Chapter 27

Metabolic Integration and Organ Specialization

Biochemistry

by

Reginald Garrett and Charles Grisham

Outline1. 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:– 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

• 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

• Only a few intermediates interconnect the major metabolic systems– Sugar-phosphates (triose-P, tetraose-P, pentose-P,

and hexose-P) -keto acids (pyruvate, oxaloacetate, and -

ketoglutarate) – CoA derivs (acetyl-CoA and suucinyl-CoA)– PEP

• ATP & NADPH couple catabolism & anabolism

• Phototrophs also have photosynthesis and CO2 fixation systems

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

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

1. Reaction 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

2. Obligate 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

3. Evolved coupling stoichiometry

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

• 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

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

ATP has two metabolic roles

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

• 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

• 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]

Energy Charge Relates the ATP Levels to the Total Adenine Nucleotide Pool

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 G°' for the reaction is -473 J/mol; Keq = 1.2.)

• 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

Key enzymes are regulated by Energy charge

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– 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

Figure 27.4 Domain structure of the AMP-activated protein kinase (AMPK) subunits.

• AMPK is an heterotrimer; the -subunit is the catalytic subunit and the -subunit is regulatory

• The -subunit has an -binding domain that brings and together

• 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

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

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)

Figure 27.7 Metabolic relationships among the major human organs.

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

BrainBrain has two remarkable metabolic features

1. very high respiratory metabolism20 % of oxygen consumed is used by the brain

2. but no fuel reservesUses 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

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• 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

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

Creatine Kinase and Phosphocreatine Provide an Energy Reserve in Muscle

Figure 27.9 Phosphocreatine serves as a reservoir of ATP-synthesizing potential.

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

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

• 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• ~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

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

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

• 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

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

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

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 1. Short-term regulator: determine individual meal2. Long-term regulator: act as stabilize the levels of

body fat deposit

• Two subset neurons1. NPY/ AgRP producing neurons -- stimulating2. Melanocortin producing neurons-- inhibiting

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 - and -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

• Insulin and leptin are long-term regulators of eating behavior

– Insulin is produced in the -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

• 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

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

• 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

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

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 PPAR (peroxisome proliferator-activator receptor- )

– PPAR is a nuclear hormone receptor that activates transcription of genes involved in adipogenesis and fat storage

• SIRT1 binding to PPAR 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

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 PPAR (peroxisome proliferator-activator receptor- )

• PPAR is a nuclear hormone receptor that activates transcription of genes involved in adipogenesis and fat storage

• SIRT1 binding to PPAR 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

Resveratrol in Red Wine is a Potent Activator of Sirtuin Activity

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.

French people enjoy longevity despite a high-fat diet. Resveratrol may be the basis of this “French paradox”.