Metabolic Integration and

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Chapter 27

• Metabolic Integration and
• Organ Specialization
• Biochemistry
• by
• Reginald Garrett and Charles Grisham

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Outline

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

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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

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Figure 27.1 Block diagram of intermediary
metabolism.
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• 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

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• 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

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• 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 derivs (acetyl-CoA and suucinyl-CoA)
• PEP
• ATP NADPH couple catabolism anabolism
• Phototrophs also have photosynthesis and CO2
  fixation systems

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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

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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

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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

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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

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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/(ADPPi)
  ratio

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• The cell maintains a very high ATP/(ADPPi)
  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

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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

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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

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• 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

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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
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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.)
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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

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

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• 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.
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• 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

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Figure 27.6 AMPK regulation of energy production
and consumption in mammals.
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27.5 How Is Metabolism Integrated in a
Multicellular Organism?

• Organ systems in complex multicellular organisms
  have arisen to carry out specific physiological
  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)

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Figure 27.7 Metabolic relationships among the
major human organs.
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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 gt triacylglycerol gt protein
• The tissues of the body work together to maintain
  energy homeostasis

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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

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Figure 27.8 Ketone bodies such as
ß-hydroxybutyrate provide the brain with a source
of acetyl-CoA when glucose is unavailable.
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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 Ca2 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

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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

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

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Figure 27.10 The transamination of pyruvate to
alanine by glutamatealanine aminotransferase.
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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

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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 of hormone-sensitive
  lipase
• Lack glycerol kinase cannot recycle the glycerol
  of TAG

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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

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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

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• 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

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

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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

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Figure 27.12 The regulatory pathways that control
eating.
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• 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
• Cholecytokinin signal satiety and tends to
  curtail further eating

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• 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

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• 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

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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

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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

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Figure 27.13 The NAD-dependent protein
deacetylase reaction of sirtuins.
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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 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 PPARg represses transcription of
  these genes, leading to loss of fat stores.
• Because adipose tissue functions as an endocrine
  organ, this loss of fat has significant hormonal
  consequences for energy metabolism

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SIRT1 is a Key Regulator in Caloric Restriction

• SIRT1 connects nutrient availability to the
  expression of metabolic genes
• SIRT1 participates in the transcriptional
  regulation of adipogenesis through interaction
  with PPARg (peroxisome proliferator-activator
  receptor- g)
• PPAR g is a nuclear hormone receptor that
  activates transcription of genes involved in
  adipogenesis and fat storage
• SIRT1 binding to PPAR g represses transcription
  of these genes, leading to loss of fat stores.
• Because adipose tissue functions as an endocrine
  organ, this loss of fat has significant hormonal
  consequences for energy metabolism

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Resveratrol in Red Wine is a Potent Activator of
Sirtuin Activity
French people enjoy longevity despite a high-fat
diet. Resveratrol may be the basis of this
French paradox.
Figure 27.14 Resveratrol, a phytoalexin, is a
member of the polyphenol class of natural
products. It is a free-radical scavenger, which
may explain its cancer preventive properties.