1. Chapter 27 Metabolic Integration and Organ Specialization Biochemistryby
Reginald Garrett and Charles Grisham
What principles underlie the integration of catabolism and energy production with anabolism and energy consumption?
How is metabolism integrated in complex organisms with multiple organ systems?

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. Macromolecular synthesis and growth
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
Macromolecular synthesis 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. Only a 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. Three types of stoichiometry in biological systems
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 electron carriers
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
C6H12O O2  6 CO 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) C6H12O NAD+ + 2 FAD + 6 H2O  6 CO NADH + 10 H+ + 2 FADH2
(b) 10 NADH + 10 H+ + 2 FADH2 + 6 O2  H2O + 10 NAD+ + 2 FAD
(24 electrons)

11. 3. Evolved coupling stoichiometry
The coupled formation of ATP by oxidative phosphorylation
C6H12O O ADP Pi  6 CO ATP 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 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 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 fashion 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.
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. The b-subunit has an ag-binding domain that brings a and g together
AMPK is an abg heterotrimer; 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 specific physiological 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. glycogen > triacylglycerol > protein
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. 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 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. Cholesterol synthesis Detoxification organ
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 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

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

49. AgRP (agouti-related peptide) Melanocortin
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
Cholecytokinin released from GI tract during eating signals satiety (the sense of fullness) and tends to curtail further eating

50. PYY3-36 inhibits eating by acting on the NPY/AgRP-producing neurons
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

51. AMPK mediates many of the hypothalamic responses to these hormones
The actions of leptin, gherlin, and NPY converge at AMPK
Leptin inhibits AMPK in the arcuate nucleus via MC4R (melanocortin-4-receptor)
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

52. 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

53. 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
Humans have seven genes analogous to SIR2, designed SIRT1 through SIRT7
SIRT is an abbreviation of sirtuin
SIRT1 cycles between cytosol and nucleus
SIRT3, 4, and 5 confined to the mitochondria

54. SIRT 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+
NAD+/NADH ratio raise
Enhances sirtuin activity
CR increases mitochondrial biogenesis and then raises the ratio

55. Sirtuin-catalyzed removal of acetyl groups from lysine residues of histones
Allow the nucleosomes to interact more strongly with DNA, making transcription more difficult
Almost all of the enzymes of energy metabolism are acetylated and potentially regulated by SIRT

56. SIRT1 is a Key Regulator in Caloric Restriction
SIRT1 connects nutrient availability to the expression of metabolic genes
A striking feature of CR is the loss of fat stores and reduction of WAT (white adipose tissue)
SIRT1 participates (prevents) in the transcriptional regulation of adipogenesis through interaction with PPARg (peroxisome proliferator-activator receptor- g)
PPARg is a nuclear hormone receptor that activates transcription of genes involved in adipogenesis and fat storage
SIRT1 binding to two PPARg corepressors, NCoR and SMRT, prevents transcription of these genes, leading to loss of fat stores.

57. SIRT1 is a Key Regulator in Caloric Restriction
Because adipose tissue functions as an endocrine organ, this loss of fat has significant hormonal consequences for energy metabolism
In liver, SIRT1 interacts with and deacetylates PGC-1a (PPAR-g corepressor-1)
CR leads to increased transcription of these genes encoding the enzymes of gluconeogenesis and repression of genes encoding glycolytic enzymes
SIRT1 connects nutrient availability to the regulation of major pathways of energy storage (glycogen and Fat) and fuel utilization

58. Resveratrol in Red Wine is a Potent Activator of Sirtuin Activity
Is a phytoalexin, is a member of the polyphenol class of natural products
Is a free-radical scavenger, which may explain its cancer preventive properties.
Is red wine is an excellent source of resveratrol
Resveratrol activates SIRT NAD+-dependent deacetylase activity
Resveratrol activates AMPK in the brain

59. Metabolic syndrome High blood pressure
Elevated blood triglyceride levels
Low blood HDL-cholesterol levels
High blood glucose levels
Central obesity
Caloric restriction reverses all of the symptoms of metabolic syndrome
Nutrients are limiting (as in CR)
Activates SIRT1 (through deacetylation) and AMPK (phosphorylation)
Down-regulates anabolic processes and activates ATP-producing catabolic processes