CHAPTER 15 Principles of Metabolic Regulation

1. CHAPTER 15Principles of Metabolic Regulation Key topics: • Principles of regulation in biological systems • Glycolysis vs. gluconeogenesis?

2. Metabolic Pathways • The biochemical reactions in the living cell — the metabolism — is organized into metabolic pathways • The pathways have dedicated purposes • Some are dedicated to extraction of energy • Some are dedicated to storage of fuels • Some are dedicated for synthesis of important building blocks • Some are dedicated to elimination of waste materials • The pathways can be represented as a map • Follow the fate of metabolites and building blocks • Identify enzymes that act on these metabolites • Identify points and agents of regulation • Identify sources of metabolic diseases

3. Map of Metabolic Pathways

5. Homeostasis • Organisms maintain homeostasis by keeping the concentrations of most metabolites at steady state • In steady state, the rate of synthesis of a metabolite equals the rate of breakdown of this metabolite

6. Principles of Regulation • The flow of metabolites through the pathways is regulated to maintain homeostasis • Sometimes, the levels of required metabolites must be altered very rapidly • Need to increase the capacity of glycolysis during the action • Need to reduce the capacity of glycolysis after the action • Need to increases the capacity of gluconeogenesis after successful action

7. Feedback Inhibition • In many cases, ultimate products of metabolic pathways directly or indirectly inhibit their own biosynthetic pathways • ATP inhibits the commitment step of glycolysis

9. Reactions Far From Equilibrium are Common Points of Regulation • Living systems thrive by keeping some metabolic reactions far from equilibrium while the levels of metabolites are in steady state

12. Rates of a Biochemical Reactions • Rates of a biochemical reactions depend on many factors • Concentration of reactants • Activity of the catalyst • Concentration of the enzyme • Intrinsic activity of the enzyme • Concentrations of effectors • Allosteric regulators • Competing substrates • pH, ionic environment • Temperature

15. Factors that Determine the Activity of Enzymes

17. Active Protein Molecules have a Finite Lifespan • Different proteins in the same tissue have very different half-lives • less than an hour to about a week for liver enzymes • The stability correlates with the sequence at N-terminus • Some proteins are as old as you are • Crystallins in the eye lens

19. Phosphorylation of Enzymes Affects their Activity • Protein phosphorylation is catalyzed by protein kinases • Dephosphorylation is spontaneous, or catalyzed by protein phosphatases • Typically, hydroxyl groups of Ser, Thr, or Tyr are phosphorylated

21. Some Enzymes in the Pathway Limit the Flux of Metabolites More than Others • Hexokinase and phosphofructokinase are appropriate targets for regulation of glycolytic flux • Increased hexokinase activity enables activation of glucose • Increased phosphofructokinase-1 activity enables catabolism of activated glucose via glycolysis

23. Control of Glycogen Synthesis • Insulin signaling pathway • increases glucose import into muscle • stimulates the activity of muscle hexokinase • activates glycogen synthase • Increased hexokinase activity enables activation of glucose • Glycogen synthase makes glycogen for energy storage

25. Regulation of Hexokinase IV by Sequestration

27. Rate of Reaction Depends on the Concentration of Substrates • The rate is more sensitive to concentration at low concentrations • Frequency of substrate meeting the enzyme matters • The rate becomes insensitive at high substrate concentrations • The enzyme is nearly saturated with substrate

29. Isozymes may Show Different Kinetic Properties • Isozymes are different enzymes that catalyze the same reaction • They typically share similar sequences • Their regulation is often different

31. Glycolysis vs. Gluconeogenesis

33. Regulation of Phosphofructokinase-1 • The conversion of fructose-6-phosphate to fructose 1,6-bisphosphate is the commitment step in glycolysis • ATP is a negative effector • Do not spend glucose in glycolysis if there is plenty of ATP

34. Phosphofructokinase-1 (PFK-1)

36. Regulation of Phosphofructokinase 1 and Fructose 1,6-Bisphosphatase • Go glycolysis if AMP is high and ATP is low • Go gluconeogenesis if AMP is low

38. Regulation by Fructose 2,6-Bisphosphate • F26BP activates phosphofructokinase (glycolytic enzyme) • F26BP inhibits fructose 1,6-bisphosphatase (gluconeogenetic enzyme)

42. Regulation by Fructose 2,6-Bisphosphate • Go glycolysis if F26BP is high • Go gluconeogenesis if F26BP is low

44. Regulation of 2,6-Bisphosphate Levels

47. Molecular Origin of Enzyme Regulation • Regulation of catalysis typically involves • Binding of inhibitors, often to the active site • Binding of regulatory protein subunits

49. Regulation of Pyruvate Kinase • Signs of abundant energy supply allosterically inhibit all pyruvate kinase isoforms • Signs of glucose depletion (glucagon) inactivate liver pyruvate kinase via phosphorylation • Glucose from liver is exported to brain and other vital organs