Erythrocyte Structure And Function (Schalm 20)

Question 1

Three procoagulant mechanisms of erythrocytes

Answer 1

Release of ADP
Release of Thromboxane A2
Exposure of phosphatidylserine on outer membrane leaflet

Question 2

Anticoagulant property of erythrocytes

Answer 2

Make fibrin network more porous

Question 3

Components of RBC membrane phospholipid bilayer

Answer 3

Phosphatidylcholine, sphingomyelin
Phosphatidylserine, phosphatidylethanolamine
(PS and PE almost exclusively on inner PL layer)

Question 4

What enzyme activities maintain the asymmetry of the RBC phospholipid membrane?

Answer 4

Flippase
Floppase
Type-IV P-type ATPases
ABC transporters

Question 5

Name three proteins localized to the lipid raft RBC membrane microdomains

Answer 5

Flotillins
Stomatin
Glycophosphatidylinositol-anchored proteins

Question 6

RBC membrane deformability and resistance to fragmentation is dependent on what factors?

Answer 6

Biconcave shape
Viscoelastic properties dependent on spectrin cytoskeleton
Red cell viscosity dependent on HGB content

Question 7

Name two RBC integral membrane proteins

Answer 7

1) Band 3 (aka anion exchange protein)
2) Gylcophorins

Question 8

What are the main functions of the Band 3 integral membrane protein?

Answer 8

1) anchors the cytoskeleton to the membrane
2) anion exchange (primarily for delivering CO2 to lungs in exchange for Cl)
3) binds glycolytic enzymes, hemoglobin, and hemichromes

Question 9

What is the main function of the RBC integral membrane proteins “Glycophorins”?

Answer 9

Carry silicone acid residues and, therefore, maintain the negative surface charge of the RBC

Question 10

What is the overall structure and molecular weight of hemoglobin?

Answer 10

Globular tetramer made of two alpha-polypeptide and two beta-polypeptide chains
Each chain contains a heme group
Approximately 64kDA in molecular weight

Question 11

What are the two components of heme?

Answer 11

1) tetrapyrrole protpoporphyrin IX
2) Central iron molecule

Question 12

What is the cytosolic storage form of iron?

Answer 12

Ferritin

Question 13

What is the initial step in heme synthesis and where does it occur?

Answer 13

Succinate and glycine are converted to delta aminolevulinic acid (ALA) by ALA synthase. Vitamin B6 (pyridoxine) is required. This step occurs in the mitochondria.

Question 14

Describe the step of heme synthesis that occurs in the cytosol, and name a element that inhibits this reaction

Answer 14

Two moles of aminolevulinic acid (ALA) are condensed to form porphobilinogen (PBG). This step is catabolized by ALA-dehydratase (ALA-D). ALA-D is strongly inhibited by lead.

Question 15

Where and how is iron added to heme protein?

Answer 15

Ferrous iron (Fe2+) is inserted into protoporphyrin IX by ferrochelatase (FER-Ch) in the mitochondria

Question 16

Where are the alpha and beta globin chains synthesized?

Answer 16

Globin chains are synthesized in the ribosomes and polyribosomes in the cytoplasm

Question 17

Describe the final construction of hemoglobin from its components

Answer 17

Addition of heme to the hydrophobic pockets of globin is followed by a dimerization of alpha and beta globin chains. This is followed by spontaneous formation of hemoglobin tetramers.

Question 18

Hemoglobin synthesis is ongoing through what erythroid stage?

Answer 18

Hemoglobin synthesis is ongoing through the reticulocyte stage

Question 19

Describe three mechanisms coordinating hemoglobin synthesis, minimizing free heme or globin in the cytoplasm of developing erythrocytes

Answer 19

1) Heme governs ribosomal translation of globin chain synthesis
2) alpha chains inhibit alpha-chain systhesis and stimulate beta-chain synthesis
3) beta-chain synthesis inhibit their own (beta-chain) synthesis

Question 20

What main protein binds free hemoglobin? What is the half-life for free hemoglobin clearance?

Answer 20

Haptoglobin. The half-time for free HGB clearance is 20-30 mins.

Question 21

WHat are the four minor pathways for free HGB clearance?

Answer 21

1) Oxidation to MetHgb that can be excreted
2) Hydrolysis of MetHgb to release ferriheme, which is complexed to hemopexin for transport to the mononuclear phagocyte system
3) Binding of ferriheme to albumin for transport to the MPS
4) HGB may be cleared by glomerular filtration in excessive intravascular hemolysis

Question 22

Describe the initial step in heme catabolism

Answer 22

Cleavage of the heme ring at the alpha-methane bridge to release linear tetrapyrrole biliverdin, iron, and carbon monoxide. This reaction is catalyzed by microsomal heme oxygenase in the presence of cytochrome P-450, oxygen, and NADPH.

Question 23

In what organ is heme oxygenase activity the highest?

Answer 23

Heme oxygenase activity is highest in the SPLEEN. Some activity also in the liver, bone marrow, and renal tubular cells

Question 24

Describe how iron is processed in heme catabolism

Answer 24

1) Iron is oxidized to ferric form (Fe3+)
2) Transported as transferrin
3) Stored as hemosiderin or ferritin in the liver, ferritin in the bone marrow

Question 25

What enzyme reduces biliverdin to bilirubin in macrophages? In what animals is this virtually absent?

Answer 25

By biliverdin reductase in the presence of NADPH. This enzyme is virtually absent in birds.

Question 26

What protein transports non-soluble bilirubin in the plasma?

Answer 26

Albumin

Question 27

What is the rate-limiting step in hepatic bilirubin metabolism?

Answer 27

Transport of bilirubin gluconoride (conjugated bilirubin) into the bile canaliculi is the rate limiting step

Question 28

What hemoglobin polymorphism exists in cats and what is its clinical significance?

Answer 28

Feline hemoglobin has 8-10 sulfhydryl groups (compared to 2-4 for other animals). These readily oxidizable groups allow easy Heinz body formation.

Question 29

What hemoglobin type is synthesized in small ruminants in response to severe anemia?

Answer 29

HgbC Usually sheep have HgbA, goats have Hgb A and B

Question 30

What is the transporter that absorbs iron present as heme?

Answer 30

Apical heme transporter (apical heme carrier protein 1)

Question 31

How is iron taken up by most cells?

Answer 31

Transferrin receptor on the cell surface (TfR). The low pH of the endosome causes release of Fe3+ from transferrin and reduces the iron to ferrous iron (Fe2+)

Question 32

What enzyme oxidizes iron so it can bind transferrin?

Answer 32

Ceruloplasmin

Question 33

What happens to iron after macrophage cytoplasm ferritin stores are maximally filled?

Answer 33

Hemosiderin accumulates in the lysosome

Question 34

What is the function of Divalent Metal Iron Transporter-1, and where is it expressed?

Answer 34

DMT-1 is expressed on enterocytes of the villi of the duodenum and macrophages. It is responsible for transporting iron from the enterocytes into the circulation, and from the endosomes of the macrophages to their cytoplasm.

Question 35

What is the function of ferrireductase duodenal cytochrome b?

Answer 35

Reduces dietary ferric Fe3+ to ferrous Fe2+ on the apical membrane of the enterocytes (so it can be transported by DMT-1)

Question 36

What is the function of Hephaestin?

Answer 36

This protein is a copper-dependent transmembrane ferroxidase similar to ceruloplasmin. It is expressed in the base lateral membrane of enterocytes and colocolized with ferroportin. It oxidizes Fe2+ to Fe3+ so it can be transported out of the enterocyte into the plasma.

Question 37

What is the function of ferroportin?

Answer 37

Ferroportin is the only mammalian cellular iron exporter. It exports iron from enterocytes and macrophages into the circulation. Expression is regulated by hepcidin and internalization of the protein.

Question 38

What is the function of hepcidin?

Answer 38

Hepcidin regulates iron systemically by binding and causing degradation of ferroportin. Synthesis is increased by iron loading (decreased intestinal absorption) or inflammation secondary to IL-6 Synthesis decreased by anemia and hypoxia (increasing absorption)

Question 39

How does anemia/hypoxia increase iron availability?

Answer 39

Erythropoietin causes production of erythroferrone (ERFE) by erythroid precursors. ERFE downregulates hepcidin expression.

Question 40

What form of iron is carried by transferrin in circulation?

Answer 40

Transferrin binds 1-2 iron atoms in their ferric (Fe3+) form

Question 41

Describe iron storage by ferritin

Answer 41

Apoferritin (the protein moiety) has a catalytic ferroxidase center that converts ferrous iron to ferric iron, which is bound to apoferritin to form ferritin. Ferritin may be composed of up to 31% iron (4500 ferric iron molecules), but usually around 18%. The ferroxidase process is reversible. Hemosiderin is a stripped down version of ferritin containing 25-30% iron by weight