,000 times/day • ~1,000,000 ATPs are used by a cell/second

23. Enzymes: Biological Catalysts • a catalyst: increases the reaction rate; is unchanged by the reaction • most biological catalysts are proteins • some (few) biological catalysts are ribozymes (RNA)

24. Ea determines the likelihood that a reaction will occurFigure 6.8

25. Enzymes: Biological Catalysts • each chemical reaction must overcome an energy barrier - activation energy (Ea) • spontaneous reactions will go - eventually • the direction is predictable • neither likelihood, nor rate is predictable

26. heatmay supplyEaFigure 6.9

27. E + S => E-S complex => E + PFigure 6.10

28. position substrates Figure 6.12 induce strain alter surface charge

29. Enzymes: Biological Catalysts • how to overcome the energy barrier? • increase kinetic energy of reactant molecules, or • decrease Ea • an enzyme binds a specific substrate molecule(s) at its active site • E + S => E-S complex => E + P • the active site > positions reactants, strains bonds, etc. to destabilize the reactants… • …lowering Ea

30. enzyme: lowers Ea, doesn’t change DGFigure 6.11

31. Enzymes: Biological Catalysts • enzymes… • efficiency experts of the metabolic world • lower activation energy • do not alter equilibrium • increase the rates of forward and reverse reactions

32. Enzymes: Biological Catalysts • substrate concentration affects reaction rate • as increased [reactant] increases reaction rate • so increased [substrate] increases reaction rate • until… • all active sites are occupied • the reaction is saturated

33. enzymatic reactions may be saturatedFigure 6.16

34. induced fit in hexokinaseFigure 6.14

35. Enzymes: Biological Catalysts • enzyme structure determines enzyme function • the active site fits the substrate • “lock & key” • “induced fit” • the rest of the enzyme • stabilizes the active site • provides flexibility

36. Figure 6.15

37. Enzymes: Biological Catalysts • enzyme structure determines enzyme function • some enzymes require non-protein groups • cofators: reversibly-bound ions • coenzymes: reversibly bound organic molecules • prosthetic groups: permanently bound groups

38. Table 6.1

39. Enzymes & Metabolism • metabolic regulation coordinates the many potential enzymatic reactions • sequential reactions form pathways • product of 1st reaction is substrate for 2nd E1 E2 E3 E4 A=> B=> C=> D=> product of pathway • regulation of enzymes in the pathway regulates the entire pathway

40. irreversible inhibition by DIPFFigure 6.17 related to Sarin gas and malathion

41. Enzymes & Metabolism • metabolic regulation coordinates the many potential enzymatic reactions • enzyme inhibitorsprovide negative control • artificial inhibitors can be pesticides • irreversible inhibition - covalent modification of active site • natural metabolic regulation is often reversible • competitive inhibition

42. cartoon versionFigure 6.18

43. Enzymes & Metabolism • metabolic regulation coordinates the many potential enzymatic reactions • enzyme inhibitorsprovide negative control • artificial inhibitors can be pesticides • irreversible inhibition - covalent modification of active site • natural metabolic regulation is often reversible • competitive inhibition • noncompetitive inhibition

44. cartoon versionFigure 6.18

45. Enzymes & Metabolism • metabolic regulation coordinates the many potential enzymatic reactions • allosteric enzymes have catalytic and regulatory subunits • active and inactive enzyme conformations are in equilibrium

46. Figure 6.19

47. Figure 6.20

48. Enzymes & Metabolism • metabolic regulation coordinates the many potential enzymatic reactions • allosteric enzymes regulate many metabolic pathways • catalyze first committed step • respond sensitively to inhibition • often inhibited by pathway end product - “end-product inhibition”

49. end-product inhibition by isoleucineFigure 6.21

50. Enzymes & Metabolism • metabolic regulation coordinates the many potential enzymatic reactions • allosteric enzymes regulate many metabolic pathways • catalyze first committed step • respond sensitively to inhibition • often inhibited by pathway end product - “end-product inhibition” • saves resources when end product is sufficient