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)

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,

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

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

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

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

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

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

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

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

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

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

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

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)

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

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

Bone marrow

Bone marrow stromal cells secrete growth factorsHematopoietc stem cells respond

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

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

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

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

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

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

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

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

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