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Chapter 27
Metabolic Integration and
Organ Specialization
Biochemistry
by
Reginald Garrett and Charles Grisham
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?
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:
1. Catabolism
2. Anabolism
3. Macromolecular synthesis and growth
•
Catabolic and anabolic pathways, occurring
simultaneously, must act as a regulated,
orderly, responsive whole
Figure 27.1 Block diagram of
intermediary metabolism.
•
Catabolism:
–
–
–
1.
2.
3.
4.
5.
Energy-yield nutrients are oxidized to CO2 and H2O and
most of the electrons are passed to O2 via electrontransport 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
•
Anabolism:
– The biosynthetic reactions
– Metabolic intermediates in catabolism are the
precursor for anabolism
– NADPH supplies reducing power
– ATP is the coupling energy
1. Gluconeogenesis
2. 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
• 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 aketoglutarate)
– CoA derivatives (acetyl-CoA and suucinyl-CoA)
– PEP
• ATP & NADPH couple catabolism & anabolism
• Phototrophs have an additional metabolic
system– the photochemical apparatus
27.2 – What Underlying Principle Relates
ATP Coupling to the Thermodynamics of
Metabolism?
Three types of stoichiometry in biological systems
1. Reaction stoichiometry - the number of each
kind of atom in a reaction
2. Obligate coupling stoichiometry - the required
coupling of electron carriers
3. Evolved coupling stoichiometry - the number
of ATP molecules that pathways have evolved
to consume or produce - a number that is a
compromise
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
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)
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
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
•
The cell maintains a very high
[ATP]/([ADP][Pi]) ratio
–
Living cells break down energy-yielding nutrient
molecules to generate ATP
1. Glycolysis requires the investment of
2ATP/glucose before any energy yields
2. 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
ATP has two metabolic roles
1. ATP is the energy currency of the cells
–
–
To establish large equilibrium constant for
metabolic conversions
To render metabolic sequence
thermodynamically favorable
2. An important allosteric effector in the
kinetic regulation of metabolism
•
•
PFK in glycolysis
FBPase in gluconeogenesis
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
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
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)
1
Energy charge =
2
2[ATP] + [ADP]
[ATP] + [ADP] + [AMP]
• If all adenylate is [ATP] , E.C.1.0
• If [AMP] is the only adenylate form, E.C. 0
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.)
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
0.85 - 0.88
Figure 27.3 Responses of regulatory enzymes to variation in energy charge.
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
1. When [ATP] is high, AMPK is inactive
2. When [AMP] is high, AMPK is allosterically
activated and phosphorylates many targets controlling
cellular energy production and consumption
– Activation of AMPK
1. Sets in motion catabolic pathways leading to ATP
synthesis
2. 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
1. It is an allosteric activator
2. AMP binding favors phosphorylation of Thr172
within the a-subunit
– The regulation is reversed if ATP displaces AMP from
the allosteric site
• 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)
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
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.
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)
Figure 27.7 Metabolic relationships among the major human organs.
• 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
The major organ systems have
specialized metabolic roles
Brain
Brain has two remarkable metabolic features
1. 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
2. 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)
Brain
During starvation, the body’s glycogen reserves
are depleted, brain can use b-hydroxybutyrate
–
–
b-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
Figure 27.8 Ketone bodies
such as β-hydroxybutyrate
provide the brain with a
source of acetyl-CoA when
glucose is unavailable.
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
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
Creatine Kinase and Phosphocreatine
Provide an Energy Reserve in Muscle
Figure 27.9 Phosphocreatine serves as a reservoir of ATPsynthesizing potential.
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+
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
Figure 27.10 The transamination of pyruvate to alanine by
glutamate:alanine aminotransferase.
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
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
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
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
Figure 27.11 Metabolic conversions
of glucose-6-phosphate in the liver.
• 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
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
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
Eating Behavior—Are you hungry
•
The hormones can be divided into
1. Short-term regulator: determine individual meal
2. Long-term regulator: act as stabilize the levels of
body fat deposit
•
Two subset neurons are involved:
1. NPY/ AgRP-producing neurons – release NPY
(neuropeptide Y) stimulating the neurons that
trigger eating behavior
2. Melanocortin-producing neurons-- inhibiting the
neurons
(-)
Figure 27.12 The regulatory
pathways that control eating.
•
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 released from GI tract during eating
signals satiety (the sense of fullness) and tends to curtail
further eating
•
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
•
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
1. AMPK phosphorylates ( inhibits) acetyl-CoA
carboxylase
2. Malonyl-CoA levels decreased
3. Low [malonyl-CoA] is associated with increased
food intake
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
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
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
1.Oxidative metabolism, which drives conversion of
NADH to NAD+
2.NAD+/NADH ratio raise
3.Enhances sirtuin activity
• CR increases mitochondrial biogenesis and then
raises the ratio
• 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
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.
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 PGC1a (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
Resveratrol in Red Wine is a Potent Activator
of Sirtuin Activity
Resveratrol:
• 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
Metabolic syndrome
1. High blood pressure
2. Elevated blood triglyceride levels
3. Low blood HDL-cholesterol levels
4. High blood glucose levels
5. 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