Hemoglobin Flashcards
Proteins that maintain RBC shape
Spectrin
Ankyrin
Actin
Band 4.1
Proteins that function as high affinity glucose transport anchors for ABO antigens
Glycophorins (GPs)
What protein is responsible for RBC osmotic balance?
Anion exchange protein (Band 3)
Where is EPO synthesized during fetal development?
The developing liver, subsequently moving to the kidney (physiologic source of EPO at the time of delivery)
What can cause the liver to reactivate EPO production?
Hepatitis/liver cancer
What renal cells normally produce EPO?
Peritubular cells in the renal cortex -
Decreased blood Hb concentration (anemia) is experienced by the cortical cells as hypoxia - hypoxia inducible factor (HIF) alpha is a transcriptional reuglator that induces EPO transcription under hypoxic conditions
Increased oxygenation - specific prolyl hydroxylases will hydroxylate proline and asparagine residues in HIFα (undergoes ubiquination by the von Hippel-Lindau (VHL protein complex)
Most common etiology of familial erythrocytosis
LOF mutation in the VHL gene
(VHL ubiquinates HIFα and designates it for the proteosome complex)
Pt’s suffer from increased RBC production out of proportion to physiologic need
Single most common genetic cause of erythrocytosis
Hemoblogin distrubtion in adult
In the normal adult:
95 – 98% of Hb is hemoglobin A (HbA), made up of two α chains and two β chains (α2β2),
2-3% HbA2 (α2δ2), and
0.5-1.5% fetal hemoglobin (HbF) (α2γ2).
How are alpha/beta Hb subunits inherited?
Two α globin genes and one β globin gene are inherited from each parent
so that the normal α globin
genotype is αα/αα and the normal β globin genotype is β/β.
R (relaxed) Hb state
OxyHb
T (tense) Hb state
deoxyHb
2,3-BPG binds to positive residues on β subunit and stabilizes the tense state
Methemoglobin (MetHb)
Hb containing heme in which iron has been oxidized to ferric iron (Fe3+) and does not bind oxygen
Cytochrome b5 reductase is an enzyme that reverse heme iron oxidation
M-hemoglobins (M-Hb)
Congenital mutation in either the α-, β-, or γ-globin chains that stabilize the heme group in the ferric (3+) state
Pt’s w/ mutated γ-globin exhibit “neonatal cyanosis” that resolves as HbF production decreases
M-Hb patients will have normal ferroprotoporphyrin IX as the heme
group on the unmutated chains, which permits adequate oxygen delivery.
M-Hb patients are asymptomatic other than a cyanosis-like appearance when deoxyHb concentration is > 5%. There is no treatment required or necessary, and methylene blue is ineffective and can be toxic.
How do RBCs transport CO2?
Most of the CO2 from tissue capillaries is carried by Hb as bicarbonate (HCO3-) generated by RBC carbonic anhydrase
10% carried as a carbamino complex bound reversibly to globin chains
Hb catabolism
Macrophages recognize damaged RBCs in marrow, liver, spleen - phagocytose RBC
Hg undergoes oxidative degradation in the cytoplasm
Heme is oxidized to ferric protoporphyrin IX (hemin) - then metabolized by heme oxygenase (HOx)
Bilberdin is reduced to bilirubin by biliverdin reductase - transported to liver
HOx is rapidly induced by the presence of heme - results in the release of carbon monoxide, ferric iron (may be stored as ferritin or released into plasma bound to transferrin), and biliveridin
What is the Hb scavenger receptor?
CD163 expressed on the surface of macrophages - endocytose haptoglobin-Hb complex
Extravascular hemolysis
Hemolysis that occurs in the spleen or liver
Intramedullary hemolysis
Hemolysis that occurs in the marrow as a result of ineffective production of RBCs and late RBC precursors that undergo degradation before they are released into circulation (ineffective erythropoiesis)
What binds heme released by extravascular/intramedullary hemolysis?
Hemopexin - gets taken up by the hepatocyte after binding to CD91
What binds heme released by intravascular hemolysis?
Haptoglobin binds free Hb in circulation - then complex binds to macrophage CD163
Subacute combined degeneration
Refers to neurologic manifestations characteristic of B12 deficiency (sometimes also folate deficiency)
Paresthesias, abnormalities in proprioception - neurologic manifestations predominantly present symmetrically in legs
Radiologic lesions in the posterior/lateral columns of the cervical spinal cord (V sign on MRI)
Chronic non-spherocytic hemolytic anemia
Clues you in to congenital hemolytic anemia due to RBC enzyme deficiencies (i.e. G6PD deficiency)
Episodic jaundice following minor illnesses
Clue to congenital hemolytic anemia - typically membrane protein defects like hereditary spherocytosis or RBC enzyme deficiency like G6PD deficiency
Can also be seen in Gilbert disease (not have hemolytic anemia)
Hemoglobinuria
Characteristic of intravascular hemolysis
Myoglobinuria
Characteristic of rhabdomyolysis
In general, how is iron homeostasis maintained?
By controllong absorption - exretion/loss of iron is not regulated
What reduces iron?
Nonheme iron is typically ferric (3+) and needs to be reduced to ferrous iron by duodenal cytochrome b (Dcytb), by ascorbic acid present in the intestinal lumen, or by other
ferrireductases.
What transports ferrous (2+) iron across the apical surface of an enterocyte?
Divalent metal transporter-1 (DMT1)
How does iron leave the enterocyte?
By means of ferroportin (FPN) - only cellular export protein for iron
Mobilization of stored cellular iron from the reticuloendothelial system and liver requires what cell protein?
Ferroportin (FPN) - the presence of FPN on the cell membrane is controlled by hepcidin; translation of FPN mRNA is regulated by the interaction of iron regulator protein (IRP) w/ iron response elements (IRE)
Regulation of iron in iron-depleted cells
IRP bind to IRE in FPN mRNA, inhibiting translation
Mobilization of iron in iron-replete cells
IRPs are released - allows FPN mRNA translation
FPN forms a complex w/ hepaestin (copper-containing ferroxidase on cell membrane)
Ferrous iron is exported by FPN and oxided by either hephaestin or by the soluble copper transport protein ceruloplasmin
Ferric iron then becomes bound to transferrin (Tf) and is transported to cell for storage or for erythropoiesis or other synthetic needs
What is the master regulatory protein of iron homeostasis
Hepcidin - produced in the liver and is part of innate immunity
Hepcidin production in response to iron deficiency
Hepcidin production is decreased by iron deificiency - permitting iron transfer from enterocyte to the circulation
Hepcidin production in response to IL-6
Hepcidin production is increased
Sequesters iron in response - innate immunity
Hepcidin production in response to increased erythroid activity in the marrow
Hepcidin production is decreased - mediated by erythroferrone (ERFE) which is released by erythroid precursor and decreases hepcidin production
How does hepcidin regulate ferroportin?
Hepcidin directly inhibits the function of ferroportin by binding to it and causing its internalization and degradation, effectively blocking iron export from cells into the bloodstream
In pyruvate kinase (PK) deficiency, 2, 3-BPG accumulates in red blood cells. This would be expected to do which of the following?
A. Increase tissue oxygen delivery to peripheral tissues
B. Decrease tissue oxygen delivery to peripheral tissues
C. Have no affect on tissue oxygen delivery to peripheral tissues
A. Increase tissue oxygen delivery to peripheral tissues
In patients with acquired cytochrome B5 reductase deficiency, which of the following is correct regarding methylene blue therapy?
A. Does not depend on NADPH availability
B. It reduces heme iron to the ferrous state
C. It oxidizes heme iron to the ferric state
D. It converts M-hemoglobin to hemoglobin
B. Methylene blue reduces heme iron to the ferrous state
In the metabolism of free heme, bilirubin is generated directly by which enzyme?
Biliverdin reductase
What is the primary function of RBC anion exchange protein (aka band 3)?
Maintenance of RBC osmotic balance
C. Maintenance of biconcave disk structure
B. GATA 1
A. Increased hemoglobin P50
D. Mutation affecting the γ globin chain
Ferric reduction to ferrous iron in cytochrome b5 reductase deficiency
Tx w/ methylene blue
Requires NADPH - those with G6PD deficiency will not respond (instead ascorbic acid is used as the reducing agent - not as effective)
D. Haptoglobin
Binds to frayed hemoglobin - haptoglobin is not readily repleted
Hemopexin - binds to heme only - not measured clinically
Steps of heme recycling
- Hb and heme are oxidized in cytoplasm
- MetHb and Hb globin chains are broken down and recycled
- Ferric ppIX is metabolized by HOx to release Fe3+ and biliverdin
- Biliverdin is reduced to bilirubin by biliverdin reductase
- Hb catabolism is the source of the majority of bilirubin
What is the only only extracellular iron carrier?
Transferrin (Tf) - binds up to two ferric atoms
apo-Tf = none bound
monoferric Tf
diferric Tf
TfR2
regulatory molecule that signals systemic iron status through BMP-6 receptor/HFE complex
regulates hepcidin production
TfR1
Receptor that delivers iron to the cytosol
Iron-bound TfR1 undergoes endocytosis
TfR1 is either utilized for cellular processes such as heme synthesis (erythroid precursors) or is storred as ferritin
What ferritin has ferroxidase activity to maintain iron in ferric form?
Heavy (H) ferritin
Hemosiderin
Partially degraded ferritin complexes that have undergone partial lysosomal degradation
Where does heme synthesis occur?
Sequentially in the mitochondria and then the cytosol of erythroid precursors
RLS of heme synthesis
5-aminolevulinic acid synthase (5-ALA)
First step produces 5-ALA and occurs in mitochondria
What heme biosynthesis pathway enzymes are inhibited by lead?
5-ALA dehydratase
&
Ferochelatase
What heme biosynthesis intermediates fluoresce in the UV spectrum and can cause cutaneous photosensitivity?
Porphobiligen and down:
Prophobilinogen
Hydroxymethylbilane
Uroporphyrinogen III
Coproporphyrinogen III
Protoporphyrinogen IX
Protoporphyrin IX + Fe2+
Heme
Heme biosynthesis intermediate that is neurotoxic
5-aminolevulinic acid
Which intermediate re-enters the mitochondrion for the final steps of heme biosynthesis?
Protoporhyrin IX
What enzyme of heme biosynthesis is responsible for inserting ferrous iron into protoporphyrin IX?
Ferrochelatase
Forms heme
Where is heme produced?
Bone marrow (~80%) and liver (~20%)
What inhibits the RLS of heme biosynthesis?
Heme and iron deficiency inhibit the production of 5-aminolevulinic acid (5-ALA) by 5-ALA synthase
Most common porhyria
Prophyria cutanea tarda
uroporphyrinogen decarboxylase (UROD) deficiency - leads to accumulation of urophorphyrin
Porphyrinogen vs. porphyrin
Sxs of porphyria cutanea tarda
Chronic blistering skin lesions
Often accompanied by elevated AST/ALT
Urine appears tea colored
Considered an example of iron overload disorder
Tx of porphyria cutanea tarda
Plebotomy (iron sxs)
and hydroxychloriquine
Acute intermittent porphyria
Deficiency of porphobilinogen deaminase (aka hydroxymethylbilane synthase)
Inherited in an autosomal dominant fashion (variable expressivity)
What affect does glucose have on 5-ALA synthase?
Activates its activity
Acute intermittent porphyria sxs
Five P’s:
Pain (abdominal)
Port-wine colored urine
Polyneuropathy
Psychological disturbances
Precipitated by drugs
What is the proccess that halts globin translation and stops further erythroid differentiation during heme deficiency?
Heme-regulated inhibitor (HRI)
phosphorylates eukaryotic initiating factor (eIF) 2α
Lead poisoning leads to an accumulation of what heme biosynthesis intermediate?
Free erythrocyte protoporphyrin (protoporphyrin IX w/o iron)
What porphyrias have nueropathy manifestations?
ALA dehydratase porphyria (ADP)
and
Acute intermittent porphryia
Lack cutaneous findings
What porphyrias can have both skin and neuropathyic features?
Variegate porphyria or hereditary coproporphyria
Frequently one predominates (either neurologic or cutaneous sxs)
What are the cutaneous porphryias?
Porphria cutanea tarda
Congenital erythropoeietic protoporphria
X-linked protoporphria
If present w/ neurologic sxs - should be investigated independently of porphria
What can increase free erythrocyte protoporphyrin (FEP)
FEP is increased in:
iron deficiency
lead poisoning
and in the erythropoietic porphria/protoporphria syndromes
free erythrocyte protoporphyrin is
predominantly zinc (Zn2+) protoporphyrin, while in protoporphyria it is predominantly metal-free.
Hepcidin in sideroblastic anemia
Hepcidin production is supressed - iron absorption is increased and Pt’s are at risk for iron overload
X-linked sideroblastic anemia
Some cases of X-linked sideroblastic anemia respond to therapy with pyridoxine (vitamin B6). This
response is rarely complete, and appears to be dependent on the role of the pyridoxine metabolite
pyridoxal-6-phosphate as an electron recipient in the final stage of 5-ALA synthesis
Increased free erythrocyte protoporphyrin results from inhibition of which of the following?
A. Heme-regulated inhibitor (HRI)
B. Ferrochelatase
C. 5-ALA synthase
D. Uroporphyrin decarboxylase
B. Ferrochelatase
What is the most specific indicator or iron deficiency?
Decreased serum ferritin concentration
Myelodysplastic syndrome w/ ring sideroblasts are characteristically associated with what mutation?
Mutations in SF3B1
Which of the following is a common exacerbating factor in porphyria cutanea tarda?
A. Lead toxicity
B. Copper deficiency
C. Iron overload
D. Excess zinc ingestion
C. Iron overload
In hereditary hemochromatosis, hepcidin production is suppressed by interference with?
Bone morphogenetic proteins-6 (BMP-6) signaling
A 36-year-old man is found to have a serum ferritin elevated at 900 ng/mL (normal 30-300 ng/mL). Serum total iron binding capacity saturation is 45% (normal 20-45%). His father and his paternal grandmother have similarly elevated ferritin. There is no history of abnormal iron findings on his mother’s side. What is the most likely diagnosis?
Ferroportin disease
A congenital mutation in HMBS/PBGD is most likely to be associated with which of the following?
A. Cutaneous pohotosensitivity
B. Bone marrow ring sideroblasts
C. Autonomic neuropathy
D. Hereditary hemochromatosis
C. Autonomic neuropathy
How is ferroportin expression affected by iron deficiency anemia due to blood loss?
Ferroportin expression is increased on enterocyte basal membranes
What is the only other thing that can lower serum ferritin besides iron-deficiency anemia?
Acidotic levels of vitamin C
Mechanism of x-linked sideroblastic anemia
Mutation in ALAS2 (erythroid ALA synthase)
Genetic inheritance of heriditary hemochromatosis
All are autosomal recessive besides Ferroportin disease
What gene mutation is associated w/ acute intermittent porphyria?
HMBS/PBGD mutation
B. RBC Hg
B. Increased erythropoietic acitivty
How would decreased matriptase-2 affect hepcidin expression?
Increased hepcidin expression
Maltriptase-2 is associated w/ iron refractory iron deficiency anemia (IRIDA)
B. Anemia of inflammation/anemia of chronic disease
Hepcidin signaling
Diagram
Illustration of iron signaling - hemocrhomatosis is a disease of hepcidin dysfunction
C. Serum iron concentration
What labs will be elevated in iron deficiency anemia?
Serum Tf/TIBC and ferritin
low Tf/TIBC saturation
What is elevated serum ferritin indicative of
Either iron overload or inflammation
A. X-linked sideroblastic anemia
Other options give you acute siderblastic anemia not chronic (as with x-linked sideroblastic anemia)
Coarse basophilic stipling associated w/ lead poisoning
C. Liver ALAS1 suppresion
C. Increased uroporphyrin I
B. Homoygous HJV mutation
A. HFE C282Y - even homozygous male would only have 25% chance of having symptomatic iron overload - also having females are protected due to menses (takes at least 35-40 years for sxs of iron overload to present with this mutation)
