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: • Energy-yield nutrients are oxidized to CO2 and H2O and most of the electrons are passed to O2 via electron-transport pathway coupled with oxidative phosphorylation, resulting in the formation of ATP • Some electrons 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 • Metabolic intermediates in catabolism are the precursor for anabolism • NADPH supplies reducing power • ATP is the coupling energy • Gluconeogenesis • Fatty acid biosynthesis • Macromolecularsynthesis and growth • Creating macromolecules • Required energy from ATP • 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 derivatives (acetyl-CoA and suucinyl-CoA) • PEP • ATP & NADPH couple catabolism & anabolism • Phototrophs have an additional metabolic system– the photochemical apparatus

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 • 6 carbons • 12 hydrogens • 18 oxygens

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 (24 electrons)

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 • The value of 38 was established a long time ago in evolution • 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) • DG0’ for ATP hydrolysis is a large negative number • ATP changes the Keq by a factor of 108 (p69-70) • 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

13. The cell maintains a very high [ATP]/([ADP][Pi]) ratio • Living cells break down energy-yielding nutrient molecules to generate ATP • Glycolysis requires the investment of 2ATP/glucose before any energy yields • Fatty acid oxidation depends on fatty acid activation by acyl-CoA synthetase • So, ATP hydrolysis can serve as the driving force for virtually all biochemical events

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 metabolic lifetime of an ATP is brief • ATP, ADP, and AMP are all important effectors in exerting kinetic control on regulated enzymes • The regulation of metabolism by adenylates in turn requires close control of the relative concentrations of ATP, ADP, and AMP

16. Adenylate Kinase Interconverts ATP, ADP, and AMP • Adenylate kinase provides a direct connection among all three members of the adenylate pool ATP + AMP 2 ADP • The free energy of hydrolysis of a phosphoanhydride bond is the same in ADP and ATP • Adenylate pool: [ATP] + [ADP] + [AMP] • The Adenylates system provides phosphoryl groups to drive thermodynamically unfavorable reactions [ATP] [ADP] [AMP] PP

17. Energy Charge Relates the ATP Levels to the Total Adenine Nucleotide Pool • Energy charge (E.C.) is an index of how fully charged adenylates are with phosphoric anhydrides (ATP=2; ADP=1) Energy charge = • If all adenylate is [ATP] , E.C.1.0 • If [AMP] is the only adenylate form, E.C. 0 2[ATP] + [ADP] 1 2 [ATP] + [ADP] + [AMP]

18. Figure 27.2 Relative 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 fashionto adenine nucleotides • For example, phosphofructokinase is stimulated by AMP and inhibited by ATP • Regulatory enzymes in energy-producing catabolic pathways show greateractivity 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 • The competition between ATP (inactivate)and AMP (activate) for binding to the AMPK allosteric sites determines the activity of AMPK • When [ATP] is high, AMPK is inactive • When [AMP] is high, AMPK is allosterically activated and phosphorylates many targets controlling cellular energy production and consumption

22. Activation of AMPK • Sets in motion catabolic pathways leading to ATP synthesis • Shuts down pathways that consume ATP energy, such as biosynthesis and cell growth • AMP binding to AMPK increases its protein kinase activity by more than 1000-fold • AMP activates AMPK in two ways • It is an allosteric activator • AMP binding favors phosphorylation of Thr172 within the a-subunit • The regulation is reversed if ATP displaces AMP from the allosteric site

23. 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. (CBS: cystathionine-b-synthase)

24. 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) → [F-2,6-BP]↑ → stimulates glycolysis • glycogen synthase → inhibit glycogen synthesis • ACC → inhibit fatty acid biosynthesis • HMG-CoA reductase → inhibit cholesterol biosynthesis • Phosphorylation of transcription factors diminishes expression of gene encoding biosynthetic enzymes

25. AMPK controls whole-body energy homeostasis AMPK is activated by hormone such as adiponectin and leptin in sketal muscle. Exercise also activates AMPK Figure 27.6 AMPK regulation of energy production and consumption in mammals.

26. 27.5 – How Is Metabolism Integrated in a Multicellular Organism? • In complex multicellular organisms, Organ systems have arisen to carry out specificphysiological functions • Each organ expresses a repertoire of metabolic pathways • Such specialization depends on coordination of metabolic responsibilities among organs so that the organism as a whole may thrive • Organs differ in the metabolic fuels they prefer as substrates for energy production (see Figure 27.7)

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

28. 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 energyhomeostasis

29. The major organ systems have specialized metabolic roles

30. Brain Brain has two remarkable metabolic features • It has a very high respiratory metabolism • 20 % of oxygen consumed is used by the brain • Only 2% of body mass • Oxygen consumption is independent of mental activity, continuing even during sleep • It is an organ with no fuel reserves • Uses only glucose as a fuel and is dependent on the blood for a continuous, incoming supply (120g per day)

31. Brain During starvation, the body’s glycogen reserves are depleted, brain can use -hydroxybutyrate -hydroxybutyrate is formed from fatty acids in the liver and converted to acetyl-CoA → enter TCA cycle This allows the brain to use fat as fuel High rate of ATP production are necessary to maintain the membrane potentials essential for transmission of nerve impulses

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

33. Muscle • Skeletal muscles is responsible for about 30% of the O2 consumed by the human body at rest • During maximal exertion, skeletal muscle can account for more than 90% of the total metabolism • Muscle contraction occurs when a motor never impulse causes Ca+2 release from endomembrane compartments (sarcoplasmic reticulum) • The muscle contraction requires hydrolysis of ATP • In relaxation, Ca2+ ions are pumped back into the sarcoplamic reticulum. Two Ca2+ ions are translocated per ATP hydrolysis

34. Creatine Kinase in Muscle Muscle at rest can utilize a variety of fuels --glucose, fatty acids, and ketone bodies Rest muscle contains about 2% glycogen and 0.08% phoshpocreatine by weight When ATP is used to drive muscle contraction, the ADP formed can be reconverted to ATP by creatine kinase at the expense of phosphocreatine Muscle phosphocreatine can generate enough ATP to power about 4 seconds of exertion

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

36. Creatine Kinase in Muscle • During strenuous exertion, once phosphocreatine is depleted, muscle relies solely on its glycogen reserves • Glycolysis is capable of explosive bursts of activity • The flux of glucose-6-P through glycolysis can increase 2000-fold almost instantaneously • The triggers for this activation are Ca2+ and the “fight or flight” hormone epinephine • Glycolysis rapidly lowers pH (not lactate accumulation), causing muscle fatigue • The conversion of glucose to 2 lactate is accompanied by the release of 2 H+

37. Muscle Protein Degradation • During fasting or excessive activity, muscle protein is degraded to amino acids so that their carbon skeletons can be used as fuel • Many amino acids are converted to pyruvate, which can be transaminated to alanine • Alanine circulates to liver, where it is converted back to pyruvate – a substrate for gluconeogenesis • Muscle protein is a fuel of last resort

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

39. 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 • Heart tissue has minimal energy reserves: a small amount of phosphocreatine and limited glycogen • Continually nourished with oxygen and free fatty acid, glucose, or ketone bodies as fuel

40. Adipose tissue • Amorphous tissue widely distributed about the body • Consist of adipocytes • Endocrine organ: secrete leptin, adiponectin… • ~65% of the weight of adipose tissue is triacylglycerol (TAG) • Have a high rate of metabolic activity, synthesizing and breaking down of TAG • Free fatty acids are obtained from the liver • Lack glycerol kinase; cannot recycle the glycerol of TAG • Glucose plays a pivotal role for adipose tissue • Glycolysis produces DHAP converted to glycerol-3-P • Pentose phosphate pathway provides NAPDH

41. Brown fat • A specialized type of adipose tissue, is found in newborn and hibernating animals • Rich in mitochondria (brown color) • 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

42. 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 • Much of the liver’s activity centers around conversions involving glucose-6-phosphate

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

44. 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

45. 27.6 What Regulates Our Eating Behavior? • Approximately two-thirds of American are overweight • One-third of Americans are clinicallyobese • 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

46. Eating Behavior • The hormones control eating behavior • Produced in the stomach, liver, pancreas,... • Move to brain and act on neurons, principally on the arcuate nucleus region of the hypothalamus • The arcuate nucleus is an anatomically distinct brain area that functions in • Homeostasis of body weight • Body temperature • Blood pressure • Other vital functions

47. Eating Behavior—Are you hungry The hormones can be divided into Short-term regulator: determine individual meal Long-term regulator: act as stabilize the levels of body fat deposit Two subset neurons are involved: NPY/ AgRP-producing neurons – release NPY (neuropeptide Y) stimulating the neurons that trigger eating behavior Melanocortin-producing neurons-- inhibiting the neurons

48. (-) Figure 27.12 The regulatory pathways that control eating.

49. 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 • Ghrelin is an appetite-stimulating peptide hormone produced in the stomach • Cholecytokininreleased from GI tractduring eatingsignals satiety (the sense of fullness) and tends to curtail further eating

50. Insulin and leptin are long-term regulators of eating behavior (Both inhibit eating) • Insulin is produced in the b-cells of the pancreas when blood glucose level raise • The major role is to stimulate glucose uptake from the blood • Insulin also stimulates fat cells to make leptin • Leptin is an anorexic (appetite-suppressing) agent and inhibits the release of NPY • NPY is a orexic (appetite-stimulating) hormone • PYY3-36 inhibits eating by acting on the NPY/AgRP-producing neurons