Download Biochem Chapter 44 [4-20

Biochem Chapter 44: Biochem of Erythrocytes and Other Blood Cells
Blood is a “liquid tissue” made of water, proteins, and cells
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Erythrocytes (red blood cells, RBCs) are the most abundant cells in the blood
o RBCs transport oxygen to the tissues and have Hgb that help buffer blood by binding
protons
o RBCs lose all their organelles during differentiation
Leukocytes (white blood cells, WBCs) have a nucleus, and do immune defense in the blood
Thrombocytes (platelets) have organelles but no nucleus, and work in clotting
Page 824 – normal concentrations of RBCs, WBCs, and platelets in the blood
WBCs can either be polymorphonuclear leukocytes (granulocytes) or mononuclear leukocytes
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Granulocytes (polymorphonuclear leukocyte) – have a multilobed, segmented nucleus (so
“poly” more than one)
o They’re called granulocytes because they have secretory granules
o Granulocytes (polymorphonuclear leukocytes) are neutrophils, eosinophils, & basophils
o When granulocytes get activated, their granule vesicles fuse with the plasma membrane
of the cell to be exocytosed, causing release of granule stuff, called degranulation
o Each granulocyte stains different:
 Neutrophils – stain pink, eosinophils – stain red, basophils – stain blue
o Neutrophils – phagocytes that respond to infection by engulfing foreign stuff and
destroying it with the respiratory burst (which creates free radicals)
o Eosinophils – fight viral infections with RNase in their granules, remove fibrin during
inflammation, and fight parasites
 Eosinophil granules are lysosomes with hydrolytic enzymes and cationic
proteins, which are toxic to parasites
 Eosinophils also work in allegies (type 1 IgE mediated hypersensitivity rxns)
o Basophils – work in hypersensitivity rxns, like allergies
 The least abundant WBC
 Basophils store histamine in their granules, and release it to increase vascular
permeability and cause smooth muscle contraction
 Basophil granules also have enzymes like proteases, β-glucuronidase, and
lysophospholipase, which all degrade microbes and help remodel tissue
Mononuclear leukocytes – have a rounded nucleus
o Mononuclear leukocytes are lymphocytes and monocytes
o Lymphocytes – small round cells with a big nucleus compared to their cytoplasm, that
are the main response to foreign stuff in the body
 3 types of lymphocytes: T cells, B cells, and NK cells
 The precursors of T cells (thymus derived cells) are made in the bone marrow,
and then migrate to the thymus, where they mature before being released into
the blood
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B cells mature in the bone marrow, and secrete antibodies
Natural killer (NK) cells target virus infected cells and malignant cells for
destruction
Monocytes – precursors to macrophage
Platelets (thrombocyte) – disc-like cells with tons of granules that help regulate clotting
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Platelets don’t have a nucleus
Platelets arise by budding of the cytoplasm of megakaryocytes, which are multinucleated cells
that hang out in the bone marrow
The major job of RBCs is to deliver oxygen to the tissues
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To do this, there needs to be enough hemoglobin (Hgb) in the RBCs
When Hgb concentration decreases, it’s called anemia
o Page 826 – normal levels of Hgb in a person
Anemias are categorized by RBC size and Hgb concentration
RBCs can be normal sized (normocytic), small (microcytic), or large (macrocytic)
RBCs with a normal Hgb concentration are called normochromic, and those with decreased Hgb
are called hypochromic
Microcytic, hypochromic anemia – caused by problems making Hgb
o Can be caused by iron deficiency, mutation that led to thalassemia, or lead poisoning
Macrocytic, normochromic anemia – caused by problems making DNA
o Can be caused by a vitamin B12 or folate deficiency, or erythroleukemia
Normocytic, normochromic anemia – caused by loss of RBCs
o Can be cause by acute bleeding, sickle cell disease, metabolic problems with RBCs, and
RBC membrane problems
Mean corpuscular volume (MCV) – average volume of the RBC
o Normal MCV is 80-100 fL
Mean corpuscular hemoglobin concentration (MCHC) – average concentration of Hgb in each
RBC
o Normal is 32-37 g/L
o Less than 32 – hypochromic cells
So a microcytic hypochromic RBC has an MCV less than 80, and an MCHC less than 32
Macrocytic normochromic cells have an MCV greater than 100, with an MCHC between 32- 37
A complete blood count (CBC) is ordered when you suspect a problem in the cell composition of the
patient’s blood
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Each cell in the collected blood is counted and typed as they pass through a detector
A laser is shown on them, and then the way the light scatters when it hits the cell is measured
A CBC gives you – total # of RBCs, MCHC, hematocrit (fraction telling you how much of the blood
is RBCs), MCV, total # of WBCs, and a count of the total # of each type of WBC
Mature RBCs don’t have any intracellular organelles, so the only enzymes a RBC has is in its cytoplasm
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The RBC cytoplasm has Hgb, and enzymes needed to prevent and repair damage done by ROS
and make energy
RBCs can only make ATP by glycolysis
The ATP is used for ion transport across the cell membrane, phosphorylation of membrane
proteins, and priming rxns of glycolysis
RBC glycolysis uses the Rapoport-Luebering shunt to generate 2,3-bisphosphoglycerate (2,3BPG)
o RBCs have way more 2,3-BPG than other cells
o 2,3-BPG is needed for the phosphoglycerate mutase rxn of glycolysis, where 3phosphoglycerate is converted to 2-phosphoglycerate – page 827
o 2,3-BPG regulates oxygen binding to Hgb that stabilizes the deoxy form of Hgb,
therefore facilitating the release of oxygen to the tissues
To bind oxygen, the iron of Hgb needs to be in the ferrous (Fe2+) form
o ROS can oxidize iron to the ferric form (Fe3+), to form methhemoglobin
o Some of the NADH made from glycolysis is used to regenerate Hgb from
methhemoglobin by NADH-cytochrome b5 methemoglobin reductase system
 Cytochrome b5 reduces the ferric iron of methemoglobin
 The oxidized cytochrome b5 is then reduced by cytochrome b5 reductase (aka
methemoglobin reductase) using NADH as the reducing agent
o Congenital methemoglobinemia – excess methemoglobin due to a deficiency in
cytochrome b5 reductase, or in people who inherited Hgb M
 In Hgb M, a mutation in the heme-binding area stabilizes the ferric oxygen
 People with congenital methemoglobinemia look cyanotic, but have few clinical
problems
o Methemoglobinemia can be acquired by ingesting oxidants like nitrites, quinones,
aniline, and sulfonamides
 Can treat acquired methemoglobinemia by giving them a reducing agent, like
vitamin C or methylene blue
Up to 10% of the glucose metabolized by RBCs is used to generate NADPH by the hexose
monophosphate shunt
o NADPH is used to keep a supply of reduced glutathione
o The glutathione cycle is the RBC’s main defense against damage from ROS
o The enzyme that catalyzes the 1st step of the hexose monophosphate shunt to generate
NADPH, is glucose-6-phosphate dehydrogenase (G6PD)
 How long the RBC lives depends on G6PD
 RBCs don’t have ribosomes, so they can’t make new G6PD
 So when G6PD activity decreases, oxidative damage accumulates, leading to
lysis of the RBC (called hemolysis)
 Hemolysis of too many RBCs leads to hemolytic anemia
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Glucose-6-phosphate dehydrogenase deficiency is the most common enzyme
deficiency in people
 Probably because these people are resistant to malaria
 RBCs that don’t have G6PD have shorter life spans, and are more likely
to lyse when exposed to oxidative stress
 Mutation to G6PD causes it to have decreased stability or lowered
activity, leading to a decreased RBC life span
o An inherited deficiency in pyruvate kinase can cause hemolytic anemia
 Since the amount of ATP made from glycolysis is decreased by half, RBC ion
transport doesn’t work, and the RBCs gain calcium, & lose potassium and water
 The water loss increases the Hgb concentration in the RBC, which increases the
internal viscosity of the RBC, making it rigid and easier to hurt by shearing
forces in circulation
 Once RBCs are damaged, they’re removed from circulation, leading to anemia
 The effects of the anemia are usually buffered by a big increase in 2,3-BPG
concentration from the blockage of conversion of phosphoenolpyruvate to
pyruvate
 Since 2,3-BPG binding to Hgb decreases the affinity of Hgb for oxygen, RBCs
remaining in circulation are very good at releasing bound oxygen to the tissues
The rest of the NADH from glycolysis goes towards converting pyruvate to lactate, so that NAD+
can be regenerated to do glycolysis again
Heme is made of a porphyrin ring attached to an atom of iron – page 828
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The porphoryin ring is made of 4 linked pyrrole rings, each of which has 2 side chains
Heme is made from glycine and succinyl CoA, which are combined to form δ-aminovulinic acid
(δ-ALA) by δ-ALA synthetase – page 829 bottom pic
o δ-ALA synthetase needs pyridoxal phosphate from vitamin B6 (pyridoxine)
o B6 deficiencies slow the rate of heme making, so that less heme is made, causing the
RBCs to be small and pale with higher iron stores, seen as a microcytic (small)
hypochromic (pale) anemia
Next, 2 δ-ALA are combined by δ-ALA dehydratase into a pyrrole called porphobilinogen
4 pyrrole rings then condense to form a linear chain of porphyrinogens
The side chains of the porphyrinogens initially have acetyl and propionyl groups
o The acetyl groups get converted to methyls
o Then the first 2 propionyl side chains are converted to vinyl gropus, forming a
protoporphyrinogen
The methyl bridges connecting the 4 pyrroles then get oxidized to form protoporphyrin 9
In the final step, ferrous iron is added to the protoporphyrin 9 by ferrochelatase (aka heme
synthase) – page 829
To make one heme, 8 glycine and 8 succinyl CoA are needed
Deficiencies in any of these enzymes cause diseases called porphyrias – p. 829
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Intermediates in the heme making pathway then accumulate and can have toxic effects
on the nervous system
o When porphyrinogens accumulate, they can be converted by light to porphyrins, which
react with oxygen to form ROS, which can hurt the skin
 So people who have excessive porphyrins are sensitive to light
o Porphyrias also show scarring and increased growth of facial hair
δ-ALA dehydratase (has zinc) and ferrochelatase, are inactivated by lead
o so in lead poisoning, δ-ALA and protoporphyrin 9 accumulate, and less heme is made
o You get anemia from the lack of Hgb, and less energy because heme is also found in
cytochromes of the electron transport chain
Heme is red, and is the reason RBCs and muscles with lots of mitochondria (P450’s) are red
Premenopausal women need more iron than postmenopausal women and men, due to menstruation
The average diet has the amount of iron the body should get in it, but only 10-15% of dietary iron is
normally absorbed, so iron deficiencies are common
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The iron in meats is in the heme form, so it’s easily absorbed
Nonheme iron in plants isn’t absorbed well, because plants have phenols that chelate or form
insoluble precipitates with the iron, preventing its absorption
Vitamin C (ascorbic acid) increases nonheme iron uptake in the GI
Iron is absorbed in the ferrous state, and then oxidized to the ferric state by ceruloplasmin,
which has copper in it
o The ferric state is the one needed for transport throughout the body
Free iron in the body is toxic, so iron is usually bound to proteins
o Iron is carried in the blood by transferrin
o Transferrin is usually only 1/3 saturated with iron
Transferrin bound to iron, binds to the transferrin receptor on the surface, and the complex gets
internalized into the cell
o The internalized membrane develops into an endosome with an acidic pH
o The iron, now in the ferrous form, is transported out of the endosome into the
cytoplasm by divalent metal ion transporter 1 (DMT-1)
 Mutation to the gene for DMT-1, which is SLC11A2, leads to iron deficiency
anemia, seen as microcytic hypochromic anemia
 The iron is trapped in endosomes and can’t be released to bind to
ferritin, so less heme is made, so less Hgb, leading to anemia
o Once in the cytoplasm, the iron is oxidized and binds to ferritin for long-term storage
Iron is stored in most cells, but especially in the liver, spleen, and bone marrow
o In these cells, apoferritin storage protein binds ferric iron to form a complex called
ferritin
o Normally ferritin is found in the blood in small amounts
o As iron stores increase, ferritin in the blood increases
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So the amount of ferritin in the blood is the most sensitive indicator of the
amount of stored iron in the body
When cells need iron, iron can be drawn from ferritin stores, transported into the blood as
transferrin, and taken up by receptor-mediated endocytosis by the cell
When excess iron is absorbed from the diet, it’s stored as hemosiderin, which is a form of
ferritin that is complexed with an additional iron that can’t be readily metabolized
So iron is absorbed from the diet, transported in the blood by transferrin, stored in ferritin, and
used to make things like Hgb – page 830
Iron is lost from the body with bleeding and sloughed off cells, sweat, urine, and feces
Heme regulates its own making by regulating δ-ALA synthetase
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Heme can suppress making of δ-ALA synthase, and directly inhibit δ-ALA synthase
So heme is made when heme levels are low, and when you get more heme, it makes it so that
less heme is made
Heme also regulates the making of Hgb by stimulating making of protein globin
Heme is broken down into bilirubin, which is carried in the blood by albumin to the liver, where it is
conjugated with glucuronic acid & excreted in bile – p. 831
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The major source of bilirubin is Hgb
At the end of the RBC life span (about 120 days), the RBC is phagocytosed
The globin is then cleaved into amino acids, and the iron is returned to the body’s iron stores
Heme is then oxidized and cleaved into carbon monoxide and biliverdin
Biliverdin is then reduced to bilirubin, which is transported to the liver by albumin
Bilirubin is then glucuronated and excreted in bile
In the colon, bacteria deconjugate bilirubin into urobilinogens
o Some urobilinogens are absorbed into blood and excreted in urine, but most is excreted
in feces, giving it its brown color
Men usually don’t have issues getting enough iron, so if a guy eating a normal diet has iron deficiency
anemia, you should suspect bleeding from the GI from ulcers or cancer
Drugs can induce ER cytochrome P450’s, which have heme, so since more are made, free heme levels
decrease and δ-ALA synthase is induced to increase the rate of heme making
Iron deficiency will show microcytic hypochromic anemia, which shows small and pale RBCs
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Unlike B6 deficiency though, which also shows microcytic hypochromic anemia, the iron stores
will be low in iron deficiency
A RBC looks like a red disc with a pale central area, described as a biconcave disc
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This biconcave disc shape facilitates gas exchange across the cell membrane
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The membrane proteins that maintain the shape of the RBC also allow the RBC to go through
very narrow capillaries, to deliver oxygen to the tissues
o Many capillary lumens are smaller than a RBC
Also, when RBCs go through the kidneys, they go through very hypertonic areas and then back
again, causing them to shrink and expand as they travel
The spleen determines if the RBC is viable or not
o RBCs pass through the spleen over 100 times a day
o In order to survive going through the spleen, the RBC needs to be able to deform
o Damaged RBCs that can’t deform get trapped in the spleen, where they get destroyed
by macrophage
Whether a RBC can deform or not depends on its shape and the organization of its membrane
proteins
o A RBC has more surface area than a typical sphere would allow, and a cytoskeleton to
support it
 This allows for stretching and getting deformed by stresses as the cell passes
through narrow spaces
 Proteins on the cytoplasm side of the membrane to make the RBC flexible
include spectrin, actin, bands 4.1 and 4.2, and ankyrin – page 833
 Spectrin is the major protein, and it has actin and band 4.1 at its end
 Many spectrin can attach to one actin, forming a cytoskeleton
 The spectrin cytoskeleton is connected to the membrane lipid bilayer by
ankyrin, which interacts between membrane protein band 3 and spectrin’s β
subunit
 Band 4.2 helps stabilize this conection
 Band 4.1 anchors the spectrin cytoskeleton with the membrane by
binding membrane protein glycophorin C and the actin complex
 When the RBC faces mechanical stress, the spectrin network rearranges so that
some spectrin get uncoiled and extended, while others get compressed
 This changes the shape of the cell, but not its surface area
 Problems with the cytoskeletal proteins leads to hemolytic anemia
 Shear stresses in the circulation cause loss of pieces of the RBC
membrane
 As these bits are lost, the RBC gets more spherical and loses deformity,
making them more likely to lyse or be trapped in the spleen
o A mature RBC can’t make new membrane stuff, but it can exchange membrane lipids
with circulating lipoprotein lipids
Things that affect oxygen binding to Hgb – 2,3-BP, protons, and carbon dioxide
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2,3-Bisphosphoglycerate (2,3-BPG) – binds to Hgb in the central cavity formed by the 4 subunits,
increasing the energy needed for the shape changes that allow oxygen to bind
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So 2,3-BPG lowers the affinity of Hgb for oxygen, so Hgb doesn’t want to bind oxygen,
and wants to release oxygen if it was already bound
Protons – when protons bind Hgb it lowers its affinity for oxygen, called the Bohr effect
o The pH of blood decreases and gets more acidic as you enter the tissues, because fo the
carbon dioxide made by metabolism being converted to carbonic acid and then proton
and bicarb
 Carbon dioxide + water carbonic acidproton + bicarb – page 835
 The higher amount of protons react with amino acids in the Hgb, causing shape
changes that promote the release of oxygen
o In the lungs, the opposite happens, and oxygen binds to Hgb, causing release of protons,
which combine with bicarb to form carbonic acid, increasing the pH of the blood
 Carbonic anhydrase cleaves carbonic acid into carbon dioxide and water
 The carbon dioxide is then exhaled
o Page 834 bottom pic – Dr. Thomas made a big deal about knowing graphs like this
Carbon dioxide – most of the carbon dioxide made from metabolism is carried to the lungs as
bicarb, but some of the carbon dioxide binds to Hgb in the tissues
o In the lungs, where there’s more oxygen, the oxygen binds the Hgb, causing it to release
the carbon dioxide, because Hgb likes oxygen better than carbon dioxide
Blood cells are made in the bone marrow, called hematopoeisis
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All blood cells are descended from hematopoietic pluripotent stem cells, which can be renewed
throughout life
o There aren’t many hematopoietic stem cells
o Signals tell the hematopoietic stem cells to proliferate, differentiate, and mature into a
blood cell – page 836
 CFU – colony forming unit
o As the hematopoietic lineages differentiate, each time it differentiates, it’s more
restricted in the # of kinds of cells it can become
The developing progenitor cells in the bone marrow grow near marrow stromal cells
o Marrow stromal cells include fibroblasts, endothelial cells, adipocytes, and macrophage
o The stromal cells form an ECM, and secrete growth factors that regulate hematopoiesis
o Most hematopoietic growth factors bind to receptors that then lead to receptor
aggregation, which induces phosphorylation of Janus kinases (JAKs)
 JAKs are tyrosine kinases that get activated by being phosphorylated
 Activated JAKs then phosphorylate the cytokine receptor, which creates areas
for more signaling molecules to bind, including signal transducer and activator
of transcription (STAT) transcription factors
 The JAKs then phosphorylate the STATs, which then dimerize and go to the
nucleus, where they activate target genes
 More signal transduction proteins bind to the phosphorylated cytokine
receptor, leading to activation fo the Ras/Raf/MAP kinase pathways
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 Other pathways are activated to inhibit apoptosis
o The response to cytokine binding is usually temporary because the cell has many
negative regulators of cytokine signaling
 Silencer of cytokine signaling (SOCS) proteins is induced by cytokine binding
 Some SOCS binds the phosphorylated receptor and prevents binding of
signal transduction proteins
 Other SOCS bind to JAKs and inhibit them
 SHP-1 is a tyrosine phosphatase found mainly in hematopoietic cells, that is
needed to form myeloid and lymphoid cells
 SHP-1 dephosphorylates JAK2, which inactivates it
 Mutations to the erythropoietin receptor that make it so that SHP-1
doesn’t work, cause uncontrolled stimulation for RBC making by
erythropoietin, so they have lots of RBCs
 Protein inhibitors of activated STAT (PIAS) bind to phosphorylated STATs to
prevent their dimerization, or break apart the dimers
Leukemias – malignancies of the blood
o Happen when a differentiated hematopoietic cell doesn’t complete its development,
and instead remains in an immature proliferative state
X linked severe combined immunodeficiency disease (SCID) – mutation to the Il-2 receptor
makes it so that the receptor can’t activate JAK3, so the cells don’t respond to growth factors
o So T cells aren’t formed, and B cells aren’t active
o Il-2 – the T cell growth factor
Problems with JAK/STAT signaling can cause lymphoid and myeloid leukemias, severe congenital
neutropenia (really low #’s of neutrophils), and Fanconi anemia (bone marrow failure)
The making of RBCs (erythropoiesis) is regulated by tissue demand for oxygen
o When tissues lack oxygen, the kidney releases the hormone erythropoietin, which
stimulates the multiplication and maturation of erythroid progenitors
o Page 838 – steps in making a RBC
o Eventually, you differentiate into a normoblast in the bone marrow, which is the first
recognizable RBC precursor
o Each normoblast undergoes 4 more divisions
 During these 4 cycles of division, the nucleus gets smaller and more condensed
o After the last division, the nucleus is lost, and the precursor is now called a reticulocyte
 Reticulocytes still have some ribosomes and mRNA, and can make Hgb
o The reticulocytes get released from the bone marrow and circulate for 1-2 days
o Reticulocytes mature in the spleen, where the ribosomes and mRNA are lost
Nutritional anemias:
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Since so many RBCs are made in a day, lack of nutrients to make them causes problems quick
o Includes iron, B12, and folate
In iron deficiency – cells are smaller and paler than normal, with less heme made
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Iron-deficient RBCs keep dividing past their normal stopping point, forming small
(microcytic cells)
o The RBCs lack Hgb, so they’re pale
o So you get microcytic, hypochromic anemia
B12 or folate deficiencies – cause megaloblastic anemia, where the cells are bigger than normal
o Folate and B12 are needed to make DNA
o When there’s no folate or B12, DNA replication and nuclear division don’t keep up with
the maturation of the cytoplasm
o So the nucleus is removed before the cell has divided enough times
o So you get a big cell, and less RBCs are made
Hemoglobinopathies – disorders in the structure or amount of globin chains
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Most mutations to Hgb are single base substitutions, causing a single amino acid replacement
The most common Hgb mutation is Hgb S (HbS), called sickle cell trait (HbA (adult Hgb)/HbS)
o Homozygotes have sickle cell anemia
o Sickle cell trait protects from malaria
Hgb C (HbC) - happens from a glu-to-lys replacement, that promotes water loss from the RBC
by activating the potassium transporter, causing a higher Hgb concentration in the RBC
o In homozygotes, it greatly lowers Hgb solubility, making the Hgb tend to precipitate in
the RBC
 Unlike sickle cell though, the RBC doesn’t become deformed
 Homozygotes of HbC get a mild hemolytic anemia
o Heterozygotes have no problems
HbC is found a lot in Africa, which also has a lot of HbS
o So Africans have these mutations more often
o When they are heterozygous for both (HbS/HbC), they enhance each other
 HbS polymerizes when HbS concentration increases
 HbC gets rid of water, making HbS concentration increase
Thalassemias – when there’s more of one subunit in Hgb than the other, leading to anemia
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For Hgb to work best, Hgb α-globin and β-globin chains must have the right structure, and be
made in a 1:1 ratio
Heterozygous thalassemias also protect from malaria
One way to form a thalassemia is for a single amino acid replacement mutation of Hgb that gives
rise to a globulin subunit with decreased stability
The more common way to cause a thalassemia is a mutation that causes decreased making of
one subunit
o α-thalassemias usually arise from complete gene deletions
 2 copies of the α-globulin gene are on each chromosome, giving you 4 total αglobulin genes per precursor cell
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If one copy gets deleted – the size and Hgb concentration of each RBC is slightly
decreased
 If 2 copies are deleted – they’ve gotten small enough and lost enough Hgb that
you get microcytic hypochromic RBCs, but no anemia yet
 Can happen either from one chromosome losing both its copies (more
common in Asians), or by both chromosomes losing one chromosome
(more common in Africans)
 If 3 copies are deleted – causes a moderate microcytic hypochromic anemia
with splenomegaly
 If all 4 copies are deleted – it’s called hydrops fetalis, and usually causes death
in utero
o β-thalassemia – can happen from deletions, promoter mutations, and splice-junction
mutations
 Heterozygotes with β+ (some β globin made)or β-null (β⁰, no β globin made), are
usually asymptomatic, but usually have microcytic hypochromic RBCs and may
have a mild anemia
 β+/β+ homozygotes have anemia
 β+/β-null heterozygotes have worse anemia
 β-null/β-null homozygotes have severe disease
 In general, missing a β-globulin is more severe than missing an α-globulin
o Excess β-chains form Hgb H (HbH), which can’t deliver oxygen to tissues because it has
a high oxygen affinity
 As RBCs age, HbH precipitates in the RBCs, forming inclusion bodies
 RBCs with inclusion bodies have shorter life spans because they’re more likely
to get trapped and destroyed in the spleen
o Excess α-chains can’t form a stable Hgb, but they do precipitate in RBC precursors,
decreasing RBC making
 It also leads to lipid oxidation by ROS
Fetal Hgb (HbF) – the main Hgb in the fetus, and has 2 α chains and 2 γ chains
o Adult Hgb (HbA) has 2 α chains and 2 β chains
o Hgb switching – going from HbFHbA
 Hgb switching isn’t 100%, and most people continue to make a small amount of
HbF throughout life
 Some clinically normal people make abnormally high amounts of HbF instead of
HbA
o People with sickle cell or thalassemias have less severe disease if they make more HbF
o Hereditary persistence of fetal Hgb (HPFH) – people who express HbF after birth
 In deletion forms of HPFH, both the entire δ and β genes have been deleted
from one copy of a chromosome, so only HbF can be made
 As long as enough HbF is still made after birth, there’s no symptoms
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People who don’t make enough HbF after birth have a δ-null/β-null
thalassemia
 Nondeletion forms of HPFH happen from mutations to the promotors for the γ
chain
 The more γ chains there are, the less severe sickle cell and thalassemias
are
o HbF also has less affinity for 2,3-BPG than HbA, and so a greater affinity for oxygen
 So oxygen released from mom’s HbA is readily picked up by HbF in the fetus
Embryonic megaloblasts (large embryo RBCs that still have a nucleus) are first made in the yolk
sac about 15 days after fertilization
o After 6 weeks, RBC making shifts to the liver
o In the last few weeks before birth, bone marrow starts making RBCs
o By 8-10 weeks after birth, bone marrow is the only place making RBCs
The α-globulin gene on chromosome 16 has embryonic zeta (ῐ) gene and 2 copies of the α gene
The β globulin gene on chromosome 11 has embryonic ε gene, 2 copies of the β gene, Gγ and
Aγ, and 2 adult genes δ and β
The order of the genes along the chromosome parallels the order of expression of the genes
during development
Embryonic Hgbs are ῐ2ε2 (Gower 1), ῐ2γ2 (Portland), and α2γ2 (Gower 2)
Fetal Hgb is mainly α2Gγ2
The major adult Hgb is HbA (α2β2)
Fetal Hgb found in adults is α2Aγ2
Hgb switching is regulated by an internal clock, and not regulated by the environment much
o Premature newborns convert HbFHbA at the same time as their gestational age
would’ve
o HS40 is a region of DNA in the ῐ gene that is the major regulator of Hgb switching
 HS40 stimulates transcription of ῐ and α genes
o Both the ῐ globulin and α globulin bind to messenger ribonucleoprotein (mRNP) stability
determining complex
 Binding to it prevents mRNA from being degraded
 α-globulin mRNA has much higher affinity for mRNP than ῐ globulin, so ῐ
globulin gets rapidly degraded
o The ε gene has the locus control region (LCR), which is needed for the β globulin to work
 Proteins at the promoters for β, ε, and γ globulins compete with each other to
bind the LCR, which has enhancers
o HPFH mutations are often at transcription factor binding sites on γ globin gene
promoter
 Causes HPFH by destroying the sites, or making new ones
 2 important sites are the stage-selector protein-binding (SSP) site and the CAAT
box
 When the SSP is bound to the promoter, the γ globin gene has a competitive
advantage over the β-globin promoter for interaction with the LCR
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o
Sp1 is a transcription factor that binds the same site on the promoter,
acting as a repressor
 So Sp1 and SSP compete to regulate the activity of the γ-globin gene
 CP1 is a transcription activator that binds the CAAT box
 CAAT displacement protein (CDP) is a repressor that binds at the same
site of the CAAT box and displaces CP1
BCL11A transcription factor is a strong repressor of γ-globin gene expression, by
interfering with interaction with the LCR
 BCL11A expression is regulated by the transcription factor KLF1, which is
essential for β-globin expression
 KLF1 increases BCL11A expression, which blocks γ-globin gene expression
Anemia makes it so that the bone marrow needs to make more RBCs
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The expansion of the marrow if the anemia isn’t fixed causes osteoporosis that lead to
compression fractures in the lumbar spine
A complication of sickle cell disease is increased formation of gallstones (cholelithiasis)
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A sickle cell crisis with hemolysis increases the amount of unconjugated bilirubin
If there’s more bilirubin than the hepatocytes can conjugate, both the total and the
unconjugated bilirubin levels increase
Unconjugated bilirubin is not water soluble, so it precipitates when excreted into the
gallbladder, forming pigmented calcium gallstones
Hereditary spherocytosis – mutation to RBC membrane proteins leads to RBCs that can’t deform, called
spherocytes (sphere is the shape of the RBC when it can’t deform)
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So you get enlarged spleen cause it gets clogged up, tons of bilirubin from their hemolysis,
leading to anemia
Most often mutation is to ankyrin, β-spectrin, or band 3