Erythrocyte Info Found on Internet

8/8/2019 Erythrocyte Info Found on Internet

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Erythrocyte metabolism

Alice Skoumalová



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



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



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.



Hereditary spherocytosis

autosomal dominant

a deficiency in a spectrin amount and its abnormalities

the presence of spherocytes in the blood

the spleenµs hemolysis



Haemoglobin



4 protein chains + 4 haem groups



Movements of the heme and the F helix during the T ± R transition in

hemoglobin:





Hemoglobin saturation curves:



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)



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)



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





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



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



Glutathione synthesis in erythrocytes



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



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)





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)



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



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)



Quinols and enols -Tocopherol (vit. E)

Ubiquinol (coenzyme Q)

Ascorbic acid (vit. C)

Carotenoids -Carotin

Lycopin

Others Glutathione

Bilirubin

Biological antioxidants:



-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



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)



Haemoglobin S (sickle-cell)

Causes a sickle-cell anemia

Erythrocytes adopt an elongated sickle shape due to theaggregation of the haemoglobin S



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



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.





Hemoglobin switching:



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

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Erythrocyte Info Found on Internet