Erythrocyte Biochemistry Flashcards
When is the majority of Hb synthesized in the RBC
Before extrusion of nucleus from the RBC
A small amount of hemoglobin is made in the ____
Reticulocyte
What form is iron in when in hemoglobin
Ferrous form (Fe2+)
What are the two chain subunits in hemoglobin
2 Alpha-globin chains
2 Beta-globin chains
Three parts of embryonic hemoglobin
Hb Gower 1
Hb Gower 2
Hb Portland
When is the embryonic hemoglobin destroyed to make way for fetal hemoglobin
8-10 weeks
Difference between fetal Hb and adult Hb
Fetal
- HbF (alpha 2, gamma 2)
Adult
- HbA (alpha 2, beta 2)
When does the gamma chain of Hb dissolve and the beta chain begin
At birth
How can fetal hemoglobin be used to treat sickle cell?
The defect leading to sickle cell is in the beta chain hemoglobin in adult cells. Research uses hydroxyurea to induce HbF, which consists of alpha and gamma chains but not beta.
Downside of inducing HbF to treat sickle cell
Hydroxyurea is a toxic chemotherapeutic agent
F8 Histidine
“Proximal histidine”
Is bound to heme; stabilizes hemoglobin
E7 Histidine
“Distal Histidine”
Stabilizes O2 on hemoglobin; the oxygen binds to the iron that is between the heme and the distal histidine
What conformational change occurs in heme after binding of oxygen
It changes the position of iron in the plane of the heme
- changes the 0.4 A angle which pulls down the proximal F8 histidine and changes the interaction with associated globin chain
What kind of dissociation curve is associated with myoglobin
Hyperbolic curve
What kind of oxygen dissociation curve is associated with hemoglobin
- why
Sigmoidal curve
- interactions between globin subunits
- Hb switches between low and high affinity
- the binding to O2 is reversible
Positive cooperativity in oxygen binding
Binding of one O2 to one heme facilitates the binding of O2 to another heme
- Conformational change in one globin subunit induces change in another
Binding of O2 to Fe of a globin subunit pulls proximal F8 histidine down
—> pulls on the globin FG-helix which changes interaction with other globin change
Bohr Effect
Decreasing the pH of actively respiring tissues (from increase in CO2) decreases the affinity of Hb for O2, which favors release of O2 to the tissues
Role of 2,3-BPG in hemoglobin binding
Reduces O2 affinity so Hb gives up more O2 to tissues
- signals Hb to let go of O2
- very high concentration in RBCs
How does exercise affect Hb and O2 binding
Exercise causes a drop in pO2 —> Hb has decreased affinity to O2 and releases it
- corresponds to the steepest part of sigmoidal O2 binding curve
HbF vs HbA in mother
Fetal hemoglobin has higher affinity for oxygen
- not regulated by 2,3-BPG
What molecule carries iron in the blood
Transferrin
Two proteins that store iron in the body
Ferritin (H2O soluble)
Hemosiderin (H2O insoluble)
What is the form of iron from animal products and plant products
Animal: ferrous
Plant: ferric
Ferric reductase
Dcytb- duodenal cytochrome-like b
Converts Fe3+ in non-heme iron to Fe2+
- uses calcium
Divalent transporter 1 (DMT1)
Takes up Fe2+ and transports it into the enterocyte
Also transports iron into the mitochondria in the TfR mediated endocytosis pathway
Ferroportin
Extrudes iron out of the cell
Feroxidase/Hephaestin/cerruloplasmin
Converts free iron (Fe2+) to Fe3+ to be then carried by transferin
Transferrin receptor (TfR)
Partakes in receptor mediated endocytosis to uptake transferrin into the cell –> transferrin enters cell to deliver iron to mitochondria
Hereditary hemochromatosis
- what is it
- what mutation
- symptoms
- treatment
Organ dysfunction due to iron overload caused by dysregulation of iron uptake and export by enterocyte
- Mutation: A. Recessive, HFE gene
- Sx: cirrhosis, arthritis, endocrinopathy, skin pigmentation, cardiomyopathy
- Tx: blood letting
Hepcidin
Regulator of iron homeostasis
- binds to ferroportin which causes internalization of ferroportin which is then destroyed, thus stopping iron absorption
Hepcidin expression and ferroportin levels when:
- iron is high
- iron is low
High iron:
- high hepcidin
- low ferroportin
- iron absorption low
Low iron:
- low hepcidin
- high ferroportin
- iron absorption high
Mutations in Hfe (Human hemochromatosis protein) causes what
- mechanism
Hemochromatosis
- if Hfe is mutated then it cannot bind to TfR2, which means hepcidin expression cannot be turned on and ferroportin activity is uncontrolled
—> iron overloading
RBC production is dependent on _____ and _____
Vitamin b12 (cobalumin) Folate (folic acid)
Deficiency in vitamin B12 and folate can cause ______ due to _______
Megaloblastic anemia due to diminished synthesis of DNA in developing RBC in bone marrow
characteristics of megaloblastic macrocytic anemia
- erythrocyte volume
- hemoglobin volume
- color
- bone marrow
- large erythrocytes
- normal hemoglobin
- normochromic cells
- bone marrow shows megaloblasts (large erythroblasts); hyper-segemented neutrophils
Hyper-segmented neutrophils, megaloblasts, and large erythrocytes would indicate what disease:
megaloblastic macrocytic anemia
3 parts of folic acid
pteridine (nitrogen containing ring)
p-amino-benzoic ring (PABA)
glutamate reside chains
Dihydrofolate reductase
reduces folate –> dihydrofolate (FH2) and dihydrofolate (FH2) –> tetrahydrofolate (THF)
DHF
dietary folic acid
Tetrahydrofolate (THF) or (FH4)
- what is it
- what vital role does it play
active form of folate; transfers carbon units from donors to acceptors
- serves vital role in DNA synthesis
- –> adds methylene (CH2) in the synthesis of nucleotides (adds to dUMP to form dTMP)
Folic acid pathway
- two carbon transfers
folate –> dihydrofolate –> tetrahydrofolate
- main carbon transfer occurs when carbon side chain of serine is transferred to THF to form N5,10-methylene-THF
- the carbon of N5,10-methylene-THF is then transferred to dUMP to form dTMP, during which DHF is made, which is then reduced to THF to start cycle again
Purpose of vitamin B12 (cobalamin) in the folic acid pathway
folate is available as N5-methyl-THF, which will not give up single methylene group to dUMP to make dTMP for DNA synthesis; B12 demethylates N5-methyl-THF in order for it to enter the cycle as THF
Folic acid deficiency leads to _____ which leads to ___
decreased DNA synthesis which leads to megaloblastic microcytic anemia
how does the folic acid pathway aid in DNA synthesis
a carbon from N5,10-methylene-THF is transferred to dUMP which forms dTMP which can be used in DNA synthesis as thymine
where is folate (DHF) found in food
most foods:
- eggs
- milk
- yeast
- leafy vegetables
where is folic acid absorbed
in the small intestine (jejunum)
once absorbed in the intestine, folic acid is reduced to
N5-methyl-THF
what happens to the methyl group that B12 removes from N5-methylene-THF
it is transferred to homocysteine to create methionine via methionine synthase
most deficiencies in B12 are due to lack of _____
intrinsic protein
B12 absorption pathway
dietary B12 binds to R-binders –> proteases degrade R binders –> releases B12 –> intrinsic factors carry B12 to ileum where receptors bring it into body
R binder proteins are made by ____
gastric mucosa cells
intrinsic factor-cobalamin is taken up from the ileum by ___
receptor-mediated endocytosis
cobalamin circulates the blood carried by
transcobalamin
pernicious anemia
- what is it
- what causes it
megaloblastic macrocytic anemia
- deficiency in B12 resulting from unavailability of intrinsic factor (IF) which results from destruction of IF and the cells producing it (ex: auto-immune mechanism)
Part one of Schilling Test
- what is it
- what do the results mean
patient is given oral dose of co-labeled B12 with an injection of unlabeled B12 cobalamin
- if B12 is absorbed from the GI, it will pass into the urine
- absent in urine = B12 not absorbed (possible pernicious anemia)
- present in urine = normal absorption (dietary deficiency)
Part two of Schilling Test
- what is it
- what do the results mean
if B12 was absent in the urine in part one, proceed to part two:
patient is given oral dose of co-labeled B12 PLUS intrinsic factor with an injection of unlabeled B12 cobalamin
- if radioactive B12 is present in urine, it means the absorption has improved, meaning the patient is deficient in intrinsic factor = patient has pernicious anemia
erythropoiesis pathway
hemocytoblast (stem cell) –> proerythroblast (committed cell) –> early erythroblast –> late erythroblast –> normoblast –> reticulocyte –> erythrocyte
what occurs in phase 1 of erythropoiesis
ribosome synthesis
what occurs in phase 2 of erythropoiesis
hemoglobin accumulation
- being cranked out from ribosomes
what occurs in phase 3 of erythropoiesis
ejection of nucleus and organelles
characteristics of heme in hemoglobin structure
- how many per subunit/how many total
- what does it do
- hydrophilic/hydrophobic?
- has an atom of ____
- 1 per subunit/4 total
- carries oxygen
- hydrophobic
- iron (Fe2+)
what is adult hemoglobin (HbA) made of
97% alpha2beta2
3% alpha2sigma2
TfR mediated endocytosis pathway
transferin is internalized via clathrin coated pits (endosome) –> endosome transiently docks on the mitochondria and transfers iron directly to mitochondria –> DMT1 transports iron out of endosome
where is heme made
in the mitochondria
