Metabolism Is the Sum of Cellular Reactions

1. Metabolism Is the Sum of Cellular Reactions • Metabolism - the entire network of chemicalreactions carried out by living cells • Metabolites- small molecule intermediates in the degradation and synthesis of polymers • Catabolic reactions - degrademolecules to create smaller molecules and energy • Anabolic reactions - synthesize molecules for cell maintenance, growth and reproduction

2. Anabolism and catabolism

3. Metabolic Pathways Are Sequences of Reactions • Metabolism includes allenzymereactions • Metabolism can be subdivided into branches • The metabolism of the four major groups of biomolecules will be considered: Carbohydrates Lipids Amino Acids Nucleotides

4. Forms of metabolic pathways (a) Linear (b) Cyclic

5. Forms of metabolic pathways (c) Spiral pathway (fatty acid biosynthesis)

6. Metabolic Pathways Are Regulated • Metabolism is highlyregulated to permit organisms to respond to changing conditions • Most pathways are irreversible • Flux - flow of material through a metabolic pathway which depends upon: (1) Supply of substrates (2) Removal of products (3) Pathway enzyme activities

7. Feedback inhibition • Product of a pathway controls the rate of its own synthesis by inhibiting an early step (usually the first “committed” step (unique to the pathway)

8. Feed-forward activation • Metabolite early in the pathway activates an enzyme further down the pathway

9. Covalent modification for enzyme regulation • Interconvertible enzyme activity can be rapidly and reversibly altered by covalentmodification • Protein kinases phosphorylate enzymes (+ ATP) • Protein phosphatases remove phosphoryl groups • The initial signal may be amplified by the “cascade” nature of this signaling

10. Regulatory role of a protein kinase, amplification by a signaling cascade

11. Major Pathways in Cells • Metabolic fuels • Three major nutrients consumed by mammals: (1) Carbohydrates - provide energy(2) Proteins - provide amino acids for protein synthesis and some energy(3) Fats - triacylglycerols provide energy and also lipids for membrane synthesis

12. Overview of catabolic pathways

13. Electrons of reduced coenzymes flow toward O2 • This produces a protonflow and a transmembranepotential • Oxidative phosphorylationis the process by which the potential is coupled to the reaction: ADP + Pi ATP Reducing Power

14. Thermodynamics and Metabolism A. Free-Energy Change • Free-energy change(DG) is a measure of the chemicalenergyavailablefromareaction • DG = Gproducts - Greactants • DH = change in enthalpy • DS = change in entropy

15. Both entropy and enthalpy contribute to DG • DG = DH - TDS • (T = degrees Kelvin) • -DG = a spontaneous reaction in the direction written • +DG= the reaction is not spontaneous • DG= 0 the reaction is at equilibrium Relationship between energy and entropy

16. The Standard State (DGo) Conditions • Reaction free-energy depends upon conditions • Standard state(DGo)- defined reference conditions • Standard Temperature = 298K (25oC) • Standard Pressure = 1 atmosphere • Standard Solute Concentration = 1.0M • Biological standard state =DGo’ • Standard H+ concentration = 10-7 (pH = 7.0) rather than 1.0M (pH = 1.0)

17. For the reaction: A + B C + D Equilibrium Constants and Standard Free-Energy Change DGreaction = DGo’reaction + RT ln([C][D]/[A][B]) • At equilibrium: Keq = [C][D]/[A][B] and DGreaction = 0, so that: DGo’reaction = -RT ln Keq

18. Actual Free-Energy Change Determines Spontaneity of Cellular Reactions • When a reaction is not at equilibrium, the actual free energy change (DG) depends upon the ratio of products to substrates • Q = the mass action ratio DG = DGo’ + RT ln Q Where Q = [C]’[D]’ / [A]’[B]’

19. Hydrolysis of ATP

20. ATP is an “energy-rich” compound • A large amount of energy is released in the hydrolysis of the phosphoanhydridebonds of ATP (and UTP, GTP, CTP) • All nucleoside phosphates have nearly equal standard free energies of hydrolysis

21. Energy of phosphoanhydrides (1) Electrostaticrepulsion among negatively charged oxygens of phosphoanhydrides of ATP (2) Solvationofproducts (ADP and Pi) or (AMP and PPi) is better than solvation of reactant ATP (3) Productsaremorestablethanreactants There are more delocalized electrons on ADP, Pi or AMP, PPi than on ATP

22. Glutamine synthesis requires ATP energy

23. Phosphoryl-Group Transfer • Phosphoryl-group-transfer potential - the ability of a compound to transfer its phosphoryl group • Energy-rich or high-energycompounds have group transfer potentials equal to or greater than that of ATP • Low-energycompounds have group transfer potentials less than that of ATP

26. Transfer of the phosphoryl group from PEP to ADP • Phosphoenolpyruvate (PEP) (a glycolytic intermediate) has a high P-group transfer potential • PEP can donate a P to ADP to form ATP

27. Structures of PC and PA

28. Nucleotidyl-Group Transfer • Transfer of the nucleotidyl group from ATP is another common group-transfer reaction • Synthesis of acetyl CoA requires transfer of an AMP moiety to acetate • Hydrolysis of pyrophosphate (PPi) product drives reaction to completion

29. Synthesis of acetyl CoA

30. Synthesis of acetyl CoA

31. Thioesters Have High Free Energies of Hydrolysis • Thioesters are energy-rich compounds • Acetyl CoA has a DGo’ = -31 kJ mol-1

32. Succinyl CoA Energy Can Produce GTP

33. Amino acids, monosaccharides and lipids are oxidized in the catabolic pathways • Oxidizing agent - accepts electrons, is reduced • Reducing agent - loses electrons, is oxidized • Oxidation of one molecule must be coupled with the reduction of another molecule • Ared + Box Aox + Bred Reduced Coenzymes Conserve Energy from Biological Oxidations

34. Diagram of an electrochemical cell • Electrons flow through external circuit from Zn electrode to the Cu electrode

36. Standard reduction potentials and free energy • Relationship between standard free-energy change and the standard reduction potential: DGo’ = -nFDEo’ n = # electrons transferred F = Faraday constant (96.48 kJ V-1) DEo’= Eo’electron acceptor - Eo’electron donor

37. Reduction Potentials

38. Actual reduction potentials (DE) • Under biological conditions, reactants are not present at standard concentrations of 1 M • Actual reduction potential (DE) is dependent upon the concentrations of reactants and products • DE = DEo’ - (RT/nF) ln ([Aox][Bred] / [Ared][Box] )

39. Most NADH formed in metabolic reactions in aerobic cells is oxidized by the respiratory electron-transport chain • Energy used to produce ATP from ADP, Pi • Half-reaction for overall oxidation of NADH: • NAD+ + 2H+ + 2e- NADH + H+(Eo’ = -0.32V) Electron Transfer from NADH Provides Free Energy

41. Example Suppose we had the following voltaic cell at 25o C: Cu(s)/Cu+2 (1.0 M) // Ag+(1.0 M)/ Ag (s) What would be the cell potential under these conditions?

42. Example Suppose we had the following voltaic cell at 25o C: Cu(s)/Cu+2 (1.0 M) // Ag+(1.0 M)/ Ag (s) What would be the cell potential under these conditions? Ag+ + e- ---> Ag0 E0red = + 0.80 v Cu+2 + 2e- ----> Cu0 E0red = + 0.337 v

44. Example: Biological Systems Both NAD+ and FAD are oxidizing agents

45. The question is which would oxidize which? OR Which one of the above is the spontaneous reaction? in which DG is negative

46. To be able to answer the question We must look into the “electron donation” capabilities of NADH and FADH2 i.e. reduction potentials of NADH and FADH2

47. Remember, DEo’= Eo’electron acceptor - Eo’electron donor For a spontaneous reaction DEo’must be positive

48. Therefore, rearrange Add the two reactions

49. electron acceptor electron donor