Erythrocyte Structure And Function (Schalm 20) Flashcards
Three procoagulant mechanisms of erythrocytes
Release of ADP
Release of Thromboxane A2
Exposure of phosphatidylserine on outer membrane leaflet
Anticoagulant property of erythrocytes
Make fibrin network more porous
Components of RBC membrane phospholipid bilayer
Phosphatidylcholine, sphingomyelin
Phosphatidylserine, phosphatidylethanolamine
(PS and PE almost exclusively on inner PL layer)
What enzyme activities maintain the asymmetry of the RBC phospholipid membrane?
Flippase
Floppase
Type-IV P-type ATPases
ABC transporters
Name three proteins localized to the lipid raft RBC membrane microdomains
Flotillins
Stomatin
Glycophosphatidylinositol-anchored proteins
RBC membrane deformability and resistance to fragmentation is dependent on what factors?
Biconcave shape
Viscoelastic properties dependent on spectrin cytoskeleton
Red cell viscosity dependent on HGB content
Name two RBC integral membrane proteins
1) Band 3 (aka anion exchange protein)
2) Gylcophorins
What are the main functions of the Band 3 integral membrane protein?
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
What is the main function of the RBC integral membrane proteins “Glycophorins”?
Carry silicone acid residues and, therefore, maintain the negative surface charge of the RBC
What is the overall structure and molecular weight of hemoglobin?
Globular tetramer made of two alpha-polypeptide and two beta-polypeptide chains
Each chain contains a heme group
Approximately 64kDA in molecular weight
What are the two components of heme?
1) tetrapyrrole protpoporphyrin IX
2) Central iron molecule
What is the cytosolic storage form of iron?
Ferritin
What is the initial step in heme synthesis and where does it occur?
Succinate and glycine are converted to delta aminolevulinic acid (ALA) by ALA synthase. Vitamin B6 (pyridoxine) is required. This step occurs in the mitochondria.
Describe the step of heme synthesis that occurs in the cytosol, and name a element that inhibits this reaction
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.
Where and how is iron added to heme protein?
Ferrous iron (Fe2+) is inserted into protoporphyrin IX by ferrochelatase (FER-Ch) in the mitochondria
Where are the alpha and beta globin chains synthesized?
Globin chains are synthesized in the ribosomes and polyribosomes in the cytoplasm
Describe the final construction of hemoglobin from its components
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.
Hemoglobin synthesis is ongoing through what erythroid stage?
Hemoglobin synthesis is ongoing through the reticulocyte stage
Describe three mechanisms coordinating hemoglobin synthesis, minimizing free heme or globin in the cytoplasm of developing erythrocytes
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
What main protein binds free hemoglobin? What is the half-life for free hemoglobin clearance?
Haptoglobin. The half-time for free HGB clearance is 20-30 mins.
WHat are the four minor pathways for free HGB clearance?
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
Describe the initial step in heme catabolism
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.
In what organ is heme oxygenase activity the highest?
Heme oxygenase activity is highest in the SPLEEN. Some activity also in the liver, bone marrow, and renal tubular cells
Describe how iron is processed in heme catabolism
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
What enzyme reduces biliverdin to bilirubin in macrophages? In what animals is this virtually absent?
By biliverdin reductase in the presence of NADPH. This enzyme is virtually absent in birds.
What protein transports non-soluble bilirubin in the plasma?
Albumin
What is the rate-limiting step in hepatic bilirubin metabolism?
Transport of bilirubin gluconoride (conjugated bilirubin) into the bile canaliculi is the rate limiting step
What hemoglobin polymorphism exists in cats and what is its clinical significance?
Feline hemoglobin has 8-10 sulfhydryl groups (compared to 2-4 for other animals). These readily oxidizable groups allow easy Heinz body formation.
What hemoglobin type is synthesized in small ruminants in response to severe anemia?
HgbC
Usually sheep have HgbA, goats have Hgb A and B
What is the transporter that absorbs iron present as heme?
Apical heme transporter (apical heme carrier protein 1)
How is iron taken up by most cells?
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+)
What enzyme oxidizes iron so it can bind transferrin?
Ceruloplasmin
What happens to iron after macrophage cytoplasm ferritin stores are maximally filled?
Hemosiderin accumulates in the lysosome
What is the function of Divalent Metal Iron Transporter-1, and where is it expressed?
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.
What is the function of ferrireductase duodenal cytochrome b?
Reduces dietary ferric Fe3+ to ferrous Fe2+ on the apical membrane of the enterocytes (so it can be transported by DMT-1)
What is the function of Hephaestin?
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.
What is the function of ferroportin?
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.
What is the function of hepcidin?
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)
How does anemia/hypoxia increase iron availability?
Erythropoietin causes production of erythroferrone (ERFE) by erythroid precursors. ERFE downregulates hepcidin expression.
What form of iron is carried by transferrin in circulation?
Transferrin binds 1-2 iron atoms in their ferric (Fe3+) form
Describe iron storage by ferritin
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
