Metabolism

1
Metabolism
2
Overview of metabolism
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• most vertebrates eat periodically
• during absorption monosaccharides, amino acids,
  lipoproteins are present in the blood in high
  concentration
• the problem is storage e.g. glucose appears in
  the urine above a blood sugar concentration of
  200 mg (11 mmol/l)
• importance of the hepatic portal circulation
• between meals the problem is mobilization
• some cells can store nutritients others rely on
  the blood supply (e.g. neurons, blood cells)
• liver (glycogen) and adipose tissue
  (triglycerides) store nutritients for the whole
  body
• muscle cells store for themselves (glycogen)
• these tissues have decisive role in regulation
• transport nutritients glucose, free fatty acids
  (FFA), ketone bodies, amino acids these
  substances are decisive in regulation

3
Regulation of metabolism
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• regulation targets key enzymes selecting between
  alternate routes
• enzymes are regulated partly by metabolites.
  partly by hormones
• in mobilization period appropriate level of
  glucose is very important as neurons can only use
  this nutrients (after a longer fasting, ketone
  bodies as well)
• therefore, concentration should be kept between
  narrow limits minimum 4,5 5 mmol/l, maximum
  9-10 mmol/l
• in this regulation, hormones of the pancreatic
  islets of Langerhans, insulin and glucagon, are
  the most important
• glucose can enter several different metabolic
  pathways

4
Membrane transport of glucose

• glucose enters enterocytes and renal epithelial
  cells through indirect active transport (Na
  co-transporter)
• through the basolateral membrane and into other
  cells it is transported by facilitated diffusion
• GLUT family 12TM transporter proteins
• GLUT 1 blood-brain-barrier endothelium, red
  blood cells high affinity, independent from
  insulin
• GLUT 2 basolateral membrane of enterocytes and
  renal epithelial cells, hepatic cells, B-cells in
  pancreas low affinity, independent from insulin
• GLUT 3 neurons, liver cells independent from
  insulin
• GLUT 4 muscles and adipose tissue dependent
  on insulin
• GLUT 5 fructose transporter
• GLUT 6 ???

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5
Glucose metabolism I.
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• transported glucose is transformed into
  glucose-6-phosphate (using ATP) inside the cell
  cannot diffuse - strong concentration gradient
• different enzyme in the other direction
  (glucose-6-phosphatase) yielding glucose and P
  no such enzyme in the muscle no glucose release
• glucose-6-phosphate can be reversibly transformed
  to glucose-1-phosphate with UTP forms
  UDP-glucose glycogen synthesis
• different enzyme in the other direction using
  inorganic P and yielding glucose-1-phosphate
• glucose-6-phosphate can be reversibly transformed
  also to fructose-6-phosphate, both can enter
  pentose phosphate cycle yielding NADPH, or in the
  glycolysis

6
Glucose metabolism II.
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• fructose-6-phosphate is transformed using ATP to
  fructose-1,6-diphosphate (phosphofructo-kinase)
  enters into glycolysis reaction is facilitated
  by ADP, AMP, P, inhibited by ATP, citrate, fatty
  acids
• different enzyme in the other direction
  (fructose-1,6-diphosphatase), yielding inorganic
  P last but one step of gluconeogenesis
  reaction is facilitated by ATP, citrate, fatty
  acids, inhibited by ADP, AMP, P
• glycolysis runs in the cytoplasm down to pyruvate
• pyruvate enters mitochondrion if O2 is available
  citrate cycle (in matrix), terminal oxidation
  (inner membrane) 38 ATP/glucose
• if no O2 is available, pyruvate is transformed to
  lactic acid using up NADH 2 ATP/glucose
• after intense physical exercise, lactic acid is
  transported to the liver and synthesized to
  glucose (Cori-cycle) energy consuming process
  oxygen debt

7
Gluconeogenesis
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• gluconeogenesis means synthesis of glucose
• during fasting nervous system needs glucose - it
  is produced by gluconeogenesis from amino acids
• gluconeogenesis is also important in turning
  accumulated lactic acid into glucose
• there are 3 irreversible steps in glycolysis
  formation of glucose-6-phosphate,
  fructose-1,6-diphosphate and pyruvate ?
• first two are reversed by dephosphorylation see
  above
• phosphoenolpyruvate synthesis from pyruvate
  through oxaloacetate
• gluconeogenesis cannot use acetyl-CoA, thus fatty
  acids as two CO2 are released in the citrate
  cycle before it runs to oxaloacetate
• glucogenic and ketogenic amino acids

8
Transformations of glucose
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GLUT transporter
ATP
P
UTP
ATP
pentose-P cycle
P
glycolysis
9
Lipid metabolism
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• absorbed lipids are transported as lipoproteins
  in the circulation
• lipoproteins are also synthesized by the liver
  and the enterocytes between absorptive phases
  using building blocks in the blood
• in the endothelium of capillaries lipoprotein
  lipase enzyme is located cutting off free fatty
  acids from triglycerides easily enter the cells
• in the mitochondria ß-oxidation NADH,
  acetyl-CoA are formed
• synthesis in the ER acetyl-CoA exits the
  mitochondria as citrate and forms acetyl-CoA
  again
• acetyl-CoA enters the cyclic synthesis as
  malonyl-CoA - NADPH is also necessary
• fatty acids form triglycerides with
  glycerol-1-phosphate coming from the glycolysis
• acetyl-CoA can be used to form ketone bodies

10
Islets of Langerhans
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• pancreas is 70-80 g, 1-2 of the gland gives the
  1-2 million islands ?
• 50-300 cells/island
• A, B, D, F cells
• B-cells forming groups surrounded by A-, and
  D-cells
• interaction through paracrine means and through
  the local circulation
• A-cells 20-25, producing glucagon
• B-cells 60-75, producing insulin
• D-cells 10, producing somatostatin
• F-cells ?, producing pancreatic peptide (?)

11
Regulation of insulin production

• insulin is synthesized as preproinsulin (signal
  proinsulin) on the rough ER
• signal is cleaved off, proinsulin is packaged
  into vesicles in Golgi C-peptide is removed, A
  and B chains remain connected by 2 disulfide
  bridges ?
• stored in the vesicles, it is released by
  exocytosis (Ca) when needed
• facilitatory effects
• increase of blood glucose level transported in
  by GLUT-2 producing ATP in glycolysis ATP
  closes K channel depolarization Ca enters
  the cell
• amino acids (arginine, leucine, lysine)
• vagal effect sweet taste in the mouth
• gut hormones (incretins GIP, CCK)
• inhibitory effects
• somatostatin
• sympathetic effect through a2-receptors
  hyperglycemia in stress is not diminished by
  insulin

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12
Insulin secretion
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13
Details of glucose effect
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glucose
ATP
Ca
14
Insulin effects I.
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• binds to tyrosine-kinase receptors ?
• autophosphorylation, then phosphorylation of
  other proteins termination through
  internalization
• response types (depending on the given cell)
• GLUT-4 is added to the membrane from storage
  vesicles (adipose and muscle cells) intake of
  glucose increases several fold
• phosphorylation and dephosphorylation of enzymes
  e.g. activation of phosphodiesterase, thus
  blockade of the effect of various hormones acting
  through cAMP glucagon, catecholamines, etc.
• modulation of gene expression, e.g. inhibition of
  proglucagon transcription in A-cells
• insulin facilitates synthetic processes,
  decreases the level of transport nutritients
  (glucose, FFA, ketone bodies, amino acids)
• inhibits the effect of hormones promoting
  catabolism

15
Insulin effects II.
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• effects on liver cells
• glycogen synthesis increases
• glycogenolysis decreases
• gluconeogenesis decreases
• synthesis of fatty acids increases
  triglycerides are transported in the circulatory
  system bound to lipoproteins
• production of ketone bodies decreases
• effects on muscle cells
• glucose uptake increases
• glycogen synthesis increases
• glycogenolysis decreases
• amino acid uptake and protein synthesis increases
• K uptake increases cause is unknown
• effects on adipose cells
• glucose uptake increases glycerol is available
  for triglyceride synthesis
• amount of lipoprotein lipase increases FFA
  uptake triglyceride synthesis increases
• lipolysis (facilitated by cAMP) is inhibited

16
Insulin effect in adipose cells
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lipoprotein lipase
trigliceride
FFA glycerol
17
Regulation of glucagon production

• proglucagon is a member of the secretin family
• produced by A-cells in the pancreas and in the
  alimentary canal
• it is not known whether the latter has glucagon
  effect in humans, but in dogs it does (see
  classic experiment of Best and Banting)
• inhibitory effects
• high glucose level
• insulin by inhibiting the transcription of the
  proglucagon gene
• somatostatin
• facilitatory effects
• arginine, and to some extent other amino acids as
  well after a protein-rich meal hypoglycemia
  might develop as insulin secretion is increased
  sweetness after a large meal
• stress reaction catecholamines, growth hormone,
  glucocorticoids the role of the latter is
  permissive, enabling proglucagon transcription

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18
Glucagon production
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19
Glucagon effects
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• all important effects of glucagon influence liver
  cells through cAMP and PKA
• glycogenolysis increases
• gluconeogenesis increases
• glucose release increases
• synthesis of ketone bodies increases
• insulin antagonizes all effects (enhanced
  degradation of cAMP)
• outcome depends on the ratio of the two hormones
• gluconeogenesis and ketogenesis require
  substrates (amino acids and fatty acids) these
  are provided from the muscles and adipose tissue
  by the low insulin level

20
Hormonal background of fasting

• following the absorptive phase tissues and organs
  have to rely on stored energy
• not all cells and tissues have their own stores
• brain is unique as it can only use glucose until
  the level of ketone bodies is not very high
• brain uses 6 g glucose/hour glucose stores of
  liver would not last for long - gluconeogenesis
• maximal tolerable length of fasting depends on
  how long gluconeogenesis can continue and how
  long triglyceride stores can provide energy for
  circulation, respiration, and renal functions
• adaptation requires
• decrease of insulin/glucagon ratio
• presence of growth hormone (STH/GH) reason?
• presence of glucocorticoids (cortisol)
  synthesis of enzymes for gluconeogenesis,
  lipolysis, secretion of glucagon permissive role

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21
Phases of fasting I.
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• most of the recent data concerning metabolic
  changes during fasting have been obtained in
  patients undergoing drastic diet protocols
  (null-calorie) in the 60s-70s following
  unexplainable fatal cases this method was
  discontinued
• post-absorptive state max. 24 hours, occurs
  every day
• insulin level decreases, glucagon slightly
  increases
• blood sugar level is maintained by glycogenolysis
  in the liver (75), and by gluconeogenesis (25)
  using lactic acid, glycerol and some amino acids
• glucose consumption decreases in tissues capable
  to use other nutritients, FFA and glycerol
  release from the adipose tissue increases
  muscle cells use that

22
Phases of fasting II.
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• short-term fasting 24-72 hours
• insulin level decreases even further, glucagon
  and GH concentration increases because of the low
  blood sugar level caused by the depletion of
  glycogen stores in the liver
• gluconeogenesis increases using amino acids
  mostly from the muscles N-excretion is rising
• lipolysis increases (low insulin level, GH), most
  cells (but not nerve and blood cells) are using
  fatty acids, ketogenesis increases in the liver,
  muscles are burning ketone bodies
• chronic fasting after 72 hours
• insulin/glucagon ratio decreases further, GH
  increases, lipolysis, ketogenesis is enhanced
• total energy consumption decreases (inactivity,
  decrease of thyroid activity), brain can use
  ketone bodies now, glucose demand decreases,
  proteolysis decreases life can go on for weeks

23
Stress state
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• stress state means the collection of the
  reactions of the body to various challenges
• catabolic state similarly to fasting, but blood
  sugar level is high glycogenolysis, lipolysis
  gluconeogenesis
• a further difference is the high sympathetic
  activation, the increased catecholamine synthesis
  and glucocorticoid secretion in the adrenal gland
  (cortex and medulla, respectively)
• catecholamines inhibit insulin and enhance
  glucagon secretion increase glycogenolysis,
  gluconeogenesis and ketogenesis in the liver, as
  well as lipolysis in the adipose tissue
• glycogenolysis in the muscles might increase
  lactic acid release, facilitating gluconeogenesis

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Diabetes mellitus
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• diabetes mellitus (mellitussweet as honey) the
  court physician of Charles I. tasted the urine of
  a patient and found it sweet this method was
  used for a long time in patients comatose for
  unknown reasons
• 1920 Banting and Best induced diabetes in dogs
  by removing the pancreas, then alleviated the
  symptoms with pancreas extraction
• 1922 successful trial in a diabetic child
• 1923 Nobel-prize for Banting and McLoed
• classical method in physiology lesion
  replacement
• this was the first identified hormone and hormone
  effect
• type I diabetes (juvenile) lack of insulin
• type II diabetes heterogeneous, unknown
  mechanism, insulin is present

25
Type I diabetes mellitus
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• B-cells are destroyed by autoimmune reaction
• first antibodies only, no symptoms, later
  decreased glucose tolerance, then endogenous
  hyperglycemia
• in insulin sensitive tissues (muscles, adipose
  tissue) no glucose uptake, overproduction of
  glucagon
• glycogenolysis, gluconeogenesis, lipolysis,
  ketogenesis, lipemia (liver is synthesizing
  lipoproteins, but lipoprotein-lipase level is low
• glycosuria, osmotic diuresis, NaCl and water
  excretion, polyuria, polydipsia, dehydration,
  hematocrit increases, circulation deteriorates,
  hypoxia
• ketoacidosis hyperventilation, loss of water,
  diabetic coma

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Symptoms of diabetes
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Lack of insulin
direct effect
overproduction of glucagon
gluco- neogenesis
glucose use
lipolízis
hyperglycemia
hyperosmolarity
ketogenesis
glucosuria
ketoacidosis
ketonuria
osmotic diuresis
hyperventilation
dehydration
vomiting
circulatory insufficiency
brain hypoxia
coma
27
Type II diabetes mellitus
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• insulin is usually present
• heterogeneous causes, not well-known
• exogenous and endogenous hyperglycemia is
  characteristic
• in some patients lack of insulin receptor or
  resistance against insulin
• insulin secretion sometimes can be induced with
  arginine, but not with glucose lack of GLUT 2
  transporter
• no glucagon inhibition in most cases symptoms
  are strengthened by hyperglucagonemia
• some patients are obese, others not
• relatively benign, but might cause complications
  atherosclerosis, myocardial infraction,
  blindness, renal insufficiency
• in the USA-ban 3-5 of whites are diabetic, in
  80 type II

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Energy turnover I.
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• there are some related terms that should not be
  confused
• turnover of materials indicates chemical
  transformations and reactions only these changes
  are accompanied by changes in energy turnover
  of energy
• turnover of materials and energy together is
  called metabolism
• anabolism is when synthesis, i.e. building up of
  materials is the principal process
• difficult to measure, but it is characterized by
  positive nitrogen balance less N is excreted
  than taken up
• catabolism is when degradation is the principal
  process complex molecules are broken up to
  smaller ones

29
Energy pathways
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chemical energyof food
30
Energy turnover II.
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• chemical transformations have a certain
  efficiency part of the energy is lost in the
  form of heat serving also for the maintenance of
  body temperature
• if there is no external work, digestion or
  absorption, growth or storage, and the organism
  is in thermal balance with the surroundings, then
  basal metabolic rate can be determined by
  measuring heat production
• chemical energy mobilized from the stores is
  completely transformed into heat, and its amount
  do not depend on the route Hess law
• the rate of enzymatic reactions change with
  temperature
• low temperature low metabolic activity,
  decreased heat production freezing to death
• high temperature high metabolic rate, increased
  heat production death by overheating

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Basal metabolic rate I.
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• direct calorimetry measurement of dissipated
  heat complicated, perspiration should be also
  measured ?
• not reliable method if metabolic rate is low,
  appropriate for small birds and mammals
• indirect calorimetry decrease of stores is
  determined by measuring O2 consumption
• composition of combusted materials can be
  determined from the respiratory quotient
  (RQCO2/O2) and the N-excretion
• sugar RQ1, fat RQ0.7, proteins RQ0.8
• as O2 energy-equivalence is almost independent
  from the combusted materials, thus determination
  of RQ is not absolutely necessary
• basal metabolic rate increases with body mass,
  but not linearly MRaMb
• b0.75 for vertebrates, invertebrates and
  unicellular organisms ?
• mass-specific metabolic rate MR/MaM(b-1)?

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Basal metabolic rate II.
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• the explanation for the precise relationship
  between body mass and metabolic rate is not known
• Rubner (1883) surface hypothesis
• heat produced during metabolism is dissipated
  through the body surface surface increases as
  the 2/3 power of the mass
• popular hypothesis, but the exponent is 0.75 not
  0.67
• equation also applies to animals with variable
  body temperature - that is not expected from the
  hypothesis no explanation yet
• metabolic rate depends on the temperature Q10
  value 2-3

33
Basal metabolic rate III.
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• basic metabolic rate is lower in women, and
  decreases with age elderly people are more
  sensitive to cold
• malfunctioning of the thyroid gland can shift
  basic metabolic rate by -40 and 80,
  respectively
• specific dynamic action 25-30 increase in basal
  metabolic rate following consumption of proteins
• metabolic rate depends mostly on the activity of
  skeletal muscles
• mental activity acts also through the skeletal
  muscles energy requirement of 1 hour intensive
  mental activity can be met by the consumption of
  a half salted peanut bean

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Regulation of food intake
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• food intake is motivated behavior depends on
  the interplay of complex processes
• in addition to the need for nutritients, many
  other regulating factors circadian rhythm,
  light-dark periods, in humans psychosocial
  interactions as well
• centers in hypothalamus
• ventromedial nucleus satiation
• lateral hypothalamus hunger
• however, lesions of these centers have
  temporary effects only
• the limbic system and various brainstem nuclei
  also participate in the regulation of food intake
• facilitation NA, GABA, NPY other peptides
• inhibition 5-HT, DA, leptin
• stimuli
• glucose-sensitive neurons, hunger contractions
• gastric distension, CCK

35
End of text
36
Glycolysis
37
Islet of Langerhans
38
Structure of the insulin
39
Insulin receptor
40
Direct calorimetry
41
Mass-specific metabolic rate I.
42
Mass-specific metabolic rate II.