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8/8/2019 Erythrocyte Info Found on Internet
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Erythrocyte metabolism
Alice Skoumalová
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Erythrocytes
deliver oxygen to body tissues and remove carbon dioxide and
protons
biconcave 7.7m
lack cell organelles
120 days
women 4,2-5,4 million/l, men 4,6-6,2 million/l
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The erythrocyte membrane
50% lipid bilayer (phospholipids, cholesterol)
50% proteins
SDS-PAGE: separation of proteins (band 1-7)
isolation and analysis (10 main proteins)Integral: Anion exchanger protein, Glycophorin A, B, C
Peripheral: Spectrin, Ankyrin, Actin
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Spectrin: the most prominent component (two isoforms ,; a tetramer; a meshwork )
fixed to the membrane- ankyrin
binding sites for several other proteins (glycophorin C, actin, band 4.1,
adducin)
This organization keeps the erythrocyte shape.
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Hereditary spherocytosis
autosomal dominant
a deficiency in a spectrin amount and its abnormalities
the presence of spherocytes in the blood
the spleenµs hemolysis
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Haemoglobin
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4 protein chains + 4 haem groups
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Movements of the heme and the F helix during the T ± R transition in
hemoglobin:
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Hemoglobin saturation curves:
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Hemoglobin autooxidation
O2 binds to Fe2+ - an intermediate structure- an electron is delocalized
between the iron ion and the O2
the side effect - every so often a molecule of oxyhaemoglobin
undergoes decomposition and release superoxide
Hem - Fe2+- O2 Hem - Fe3+ - O2-
Methemoglobin (Fe3+) is unable to bind O2
(methaemoglobin reductase)
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Erythrocyte exceptions
They lack organelles
no ATP production in oxidative phosphorylation
no ability to replace damaged lipids and proteins (low metabolic activities,
with no ability to synthesize new proteins or lipids)
Free radicals exposure
haemoglobin autoxidation (O2- release)
a cell membrane rich in polyunsaturated fatty acids (susceptible to lipid
peroxidation)
deformation in tiny capillaries; catalytic ions leakage (cause of lipid
peroxidation)
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Erythrocyte metabolism
Glucose as a source of energy
Glycolysis generates ATP and 2,3-bisphosphoglycerate
The pentose phosphate pathway produces NADPH
Glutathione synthesis- the antioxidant defence system
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Glucose- source of energy
Glucose transporter:
integral membrane protein (12 membrane-spanning helices)
a channel for the glucose transport
insulin-independent transporter
Glycolysis in erythrocytes
1. Source of ATP
Lactate- the end product
Cover energy requirement
2. Generate 2,3-bisphosphoglycerate (2,3-BPG)
a major reaction pathway for the consumption of glucose in erythrocytes
the specific binding of 2,3-BPG to deoxyhemoglobin decreases the oxygen
affinity of hemoglobin and facilites oxygen release in tissues
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2,3-bisphosphoglycerate
Allosteric effector of haemoglobin: ± binds to deoxyhaemoglobin (a central cavity capable of binding 2,3-
BPG)
± decreases haemoglobinµs O2 affinity
Clinical aspects: ± In people with high-altitude adaptation or smokers the concentration of
2,3-BPG in the blood is increased (low oxygen supply)
± Fetal haemoglobin has low BPG affinity - the higher O2 affinity -
facilitates the transfer of O
2 to the fetus via the placenta
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Glutathione synthesis in erythrocytes
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GlutathioneElimination of H2O2 and organic hydroperoxides
1. Cofactor for the glutathione peroxidase (removes H2O2 formed in
erythrocytes)
2. Involved in ascorbic acid metabolism
3. Prevents protein ±SH groups from oxidizing and cross-linking
Gly
Cys
Glu
Gly
Cys
Glu
Gly
Cys SH
Glu
S S
Oxidized form of glutathione
(dimer, disulphide)
Reduced form of glutathione
(monomer)
Glutathione peroxidase
Glutathione reductase
+ R-O-O-H
+ H2O
+ NADPH
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T he pentose phosphate pathway inerythrocytes
Generates NADPH - reduction of glutathione (eliminates H2O2 formed in
erythrocytes)
Clinical apect:
Glucose-6-phosphate dehydrogenase deficiency
± Causes hemolytic anemia (decreased production of NADPH - reduced
protection against oxidative stress - haemoglobin oxidation and Heinz
bodies formation, membrane lipid peroxidation and hemolysis)
± Hemolytic crises are evocated by drugs (primaquine, sulphonamide
antibiotics) and foods (broad beans)
± The most common enzyme deficiency disease in the world (100 million
people)
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Haemoglobin autoxidation 3% of the haemoglobin undergoes oxidation every day
a constant flux of O2-
Hem - Fe2+- O2
Hem - Fe3+ - O2
-
Methaemoglobin reductase Converts methaemoglobin back to ferrous haemoglobin to permit
continued O2 transport
System containing FAD, cytochrome b5 and NADH (glycolysis)
Methaemoglobinemia1. Congenital type
± methaemoglobin reductase deficiency (AR)
± variant haemoglobin M (HbM)- mutation; tend to be oxidized tomethaemoglobin
2 . Acquired type- drugs or chemicals (sulphonamides, aniline)
Visual indicator- a blue tint to the skin (10% of metHb)
Treated- reductants (methylene blue, ascorbic acid)
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Superoxide dismutase (SOD)
a high concentration in erythrocytes
accelerates the dismutation O2- to H2O2
H2O2 remove
1. Catalase2. Glutathione peroxidase
1. Catalase
a ferric haem group bound to the active site
catalyses decomposition of H2O2 to water and oxygen:
2H2O2 2H2O+O2
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2. Glutathione peroxidase
removes H2O2 by coupling its reduction to H2O with oxidation of reduced
glutathione (GSH)H2O2+2GSH GSSG+2H2O
Glutathione reductase
reduces oxidized glutathione back to reduced
GSSG+NADPH+H+
2GSH+NADP+
NADPH- the pentose phosphate pathway (glucose-6-phosphatedehydrogenase)
Cooperation of glutathione peroxidase and catalase
The concentration of H2O2 is raised- catalase becomes more important(high Km for H2O2)
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Quinols and enols -Tocopherol (vit. E)
Ubiquinol (coenzyme Q)
Ascorbic acid (vit. C)
Carotenoids -Carotin
Lycopin
Others Glutathione
Bilirubin
Biological antioxidants:
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-Tocopherol (vitamin E) Present in the erythrocyte membrane
Prevents lipid peroxidation (chain-breaking antioxidant)
-TocH+LO2 -Toc+LO2H
Ascorbic acid (vitamin C)
Present in the cytoplasm
Recycles -tocopherol
Dehydroascorbate reductase (GSH-dependent) regeneratesascorbate
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Haemoglobinopathy abnormal structure of the haemoglobin (mutation)
large number of haemoglobin mutations, a fraction has deleterious
effects sickling, change in O2 affinity, heme loss or dissociation of tetramer
haemoglobin M andS, and thalassemias
Haemoglobin M replacement of the histidine (E8 or F7) in or -chain by the tyrosine
the iron in the heme group is in the Fe3+ state (methaemoglobin)
stabilized by the tyrosine
methaemoglobin can not bind oxygen
Thalassemias
genetic defects- or -chains are not produced ( or -thalassemia)
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Haemoglobin S (sickle-cell)
Causes a sickle-cell anemia
Erythrocytes adopt an elongated sickle shape due to theaggregation of the haemoglobin S
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Replacing Glu with the less polar amino acid Val - forming Äanadhesive region³ of the chain
The hydrophobic Val fits to the region of the another chain indeoxy (not oxy) haemoglobin and thus ad jacent haemoglobinmolecules can fit together and aggregate into a long rodlike helicalfiber
Cross section
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Red blood cells adopt a sickle shape in a consequence of the forming haemoglobin Sfibers
The elongated cells tend to block capillaries, causing inflammation and considerablepain; they are fragile what leads to anemia
The high incidence of sickle-cell disease coincides with a high incidence of malaria
Individuals heterozygous in haemoglobin S have a higher resistance to malaria; themalarial parasite spends a portion of its life cycle in red cells, and the increased fragility of the sickled cells tends to interrupt this cycle
Scanning electron micrograph of a sickled erythrocyte.
The haemoglobin S fibers can be seen within the
distorted cell. The cell has ruptured and haemoglobin
fibers are spilling out.
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Hemoglobin switching:
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Glycosylated haemoglobin (HbA1)
formed by hemoglobin's exposure to high plasma levels of glucose
non-enzymatic glycolysation (glycation)- sugar bonding to a protein
normal level HbA1- 5%; a buildup of HbA1- increased glucose concentration
the HbA1 level is proportional to average blood glucose concentration over previous weeks; in individuals with poorly controlled diabetes, increases inthe quantities of these glycated hemoglobins are noted (patientsmonitoring)
Sugar CHO + NH2 CH2 Protein
Sugar CH N CH2 Protein
Sugar CH2 NH CH2 Protein
Schiff base
Glycosylated protein
Amadori reaction