Chapter 27

1. Chapter 27 Metabolic Integration and Organ Specialization Biochemistry by Reginald Garrett and Charles Grisham

2. Outline • Can systems analysis simplify the complexity of metabolism? • What underlying principle relates ATP coupling to the thermodynamics of metabolism? • Is there a good index of cellular energy status? • How is overall energy balance regulated in cells? • How is metabolism integrated in a multicellular organism? • What regulates our eating behavior? • Can you really live longer by eating less?

3. 27.1 – Can Systems Analysis Simplify the Complexity of Metabolism? • The metabolism can be portrayed by a schematic diagram consisting of just three interconnected functional block: • Catabolism • Anabolism • Macromolecular synthesis and growth • Catabolic and anabolic pathways, occurring simultaneously, must act as a regulated, orderly, responsive whole

4. Figure 27.1 Block diagram of intermediary metabolism.

5. Catabolism: • Foods are oxidized to CO2 and H2O • The formation of ATP • Reduce NADP+ to NADPH • The intermediates serve as substrates for anabolism Glycolysis The citric acid cycle Electron transport and oxidative phosphorylation Pentose phosphate pathway Fatty acid oxidation

6. Anabolism: • The biosynthetic reactions • The chemistry of anabolism is more complex • Metabolic intermediates in catabolism are the precursor for anabolism • NADPH supplies reducing power • ATP is the coupling energy • Macromolecular synthesis and growth • Creating macromolecules • Macromolecules are the agents of biological function and information • Growth can be represented as cellular accumulation of macromolecules

7. Onlya few intermediates interconnect the major metabolic systems • Sugar-phosphates (triose-P, tetraose-P, pentose-P, and hexose-P) • a-keto acids (pyruvate, oxaloacetate, and a-ketoglutarate) • CoA derivs (acetyl-CoA and suucinyl-CoA) • PEP • ATP & NADPH couple catabolism & anabolism • Phototrophs also have photosynthesis and CO2 fixation systems

8. 27.2 – What Underlying Principle Relates ATP Coupling to the Thermodynamics of Metabolism? Three types of stoichiometry in biological systems • Reaction stoichiometry - the number of each kind of atom in a reaction • Obligate coupling stoichiometry - the required coupling of electroncarriers • Evolved coupling stoichiometry - the number of ATP molecules that pathways have evolved to consume or produce - a number that is a compromise

9. 1. Reaction stoichiometry The number of each kind of atom in any chemical reaction remains the same, and thus equal numbers must be present on both sides of the equation C6H12O6 + 6 O2 6 CO2 + 6 H2O

10. 2. Obligate coupling stoichiometry Cellular respiration is an oxidation-reduction process, and the oxidation of glucose is coupled to the reduction of NAD+ and FAD (a) C6H12O6 + 10 NAD+ + 2 FAD + 6 H2O  6 CO2 + 10 NADH + 10 H+ + 2 FADH2 (b) 10 NADH + 10 H+ + 2 FADH2 + 6 O2 12 H2O + 10 NAD+ + 2 FAD

11. 3. Evolved coupling stoichiometry • The coupled formation of ATP by oxidative phosphorylation C6H12O6 + 6 O2 + 38 ADP + 38 Pi  6 CO2 + 38 ATP + 44 H2O • Prokaryotes: 38 ATP • Eukaryotes: 32 or 30 ATP

12. ATP coupling stoichiometry determines the Keq for metabolic sequence • The energy release accompanying ATP hydrolysis is transmitted to the unfavorable reaction so that the overall free energy for the coupled process is negative (favorable) • The involvement of ATP alters the free energy change for a reaction • the role of ATP is to change the equilibrium ratio of [reactants] to [products] for a reaction • The cell maintains a very high [ATP]/([ADP][Pi]) ratio

13. The cell maintains a very high [ATP]/([ADP][Pi]) ratio • ATP hydrolysis can serve as the driving force for virtually all biochemical events • Living cells break down energy-yielding nutrient molecules to generate ATP

14. ATP has two metabolic roles • ATP is the energy currency of the cells • To establish large equilibrium constant for metabolic conversions • To render metabolic sequence thermodynamically favorable • An important allosteric effector in the kinetic regulation of metabolism • PFK in glycolysis • FBPase in gluconeogenesis

15. 27.3 – Is there a good index of cellular energy status?? • Energy transduction and energy storage in the adenylate system– ATP, ADP, and AMP – lie at the very heart of metabolism • The regulation of metabolism by adenylates in turn requires close control of the relative concentrations of ATP, ADP, and AMP • ATP, ADP, and AMP are all important effectors in exerting kinetic control on regulated enzymes

16. Adenylate kinase interconverts ATP, ADP, and AMP ATP + AMP  2 ADP • Adenylate kinase provides a direct connection among all three members of the adenylate pool • Adenylate pool: [ATP] + [ADP] + [AMP] • Adenylates provide phosphoryl groups to drive thermodynamically unfavorable reactions

17. Energy Charge Relates the ATP Levels to the Total Adenine Nucleotide Pool • Energy charge is an index of how fully charged adenylates are with phosphoric anhydrides Energy charge = • If [ATP] is high, E.C.1.0 • If [ATP] is low, E.C. 0 [ATP] + ½ [ADP] [ATP] + [ADP] + [AMP]

18. Figure 27.2Relative concentrations of AMP, ADP, and ATP as a function of energy charge. (This graph was constructed assuming that the adenylate kinase reaction is at equilibrium and that DG°' for the reaction is -473 J/mol; Keq = 1.2.)

19. Key enzymes are regulated by Energy charge • Regulatory enzymes typically respond in reciprocal fashing to adenine nucleotides • For example, phosphofructokinase is stimulated by AMP and inhibited by ATP • Regulatory enzymes in energy-producing catabolic pathways show greater activity at low energy charge • PFK and pyruvate kinase • Regulatory enzymes of anabolic pathways are not very active at low energy charge • Acetyl-CoA carboxylase

20. 0.85 - 0.88 Figure 27.3 Responses of regulatory enzymes to variation in energy charge.

21. 27.4 – How is Overall Energy Balance Regulated in Cells? • AMP-activated protein kinase (AMPK)is the cellular energy sensor • Metabolic inputs to this sensor determine whether its output (protein kinase activity) takes place • When ATP is high, AMPK is inactive • When ATP is low, AMPK is allosterically activated and phosphorylates many targets controlling cellular energy production and consumption • The competition between ATP and AMP for binding to the AMPK allosteric sites determines the activity of AMPK

22. AMPK is an abgheterotrimer; the a-subunit is the catalytic subunit and the g-subunit is regulatory • The b-subunit has an ag-binding domain that brings a and g together Figure 27.4 Domain structure of the AMP-activated protein kinase (AMPK) subunits.

23. AMPK targets key enzymes in energy production and consumption • Activation of AMPK leads to phosphorylation of many key enzymes in energy metabolism • Include phosphorylation of PFK-2 (in liver); glycogen synthase; ACC; HMG-CoA reductase • Phosphorylation of transcription factors diminishes expression of gene encoding biosynthetic enzymes • AMPK controls whole-body energy homeostasis

24. Figure 27.6 AMPK regulation of energy production and consumption in mammals.

25. 27.5 – How Is Metabolism Integrated in a Multicellular Organism? • Organ systems in complex multicellular organisms have arisen to carry out specificphysiological functions • Such specialization depends on coordination of metabolic responsibilities among organs so that the organism as a whole can thrive • Organs differ in the metabolic fuels they prefer as substrates for energy production (see Figure 27.7)

26. Figure 27.7 Metabolic relationships among the major human organs.

27. 27.5 – How Is Metabolism Integrated in a Multicellular Organism? • The major fuel depots in animals are glycogen in live and muscle; triacylglycerols in adipose tissue; and protein, mostly in skeletal muscle • The usual order of preference for use of these is glycogen > triacylglycerol > protein • The tissues of the body work together to maintain energy homeostasis

29. Brain Brain has two remarkable metabolic features • very high respiratory metabolism 20 % of oxygen consumed is used by the brain • but no fuel reserves Uses only glucose as a fuel and is dependent on the blood for a continuous incoming supply (120g per day) In fasting conditions, brain can use -hydroxybutyrate (from fatty acids in liver), converting it to acetyl-CoA for the energy production via TCA cycle Generate ATP to maintain the membrane potentials essential for transmission of nerve impulses

30. Figure 27.8 Ketone bodies such as β-hydroxybutyrate provide the brain with a source of acetyl-CoA when glucose is unavailable.

31. Muscle • Skeletal muscles is responsible for about 30% of the O2 consumed by the human body at rest • Muscle contraction occurs when a motor never impulse causes Ca+2 release from endomembrane compartments • Muscle can utilize a variety of fuels --glucose, fatty acids, and ketone bodies • Rest muscle contains about 2% glycogen and 0.08% phoshpocreatine

32. Creatine Kinase in Muscle • About 4 seconds of exertion, phosphocreatine provide enough ATP for contraction • During strenuous exertion, once phosphocreatine is depleted, muscle relies solely on its glycogen reserves • Glycolysis is capable of explosive bursts of activity, and the flux of glucose-6-P through glycolysis can increase 2000-fold almost instantaneously • Glycolysis rapidly lowers pH (lactate accumulation), causing muscle fatigue

33. Creatine Kinase and Phosphocreatine Provide an Energy Reserve in Muscle Figure 27.9 Phosphocreatine serves as a reservoir of ATP-synthesizing potential.

34. Muscle Protein Degradation • During fasting or excessive activity, amino acids are degraded to pyruvate, which can be transaminated to alanine • Alanine circulates to liver, where it is converted back to pyruvate – a substrate for gluconeogenesis • This is a fuel of last resort for the fasting or exhausted organism

35. Figure 27.10 The transamination of pyruvate to alanine by glutamate:alanine aminotransferase.

36. Heart • The activity of heart muscle is constant and rhythmic • The heart functions as a completely aerobic organ and is very rich in mitochondria • Prefers fatty acid as fuel • Continually nourished with oxygen and free fatty acid, glucose, or ketone bodies as fuel

37. Adipose tissue • Amorphous tissue widely distributed about the body • Consist of adipocytes • ~65% of the weight of adipose tissue is triacylglycerol • continuous synthesis and breakdown of triacylglycerols, with breakdown controlled largely via the activation ofhormone-sensitive lipase • Lack glycerol kinase; cannot recycle the glycerol of TAG

38. Brown fat • A specialized type of adipose tissue, is found in newborn and hibernating animals • Rich in mitochondria • Thermogenin, uncoupling protein-1, permitting the H+ ions to reenter the mitochondria matrix without generating ATP • Is specialized to oxidize fatty acids for heat production rather than ATP synthesis

39. Liver • The major metabolic processing center in vertebrates, except for triacylglycerol • Most of the incoming nutrients that pass through the intestines are routed via the portal vein to the liver for processing and distribution • Liver activity centers around glucose-6-phosphate

40. Glucose-6-phosphate • From dietary carbohydrate, degradation of glycogen, or muscle lactate • Converted to glycogen • released as blood glucose, • used to generate NADPH and pentoses via the pentose phosphate pathway, • catabolized to acetyl-CoA for fatty acid synthesis or for energy production in oxidative phosphorylation • Fatty acid turnover • Cholesterol synthesis • Detoxification organ

41. Figure 27.11Metabolic conversions of glucose-6-phosphate in the liver.

42. 27.6 What Regulates Our Eating Behavior? • Approximately two-thirds of American are overweight • One-third of Americans are clinically obese • Obesity is the most important cause of type 2 diabetes • Research into the regulatory controls on feeding behavior has become a medical urgency • The hormones that control eating behavior come from many different tissues

43. Are you hungry • The hormones control eating behavior • Produced in the stomach, liver,…. • Move to brain and act on neurons within the arcuate nucleus region of the hypothalamus • The hormones are divided into • Short-term regulator: determine individual meal • Long-term regulator: act as stabilize the levels of body fat deposit • Two subset neurons • NPY/ AgRP producing neurons -- stimulating • Melanocortin producing neurons-- inhibiting

44. Figure 27.12 The regulatory pathways that control eating.

45. AgRP (agouti-related peptide) • Block the activity of melanocortin-producing neurons • Melanocortin • Inhibit the neurons initiating eating behavior • Including a- and b-MSH (melanocyte-stimulating hormone) • Ghrelin and cholecytokinin are short-term regulators of eating behavior • Ghrelinis an appetite-stimulating peptide hormone produced in the stomach • Cholecytokininsignal satiety and tends to curtail further eating

46. Insulin and leptin are long-term regulators of eating behavior • Insulin is produced in the b-cells of the pancreas when blood glucose level raiseinsulin • Insulin stimulates fat cells to make leptin • Leptin is an anorexic (appetite-suppressing) agent • NPY is a orexic (appetite-stimulating) hormone • PYY3-36 inhibits eating by acting on the NPY/AgRP-producing neurons

47. AMPK mediates many of the hypothalamic responses to these hormones • The actions of leptin, gherlin, and NPY converge at AMPK • Leptin inhibits AMPK • Gherlin and NPY activate hypothalamic AMPK • The effects of AMPK may be mediated through changes in malonyl-CoA levels • AMPK phosphorylates ( inhibits) acetyl-CoA carboxylase • malonyl-CoA levels decreased • Low [malonyl-CoA] is associated with increased food intake

48. 27.7 Can You Really Live Longer by Eating Less? Caloric restriction leads to longevity • For most organisms, caloric restriction results in • lower blood glucose levels • declines in glycogen and fat stores • enhanced responsiveness to insulin • lower body temperature • diminished reproductive capacity • Caloric restriction also diminishes the likelihood for development of many age-related diseases, including cancer, diabetes, and atherosclerosis

49. Mutations in the SIR2 Gene Decrease Life Span • Deletion of a gene termed SIR2 (silent information regulator 2) abolishes the ability of caloric restriction to lengthen life in yeast and roundworms • This implicates the SIR2 gene product in longevity • The human gene analogous to SIR2 is SIRT1, for sirtuin 1 • Sirtuins are NAD+-dependent protein deacetylases • The tissue NAD+/NADH ratio controls sirtuin protein deacetylase activity • Nicotinamide and NADH are inhibitors of the deacetylase reaction • Oxidative metabolism, which drives conversion of NADH to NAD+, enhances sirtuin activity

50. Figure 27.13 The NAD+-dependent protein deacetylase reaction of sirtuins.