Hematology 425, RBC Metabolism, Hgb and Iron

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Hematology 425, RBC Metabolism, Hgb and Iron

• Russ Morrison
• October 3, 2006

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RBC Metabolism, Hgb and Iron

• The RBC survives for approximately 120 days
  through the process of glycolysis
• The main function of the RBC is transport of
  oxygen and CO2 to and from the tissues this
  function does not require consumption of energy
  (ATP)
• RBCs lack a nucleus and other organelles and can
  not utilize proteins and lipids for energy they
  obtain energy only from carbohydrates through the
  EM pathway

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RBC Metabolism, Hgb and Iron

• RBC Process which Require Energy
• Maintenance of intracellular cationic
  electrochemical gradients (Na, K, Ca pump and
  equilibrium)
• Maintenance of membrane phospholipid
• Maintenance of skeletal protein plasticity
• Maintenance of ferrous hemoglobin
• Protection of cell proteins from oxidative
  denaturation

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RBC Metabolism, Hgb and Iron

• RBC Process which Require Energy
• Initiation and maintenance of glycolysis
• Synthesis of glutathione
• Mediation of nucleotide salvage reactions

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RBC Metabolism, Hgb and Iron

• RBC energy is stored and available as ATP, ADP
  and AMP review structure calorie bank
• Glycolysis generates ATP from ADP and ATP is the
  greatest reservoir of energy in the RBC
• 15 of ATP production is consumed through
  membrane exchange pathways that allow maintenance
  of Na, K, Ca levels
• High K and low Na and Ca intracellularly and
  low K and high Na and Ca extracellularly.
• If deprived of ATP energy, cation balance goes
  awry, the RBC swells and is destroyed

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RBC Metabolism, Hgb and Iron

• Plasma glucose enters the RBC glucose catabolic
  process through facilitated membrane transport.
• 90-95 of glucose consumption is anaerobic
  through the EM pathway.
• Through the EM pathway, glucose is metabolized to
  lactic acid using 2 ATP and generating 4 ATP
  molecules per molecule of glucose for a net gain
  of 2 ATP.

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RBC Metabolism, Hgb and Iron

• A diversion shunt off the EMP (Luebering-Rapaport
  pathway) provides 2,3-BPG.
• 2,3-BPG regulates O2 delivery to tissues
• The methemoglobin reductase pathway, another EMP
  bypass, produces NADH
• NADH helps maintain hemoglobin in the
  functionally reduced state.

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RBC Metabolism, Hgb and Iron

• 5-10 of glucose consumption occurs through
  aerobic glycolysis through another diversion
  pathway hexose monophosphate pathway.
• The hexose monophosphate pathway provides a pool
  of reduced glutathione to combat potential
  oxidant injury to the RBC

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RBC Metabolism, Hgb and Iron

• The enzyme deficiency in the EMP responsible for
  most cases of hereditary nonspherocytic hemolytic
  anemia is pyruvate kinase
• The enzyme within the hexose monophosphate
  pathway that is most likely to give rise to
  deficient HMP function is G-6-PD

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RBC Metabolism, Hgb and Iron

• Hemoglobin
• The Hgb molecule consists of four heme groups and
  two pairs of UNLIKE polypeptide chains
• Hgb is the main component of the red blood cells,
  its concentration within the red blood cells is
  around 34 g/dL
• Hgb is a red pigment with mw of 68,000 daltons
• The vehicle for O2 transport in the body

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RBC Metabolism, Hgb and Iron

• See figure 9-2 of text
• Heme consists of a ring of carbon, hydrogen and
  nitrogen (protoporphyrin IX) with an atom of
  ferrous (Fe2) iron attached, entire structure is
  called ferroprotoporphyrin.
• Each heme group is positioned in a pocket of the
  polypeptide chain near the surface of the Hgb
  molecule.
• Heme combines reversibly with one O2 molecule
• Heme gives blood its red pigment

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RBC Metabolism, Hgb and Iron

• The globin of the hemoglobin molecule is made up
  of two pairs of polypeptide chains
• Chains are 141-146 amino acids each
• Variations in the amino acid sequence give rise
  to different types of polypeptide chains
• Each type of polypeptide chain is designated by a
  Greek letter

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RBC Metabolism, Hgb and Iron
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RBC Metabolism, Hgb and Iron

• See figure 9-3each polypeptide chain is divided
  into eight helices and 7 nonhelical segments
• Helices are designated A to H and are rigid and
  linear
• Nonhelical segments are more flexible and lie
  between the helical segments, designated NA, CD,
  etc. through HC

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RBC Metabolism, Hgb and Iron

• Globin chains are looped to form a cleft pocket
  for heme
• Heme is suspended between the E and F helices
• The Fe at the center of the protoporphyrin IX
  ring forms a bond with F8 and through the linked
  oxygen with E7
• Amino acids in this cleft are hydrophobic and
  each chain contains a heme group with iron
  positioned between two histidine radicals

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RBC Metabolism, Hgb and Iron

• Amino acids on the outside of the cleft are
  hydrophilic, making the molecule water-soluble
• The arrangement of amino acids also helps iron
  stay in the ferrous form
• The complete hgb molecule is spherical, has 4
  heme groups attached to 4 polypeptide chains and
  may carry up to 4 oxygen molecules

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RBC Metabolism, Hgb and Iron

• The biosynthesis of heme takes place in the
  mitochondria and cytoplasm of the RBC precursors
  from pronormoblast to reticulocyte in the bone
  marrow.
• Mature RBCs can not make hgb because they have no
  mitochondria and lose the capability of using the
  tricarboxylic acid cycle necessary for hgb
  synthesis

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RBC Metabolism, Hgb and Iron

• Assembly of heme occurs at the mitochondria where
  protoporphyrin IX is built.
• Transferrin carries iron in the ferric (Fe3)
  form to developing RBCs
• Fe goes through the RBC membrane to the
  mitochondria and is united with protoporphyrin IX
  to make heme
• Heme leaves the mitochondria and is joined to
  globin chains in the cytoplasm.

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RBC Metabolism, Hgb and Iron

• 6 genes control synthesis of 6 globin chains
• Alpha and zeta genes are on C16
• Gamma, beta, delta and epsilon genes are on C11
• Each set of single chains is synthesized in equal
  amounts at the ribosomes
• Globin chains are released from the ribosomes
  into the cytoplasm

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RBC Metabolism, Hgb and Iron

• In the cytoplasm, globin chains bind hemes and
  then pair off
• An alpha chain and non-alpha chain combine to
  form dimers
• 2 dimers combine to form tetramers, completing
  the hemoglobin molecule
• 2 alpha and 2 beta chains in a Hgb molecule is
  called HgbA
• 2 alpha and 2 delta is Hgb A2, while 2 alpha and
  2 gamma is HgbF.
• Globin chains exhibit different charge and may be
  separated electrophoretically

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RBC Metabolism, Hgb and Iron

• Progression of Hgb Production see figure 9-6
  and table 9-2
• Hemoglobin F, predominant in-utero, has a higher
  affinity for oxygen and is able to extract
  oxygen across the placenta from the mother to the
  fetus

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RBC Metabolism, Hgb and Iron

• A modified form of hemoglobin A is formed by
  postsynthetic, nonenzymatic reactions of sugars
  with amino groups of the globin chains, Hgb A1.
• The most common form of modified Hgb A1 is Hgb
  A1c in which glucose is added to the N terminus
  of the beta chain.
• Hgb A1c is normally 4-6 of Hgb A and is an
  important marker in management of diabetes (A1c
  becomes increased and reflects management during
  span of RBC life cycle)

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RBC Metabolism, Hgb and Iron

• Regulation of hemoglobin production
• Regulation of heme takes place in the heme
  production pathway
• Rate limiting step is thought to be the initial
  formation of aminolevulinic acid (ALA)
• ALA synthesis is inhibited by heme leading to
  decreased heme production (negative feedback)
• Other feedback mechanisms may play a role

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RBC Metabolism, Hgb and Iron

• Regulation of hemoglobin production
• Globin production is regulated by the rate at
  which the DNA code is transcribed to mRNA
• The amount of globin produced is proportional to
  the amount of mRNA
• Heme (hemin) controls the rate of globin
  synthesis in intact reticulocytes and in its
  absence, globin production decreases
• Normal mature RBCs contain complete Hgb molecules
  and pools of free heme or free globin chains are
  minute.

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RBC Metabolism, Hgb and Iron

• Hgb synthesis is stimulated by tissue hypoxia
• Hypoxia causes the kidneys to produce increased
  EPO which in turn stimulates production of Hgb
  and RBCs
• Hgb Reference ranges
• adult male 14.0-18.0 g/dL
• adult female 12.0-15.0 g/dL
• newborn 16.5-21.5 g/dL

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RBC Metabolism, Hgb and Iron

• The function of Hgb is to bind oxygen readily in
  the lung, transport oxygen, and unload oxygen in
  the tissues
• The affinity of Hgb for oxygen depends on pH, 2,3
  BPG, pCO2, temperature, Hgb variants.
• Review oxygen dissociation curve, figure 9-7
• Shifts in the oxygen dissociation curve due to pH
  is known as the Bohr effect.

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RBC Metabolism, Hgb and Iron

• Iron
• Iron is essential for sustained life in all
  living organisms except Lactobacillus and
  Bacillus species.
• Most functional iron in humans is in hemoglobin
  or myoglobin,, which carry oxygen
• About one fourth of iron is in a storage form

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RBC Metabolism, Hgb and Iron

• Functions of iron
• Carrier of electrons used to bind with cofactors
  essential to basic metabolic oxidative and
  reductive reactions
• Catalyst for oxygenation, hydroxylation and other
  metabolic processes
• Ability to cycle between ferrous and ferric forms
  makes iron useful in many biochemical reactions

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RBC Metabolism, Hgb and Iron

• Iron must be carefully regulated due to its
  potential toxicity
• Regulation is complex to preserve iron needed,
  while not allowing toxicity
• Too little iron causes cellular functions to be
  suboptimal
• Too much iron may produce organ damage and death

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RBC Metabolism, Hgb and Iron

• Iron status is dependent on iron intake, iron
  bioavailability and iron losses
• We have mechanisms for absorbing dietary iron
  efficiently, but not for eliminating excess iron
  effectively
• Understanding of molecular mechanisms involved in
  iron metabolism are just beginning to be
  understood

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RBC Metabolism, Hgb and Iron

• Iron is obtained through dietary means via heme
  (Fe2), organic and nonheme (Fe3), inorganic
  iron.
• More iron is absorbed from the heme form of iron
  (meats) than from the nonheme form (legumes and
  leafy vegetables).
• 5-35 of heme iron is absorbed from a meal, while
  2-20 of nonhem iron will be absorbed.

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RBC Metabolism, Hgb and Iron

• Maximal absorption of iron takes place in the
  duodenum and the jejunum.
• Iron from food must be in the ferrous form to be
  bound to the enterocytes of the mucosal
  epithelium where it is internalized.
• Review figure 10-1

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RBC Metabolism, Hgb and Iron

• Once absorbed, iron is transported in plasma
  bound to a carrier protein, transferrin.
• Transferrin receptors located on all cells in the
  body (except mature RBCs), aid in providing
  transferrin-bound iron access into cells and also
  play a critical role in the release of iron from
  transferrin within the cell.
• IRE-BP regulates the amoount of transferrin,
  transferrin receptor and ferritin in the body.
• The transferrin gene is on C3

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RBC Metabolism, Hgb and Iron

• Ferritin and hemosiderin are storage forms of
  iron.
• Most storage of iron is in the ferritin form, a
  water-soluble complex.
• Hemosiderin is water-insoluble, made up of
  ferritin aggregates and found in macrophages in
  the bone marrow and liver.

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Laboratory Assessment of Iron
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Additional Laboratory Tests

• Bone Marrow or liver biopsy with specific
  staining and qualitative/semi-quantitative
  assessment of available/stored iron.
• In BM, the type of iron in macrophages is
  hemosiderin, the degenerative product of ferritin
  molecules that have been incorporated into
  lysosomes of the macrophagees.
• Reticulocytes in the bone marrow that contain
  iron are termed siderocytes.
• Serum transferrin receptor analysis
• Red cell protoporphyrin test