Chapt. 44 Ch. 44 Biochemistry of Erythrocytes Student Learning Outcomes : Describe the structure/...
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Chapt. 44
Ch. 44 Biochemistry of ErythrocytesStudent Learning Outcomes:
• Describe the structure/ function of blood cell types:• Erythrocytes, leukocytes, thrombocytes
• Explain the metabolism of the red blood cell• Explain basics of hematopoiesis from bone marrow• Describe some errors of hemoglobin function,
anemias, hemoglobin switching• Describe the structure/ function of blood group
antigens (Ch. 30)
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Blood cells
Table 1 Blood cells (cells/mm3):• Erythrocytes 5.2 x 106 men carry oxygen 4.6 x 106 women
• Neutrophils 4300 granules; phagocytic, O2 burst kills
• Lymphocytes 2700 immune response, B- and T-cells, NK• Monocytes 500 macrophages for bacteria, damage• Eosinophils 230 granules destroy parasites (worms)• Basophils 40• granules hypersensitivity, allergic
histamine, proteases,
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Hematopoiesis
Fig. 15
Hematopoiesis:• Stem cells in bone marrow (1/105)• Proliferate, differentiate, mature by growth factors, hormones signal transduction paths• Myeloid, lymphoid lines• Leukemias: immature cells keep proliferating; defined by cell type
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Anemia
Anemias: hemoglobin concentration is low:• Normal Hb g/dL: men 13.5-17.5; women 11.5-15.5
Anemias classified by red blood cell morphology:
Rbc morphology functional deficit possible causeMicrocytic, impaired Hb thalassemia, lead, hypochromic synthesis iron deficiency
Macrocytic impaired DNA vit B12 or folic acidnormochromic synthesis deficient, erythroleukemia
Normocytic red cell loss acute bleeding, normochromic sickle cell defects
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Erythrocyte metabolism
Erythrocyte metabolism: Only glycolysis• ATP for Na+/K+, Ca2+
• HMP shunt makes NADPH G6PD is 1st enzyme
Lifetime rbc by G6PD activity• 2,3-BPG modulates O2 binding• Need Fe2+ Hb bind O2; If ROS made Fe3+, NADH can reduce
Fig. 1
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Heme synthesis
Heme synthesis in erythrocyte precursor:• Heme = porphryn ring, coordinated to Fe• Complexed to proteins in hemoglobin, myoglobin and
cytochromes; most common porphryn in body• 4 pyrrole rings with –CH- joining• Various side chains• Heme is red color
Fig. 2
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Heme synthesis
Fig. 3
Heme synthesis:Glycine, succinyl CoA form -Aminolevulinic acid (-ALA) Each heme needs 8 of each
Final step is Fe2+Heme regulates:
inhibit 1st enzymerepress synthesis
Porphyria diseases from defective enzymes intermediates accumulate photosensitive, toxic products
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Heme synthesis
Fig. 4
Heme synthesis begins with -ALA:• Decarboxylation by -ALA synthase
• PLP is pyridoxal phosphate• Dehydratase joins 2 -ALA• 4 pyrroles form porphyrinogen
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Sources of iron and hemeIron is essential from diet – 10-15 mg/day recommended
Iron is not readily absorbed from many sources
Iron in meats is form of heme, readily absorbedNonheme iron of plants not as easily absorbed becauuse other compounds precipitate iron
Iron absorbed in ferrous state (Fe2+), oxidized by ferroxidase to Fe3+ for transport
Apotransferrin binds Fe3+ = TransferrinStored as ferritin in cells
Heme stimulates synthesis of globin proteins from ribosomes
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Iron metabolism
Fig. 6RE = reticulo-endothelial system
Iron metabolism:• Transferrin carries Fe3+ to cells; stored as ferritin• Transferrin taken up by R-mediated endocytosis• Hemosiderin stores excess
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Degradation of hemoglobin
Figs. 7,8
Heme is degraded to bilirubin:• Bilirubin is congugated to glucuronate (more soluble),excreted• Rbc only live ~120 days• Globin is degraded to amino acids
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Red blood cells
Erythrocyte cell membrane:• Red disc, pale center• Biconcave shape
• Maximizes surface area• 140 um2 vs. 98 um2 sphere
• Deforms to enter tissues• Spleen destroys damaged red blood cells
Fig. 9
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Cytoskeleton of erythrocyte
Fig. 10 general side view; inside cell view up
Erythrocyte cytoskeleton• provides shape, structure, permits stretch• 2-D lattice of proteins links to membrane proteins:
• spectrin ()• actin• ankyrin• band 4.1
• membrane proteins:• glycophorin• band 3 protein
•Mature rbc does not synthesize new proteins• Gets lipids from circulating LDL
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Agents affect oxygen binding of hemoglobin
Fig. 11,12, 14
Agents affect oxygen binding of hemoglobin:
• 2, 3-BPG (glycolysis intermediate) binds between 4 subunits of Hb, lowers affinity for O2, releases O2 to tissues
• Proton (Bohr) effect: ↑H+ lowers affinity of Hb for O2:
• CO2 can bind to Hb (not only bicarbonate)
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Effect of H+ on oxygen binding to Hb
Effect of H+ on oxygen binding to Hemoglobin:• Tissues: CO2 released → carbonic acid, H+
• H+ bind Hb → release O2 to tissues• Lungs reverse: O2 binds H+Hb → release H+
• H2CO3 forms, releases CO2 to exhale
Fig. 13
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Hematopoiesis
Fig. 15
Hematopoiesis:• Stem cells in bone marrow
• proliferate• differentiate• mature
• myeloid vs. lymphoid
• Stromal cells secrete growth factors• Cytokines signal via membrane receptors
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Bone marrow
Bone marrow stromal cells secrete growth factorsHematopoietc stem cells respond
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Hematopoiesis involves cytokine signaling
Figs. 16; 11.15
Growth factors signal through membrane receptors:• Ligand causes receptors to aggregate• Activates JAK (kinases) by phosphorylation (cytoplasmic RTK)• JAK phophorylates cytokine receptor on Tyr• Other signaling molecules bind, including STAT (signal transducer and activator of transcription) → nucleus transcription• Also RAS/Raf/MAP kinase activated • Overactive signal → cancer
• Transient signal:SOCS silences
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Erythropoiesis
Fig. 17
Erythropoiesis:Erythropoietin from kidney increases red blood cell proliferation (if low
oxygen)• Reticulocytes still have ribosomes, mRNA to make Hb
Mature in spleen, lose ribosomes• Make 1012 rbc/day• Anemia if not appropriate diet• Iron, vitamin B12, folate
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Hemoglobin genes
Hemoglobinopathies, hemoglobin switching:• Order of genes parallels development, controls• >700 mutant Hb (often base subsittution)• HbS sickle cell (Hb Glu6Val)• HbC (Hb Glu6Lys)Both ↑ malaria resistance
Fig. 18
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Thalassemias
Thalassemias: unequal production of of Hb:• need a:b 1:1• has 2 genes each chromosome; only 1• can have amino acid substitutions, promoter
mutations, gene deletions, splice• Improper synthesis cause instability, or aggregation+ has some ; 0 makes none
• People offten survive if hereditary persistence of fetal hemoglobin: HPFH (22 = HbF)
• Treatments of -thalassemia or sickle cell:
increase Hb transcription
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VI. Hemoglobin switchingHemoglobin switching:• embryo blast synthesis yolk• fetus liver synthesis• adult bone marrow
Multiple genes for HbOrder of genes parallels developmentProblems if deletions, other mutationsProblems if imbalance
Fig. 18
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Transcription factors control Hb switching
-globin locus about 100 kb; HS40 control region
-globin locus has LCR control region• Promoter of gene has many transcription factors that bind;
HPFH mutations often map promoter• Mutated repressor (CDP) or site• SSP and SP1 compete for binding near TATA
Fig. 19
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Blood types reflect erythrocyte glycolipids
Fig. 30.16,17
Blood group substances are glycolipids or glycoproteins on cell surface of erythrocytes:
• Glycosyltransferases add sugars, detemine blood type• Two alleles (three choices) iA, iB, i• Produced in Golgi, lipid part of membrane of vesicle, fuses
and carbohydrate extends extracellular
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Key concepts
Key concepts:• Blood contains distinct cell types• Erythrocytes transport O2 and return CO2 to lung
• Limited metabolism• Heme synthesis in rbc precursos• Oxygen binding
• Hematopoiesis from bone marrow• Leukocytes include monocytes, polymorphonuclear• Hemoglobin mutant proteins, expression
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Review question
Review question:
1.A compensatory mechanism to allow adequate oxygen delivery to tissues at high altitudes, where oxygen concentrations are low, is which of the following?
a.Increase in 2,3-bisphosphoglycerate synthesis by rbc
b.Decrease in 2,3-bisphosphoglycerate synthesis by rbc
c.Increase in hemoglobin synthesis by rbc
d.Decrease in hemoglobin synthesis by rbc
e.Decreasing the blood pH