Download Biochemistry Ch. 44 823-845 [4-20

Biochemistry Ch. 44 823-845
Biochemistry of Erythrocytes and Other Blood Cells
Cells of the Blood – blood is liquid consisting of H2O, proteins, and RBC, WBC, and thrombocytes
Leukocytes are classified as polymorphonuclear leukocytes (granulocytes) or mononuclear leukocytes
1. Granulocytes – named after secretory granules visible during staining that respond to stimuli by
vesicles binding to plasma membrane and releasing contents. These mediate inflammatory
responses
a. Neutrophils – pink-staining phagocytic cells that quickly move to infection or tissue
damage and phagocytize/destroy foreign particles by creating respiratory burst
b. Eosinophils – fight viral infections (by releasing RNase), removing fibrin during
inflammation, and protect against parasites. Granules contain lysosomes
c. Basophils – least abundant, involved in allergy and hypersensitivity responses.
Histamine is stored in secretory granules of basophils to stimulate smooth muscle
contraction and increase vascular permeability
2. Mononuclear Leukocytes – consist of lymphocytes and monocytes
a. Lymphocytes – small round cells in lymph that separated into T cells, B cells, and NK
cells
i. T cells – thymus-derived cells produced in the bone marrow and move to the
thymus for maturation
ii. B cells – mature in the bone marrow and secrete antibodies
iii. NK cells – target virally infected and malignant cells for destruction
3. Thrombocytes – platelets are disc cells that aid in intravascular clotting – lack a nucleus and
arise by budding from megakaryocytes
Anemia – patient is considered anemic if hemoglobin concentration of RBC falls below normal
-RBCs can be normocytic, microcytic, or macrocytic (relating to size)
-RBC can be normochromic or hypochromic, relating to decreased hemoglobin concentration
-Mean Corpuscular Volume (MCV) and mean corpuscular hemoglobin concentration (MCHC) can classify
anemias
Erythrocyte Metabolism
1. Mature Erythrocyte – contains no organelles, so metabolism occurs 100% in the cytoplasm,
including generation of energy and repair of damage done by reactive oxygen species
a. ATP in RBC is used to transport ions across membrane (Na, K, Ca), phosphorylation of
proteins, and priming for glycolysis
b. RBC uses rapoport-luebering shunt to generate 2,3-bisphosphoglycerate (2,3-BPG) which
other cells don’t contain. 2,3-BPG is used to modulate oxygen binding to hemoglobin
c. To bind O2, Fe of hemoglobin must be in ferrous state (2+); ferric state = methemoglobin
d. Some of NADH for glycolysis is used to regenerate hemoglobin from methemoglobin using
cytochrome b5 (methemoglobin reductase)
e. 5-10% of glucose used to generate NADPH by hexose monophosphate shunt to maintain
glutathione in its reduced state. Glutathione cycle defends RBC from reactive O2 species
i. Enzyme that catalyzes first step in hexose monophosphate shunt is glucose-6phosphate dehydrogenase (G6PD). Life of RBC dependent on G6PD activity; when
RBC lysis exceeds rate of production, hemolytic anemia ensues
Glucose  Glucose-6-P  Fructose-6-P  Fructose-1,6-P  Glyceraldehyde-3-P  1,3 BPG  3phosphoglycerate  2-phosphoglycerate  PEP  Pyruvate  Lactate
Heme Structure – heme consists of porphyrin ring (4 pyrrole rings) coordinated with an iron
-heme is synthesized from glycine + succinyl coA. Steps are:
Succinyl CoA + Glycine  δ-aminolevulinic acid (δ-ALA)  porphobilinogen 
hydroxymethylbilane  uroporphyrinogen III  coproporphyrinogen III  protoporphyrinogen
IX  protoporphyrin IX  heme
-lead poisoning inactivates δ-aminolevulinic acid dehydratase and ferrochelatase to cause δ-ALA and
protoporphyrin IX to accumulate and decrease heme production to cause anemia
**See schematic drawing attached
-Fe can be absorbed from the diet (10-15%) and Fe deficiencies are common; meats have iron in heme
and are absorbed, but plant iron is not readily absorbed
-vitamin C increases nonheme iron uptake from GI tract
-Iron is absorbed in the 2+ state but converted to 3+ by ceruloplasmin for blood transport
-Free iron is toxic, so it is normally bound to apotransferrin in the blood to form transferrin
-transferrin binds to transferrin receptor on target cell and is internalized into an endosome, but
FE is removed from endosome by a metal ion transporter DMT-1 and binds to ferritin in
cytoplasm for storage in liver, spleen, and bone marrow
-Fe can be drawn from ferritin stores and transported in the blood as transferrin
-Fe absorbed from diet in excess is stored as hemosiderin
-Heme regulates its own synthesis by mechanisms that affect the first enzyme (δ-ALA synthase). Heme
represses synthesis of δ-ALA and also directly inhibits activity allosterically
-Heme stimulates synthesis of globin to form hemoglobin, and it also maintains ribosomal initiation
complex for globin synthesis in an active state
-Heme is degraded into bilirubin which is conjugated to glucuronic acid and excreted in bile
-heme from cytochromes and myglobin is also converted to bilirubin
-major source of bile pigment is hemoglobin
-RBC phagocytosed  heme cleaved to CO2 and biliverdin  bilirubin  moves to liver and
complexed with serum albumin
-in liver, bilirubin reacts with UDP-glucuronate  bilirubin monoglucuronide  diglucuronide
and excreted
-in the intestine, bacteria unconjugates bilirubin diglucuronide and converts to urobilinogen
which is absorbed into blood and excreted in urin, but also converted to stercobilin and
excreted in feces
The RBC Membrane – red disc with pale central area (biconcave disc) facilitates gas exchange and allows
it to traverse capillaries
-spleen determines viability of RBC by having tiny 3um pores for RBC to pass  if RBC is highly
deformable, it will pass through. Damaged RBC will not pass through and will be degraded
-Major proteins in cytoplasmic side are spectrin, actin, band 4.1, band 4.2, and ankyrin
-Spectrin is a heterodimer (a and B subunits wound around) bound near band 4.1. multiple
spectrins can bind actin for a branched cytoskeleton
-Spectrin is stabilized to the membrane by ankryin on B-spectrin and Band 3 helped by band 4.2
-mechanical stress causes spectrin network to rearrange shape of cell but not surface area
-glutathione system protects proteins and lipids from oxidative damage
Agents that Affect O2 Binding
1. 2,3 Bisphosphoglycerate (2,3-BPG) – formed in RBC from 1,3-bisphosphoglycerate. 2,3-BPG
binds hemoglobin to lower affinity for oxygen… Therefore 2,3-BPG = less O2
2. Proton Binding (Bohr effect) – binding of H+ to hemoglobin lowers affinity for O2 (bohr effect).
CO2 released into blood from the tissues is converted to H2CO3 by carbonic anhydrase and
dissociates into H+ and HCO3-. H+ binds to hemoglobin to promote release of oxygen
3. CO2 – some CO2 covalently bound to hemoglobin, an in the lungs where PO2 is high, O2
replaces CO2
Hematopoiesis – all blood cells are descended from stem cells in bone marrow which differentiate in a
hierarchical manner into any of the blood cells (CFU-E is progenitor for erythrocytes)
1. Cytokines – progenitor cells in marrow grow next to fibroblasts, endothelial cells, adipose
cells, and macrophages which secrete growth factors to affect hematopoiesis
a. Cytokine growth factors bind JAK receptors to phosphorylate them and start STAT
signal cascade, which dimerizes stat after phosphorylation and enters nucleus to
activate genes
b. Cytokine binding is transient because cell contains multiple negative regulators like
Silencer of Cytokine Signaling (SOCS) proteins which bind to phosphorylated
receptor and prevents docking of signal proteins. Other SOCS proteins bind JAKs to
inhibit them
c. SHP-1 – tyrosine phosphatase in hematopoietic stem cells that is necessary for
development of myeloid and lymphoid lineage – dephosphorylates JAK to inactivate
d. STATs- also inactivated by protein inhibitors of activated STAT (PIAS) which bind to
STAT and prevent their dimerization
-Leukemias are malignancies of the blood in every hematopoietic lineage
-X-linked severe combined immunodeficiency syndrome (SCID) is where circulating T lymphocytes are
not formed and B cells are not active  deficient IL-2 receptor in the gamma chain unable to activate
JAK3 and unresponsive to cytokines
-some people have mutant erythropoietin (EPO) receptor unable to bind SHP-1 leading to higher % RBC
in circulation since EPO cannot deactivate SHP-1  super active JAK 2 and STAT5
-JAK/STAT interruptions associated with leukemia development and severe congenital neutropenia
(where neutrophil count is reduced), and also Fanconi Anemia – bone marrow failure
-complication of sickle cell disease is increased gallstone formation due to RBC destruction in sickle cell
disease
Erythropoiesis – number of RBCs regulated by O2 demands in tissues. Kidneys secrete erythropoietin in
response to low tissue O2 to stimulate multiplication of erythroid progenitors
Stem cell  mixed myeloid progenitor (CFU-GEMM)  burst-forming unit-erythroid (BFU-E)  CFU-E
 normoblast  4 rounds of division where nucleus is condensed and becomes reticulocyte once
nucleus is extruded (still contain ribosomes and mRNA and can synthesize hemoglobin). Reticulocytes
mature in the spleen where ribosomes and mRNA are lost
Nutritional Anemias – deficiencies in iron, vit B12, and folate prevent RBC formation
-in Iron deficiency, cells are smaller and paler and lack heme synthesis  decreased globin synthesis
-normal maturing cells stop dividing when hemoglobin reaches normal concentration. Iron
deficient, developing RBCs will continue dividing past the normal point and will develop
microcytic, hypochromic anemia
-Vit B12 and folate deficiency causes megaloblastic anemia where cells are larger. Folate and
B12 required for DNA synthesis and if missing, nucleus will be extruded before normal cell
divisions occur, resulting in BIG cells
Hemoglobinopathies – most mutations of hemoglobin are single base pair SNPs, most common of which
is hemoglobin S (HbS, sickle cell anemia) is a bad disease
-hemoglobin C (HbC) is another common variant resulting from glu-lys switch which promotes
water loss from cell by activating K+ transporter  higher than normal hemoglobin. Also, the
substitution lowers hemoglobin solubility resulting in precipitation intracellularly (no deformity
though)
-HbC mutations cause mild hemolytic anemia
Thalassemias - hemoglobin is made of 1:1 ratio of alpha and beta chains. If one chain is in large excess
over the other, thalassemias form (provide resistance to malaria just like sickle cell)
-hemoglobin SNP give rise to decreased stability of one of the chains to cause thalassemia
-most common is a mutation that results in decreased synthesis of one chain
-α-thalassemias – complete deletion of the gene (4 copies on both chromosome 16s), if one
copy is deleted, nothing bad happens… If 2 copies are deleted, cells become microcytic,
hypochromic. If 3 copies are deleted, it causes severemicrocytic hypochromic anemia with
splenomegaly. 4 deletions are fatal
-β-thalassemias – insufficient B-globin synthesis can result from deletions, promoter mutations,
and splice-junction mutations. Heterozygotes where some B-globin is made (B+) or no B-globin
(B0) is asymptomatic but results in microcytic, hypochromic RBC with mild anemia
-B+/B+ have anemia of mild severity, B+/B0 are more severe, and B0/B0 is very severe
-Excess B-chain is more severe than excess alpha chain because homotetramer of Bchain (HbH) cannot deliver O2 and will precipitate over time to form inclusion bodies 
more likely to be trapped in spleen
Hereditary Persistence of Fetal Hemoglobin – fetal hemoglobin (HbF) is predominant in fetal period
consisting of 2 alpha and 2 gamma chains. Conversion between fetal and adult hemoglobin is called
hemoglobin switching.
-switching is not always 100%, and some people persist with some HbF
-Patients with B-thalassemia or sickle cell anemia have less severe conditions if they produce
some HbF
-Goal for research is to activate gamma chains when B chains are defective
1. Non-deletion forms of hereditary persistence of fetal hemoglobin (HPFH) are those that come from
SNPs in Agamma and Ggamma promoters  if B chain is defective or sickle cell exists, gamma chains
can ameliorate this condition
2. Deletion forms of HPFH have entire delta and beta genes deleted from one copy of chr. 11 and only
HbF can be produced.
-in some people, enough HbF is produced to remain normal, in others that don’t produce
enough, it forms δ0β0-thalassemia, difference is the site at which deletions occur in the B-globin
Hemoglobin Switching – embryonic megaloblasts (embryonic RBC) produced in yolk sac 15 days postfertilization, and after 6 weeks, erythropoiesis shifts to the liver  later the bone marrow
-Alpha globin is on chr 16
-Beta globin is on chromosome 11
-Fetal hemoglobin is designated α2Gγ2, and becomes α2Aγ2 if still present in adults