Hemoglobin Synthesis and Catabolism 8/22

Question 1

Hemoglobin

Answer 1

• a large protein with two major components: heme and globin proteins
• Heme is comprised of a porphyrin ring structure with one iron (Fe) atom chelated in the center by 4 nitrogen atoms. This is the site of reversible oxygen attachment.
• Each hemoglobin molecule contains a heme group attached to each of the four globin chains (4 total heme/molecule) which is capable of carrying up to four molecules of oxygen at one time. (one on each heme)

Question 2

Hemoglobin Synthesis

Answer 2

• requires coordinated production of heme and globin
• heme is synthesized from mitochondria–>cytosol –>mitochondria
• two distinct globin chains (each with a heme molecule) combine to form hemoglobin: one is alpha, second is non-alpha

Question 3

Heme Synthesis

Answer 3

first step: formation of porphobolinogen

• succinyl CoA + glycine –> ALA (mitochondria) through *ALA synthase* (this is the RLS, which is inhibited by Heme end product)
• ALA (cytosol) –>Porphobolinogen (PBG) via *PBG Synthase*

Second Step: formation of heme from protoporyphryin ring and Fe

• 4 Porphobilinogen –> protoporphyrin ring
• Fe + Protoporphyrin –> Heme via *ferrocheletase*

Question 4

Problems in Heme Synthesis

Answer 4

• Pb poisoning: affects ALA dehydratase/PBG sythase and ferrochelatase: results in basophilic stipling (purple dots in RBCs) due to an accumulation inside of RBC’s
• Porphyria: mutations in ALA dehydrogenase or PBG deaminase: results in inability to go out in sun, because they are ultra sensitive
• Sideroblastic anemia: mutation at ALA Synthase: don’t have heme molecule to attache Fe to, thus Fe precipitates outside of erythrocyte resulting in “ringed sideroblast”
• Protoporphyria: mutation in ferrochelatase

Question 5

Globin Synthesis

Answer 5

• occurs in cytoplasm of normoblasts and immature erythrocytes
• alpha chains: made on chromosome 16
• beta chains: made on chromosome 11

–> disruption of chain balance = thalassemia

Question 6

Heme and Globin Get together…..

Answer 6

• Hemoglobin synthesis occurs in immature red blood cells in the bone marrow.
• Normal synthesis depends on:
  1. an adequate supply of Fe (transferred to the marrow in plasma via transferrin from sites of absorption/storage)
  2. normal heme (synthesized in mitochondria) and….
  3. normal globin synthesis (synthesized in the cytoplasmic ribosomes)
• Heme leaves the mitochondria and is joined to the globin chains in the cytoplasm.
• Normal adult hemoglobin (HbA) consists of four heme groups and four globin chains (two α and two β) twisted together so the heme groups are exposed on the outside of the molecule

Question 7

Variability of Hemoglobins

Answer 7

• This is due to the variability in globin chain:

Hemoglobin A (2α & 2β globin chains) - 97%
 Hemoglobin F (2α & 2γ globin chains) - 1% (fetal)
 Hemoglobin A2 (2α & 2δ globin chains) - 2%

Embryonic hemoglobin produced by fetus rapidly drops off during months 1 to three – composed of embryonic and zeta chains. After embryonic drops off, gamma chain is rapidly produced to make fetal Hemoglobin. At birth, a baby still has a large percent of fetal hemoglobin present – problem is that it grabs onto O2 and doesn’t let it go. From birth to six months HemF drops and B chains start being produced. No one know what causes this switch.

Question 8

2 Normal Types of Hemoglobin found in Blood:

Answer 8

1. Deoxyhemoglobin- reduced hemoglobin
2. Oxyhemoglobin- hemoglobin carrying oxygen
   - –>Can be measured via pulse oximetry

Question 9

Methemoglobin

Answer 9

• *Methemoglobin**- (usually <3% of total hgb) hemoglobin carrying oxidized (ferric) iron… loses its ability to carry oxygen & becomes non-functional
• If Fe2+ is oxidized to Fe3+, (can be due to oxidizing drugs such as nitrites or sulfonamides), methemoglobin is formed and is _incapable of binding oxygen. _
• The erythrocyte has a protective enzyme, methemoglobin reductase, which converts methemoglobin back to hemoglobin.

Question 10

Sulfhemoglobin

Answer 10

Sulfhemoglobin- (Usually not present in body – quite abnormal) oxidized, partially denatured hemoglobin which may result in RBC destruction & hemolysis. Usually due to sulfur-containing drugs (sulfonamides) or aromatic amine drugs (phenacetin, etc.). Cannot carry O2

Question 11

Carboxyhemoglobin

Answer 11

Carboxyhemoglobin- (Usually <3% of total hemoglobin)hemoglobin carrying CO produced during heme degradation to bilirubin. CO is eliminated via respiration. Can also be formed due to CO poisoning.

Question 12

What are the six coordination bonds of ferrous iron?

Answer 12

four attach to heme
one attaches to the globin chain and…
one reversibly binds oxygen

Question 13

Where does non-heme iron come from? What is the process for absorption of iron from your food? (5 steps)

Answer 13

1. Ferric Reductase (Dcytb) - Dietary non-heme iron (Fe3+ ) must be reduced for transport across the apical brush border. Dcytb reduces Fe3–> Fe2+ at apical membrane.
2. DMT1 cotransports (the dietary heme iron) Fe2+ and H+ into the cells
3. Mobilferrin (Ferroportin): Fe2+ moves into the cell and it binds to ferroportin at basolateral membrane and it is then transported into the blood
4. Expression of ferroportin is regulated by Hepcidin: if Hepcidin attaches to ferroportin it causes disentegration and no iron absorption occurs (if have high iron in body, hepcidin cuts off ferroportin = regulation point)
5. After Fe+2 exits the cell, it is converted back to Fe+3 and binds to transferrin for transport to all body tissues

Question 14

Hemochromatosis

Answer 14

= overload of iron in body because of problem with hepcidin - Ferroportin levels are not monitored thus people have too much iron in blood. Patients are given therapeutic flebotamies to cure this.

Question 15

Iron Metabolism of Heme Iron

Answer 15

This is due to the breakdown of myoglobin (meats) and hemoglobin (RBC’s)

1. Heme is absorbed by duodenal epithelial cells via binding or endoscytosis.
2. Inside cells, heme oxygenase splits heme iron and releases free Fe3+ (this allows iron to enter the same pool as non-heme iron).
3. Enterocytes convert Fe3+ to Fe2+ and iron is handled the same way as nonheme iron, it is exported through ferroportin where it is oxidized in the blood to Fe3+ for incorporation into serum transferrin.

Question 16

Where is Iron stored in the body?

Answer 16

• circulating Fe is primarily deposited in the liver and the reticuloendothelia system (RES)(spleen) ; by binding to protein apoferritin to form ferritin (the storage form of iron)

Apoferritin has been called the “iron buffer system” because it can take up excess circulating iron for storage or release iron when circulating levels are too low… therefore maintaining a constant serum iron level.

Question 17

Apofferitin

Answer 17

Apoferritin binds to free ferrous iron and stores it in the ferric state.

Apoferritin takes up excess circulating iron from transferrin for storage or release iron when circulating levels are too low.

Question 18

Ferritin

Answer 18

• Ferritin serves to store iron in a non-toxic form, to deposit it in a safe form, and to transport it to areas where it is required.
• Ferritin is a ubiquitous intracellular protein that stores iron and releases it in a controlled fashion. The amount of ferritin stored reflects the amount of iron stored.
• anemia: If the ferritin level is low, there is a risk for lack of iron, which could lead to anemia. Low serum ferritin is the most specific lab test for iron deficiency anemia.
• hemochromatosis: If ferritin is high, there is iron in excess or else there is an acute inflammatory reaction in which ferritin is mobilized without iron excess.

Question 19

What organelles do RBC’s have?

Answer 19

RBC’s have no nucleus, mitochondria or ER, no ribosomes.

Question 20

How do RBC’s get energy?

Answer 20

Don’t have mitochondria, thus must utilized two inefficient pathways;

1. Glycolysis: producing ATP and NADH (re-oxidation of hemoglobin)
2. Pentose Phosphate Cycle (PPC): supplies NADPH to maintain reduced state of glutathionine and sulfhydryl groups which is important for RBC cellular integrity.

Question 21

How are RBC’s senesced?

Answer 21

Red cells lose size (20 %) and become more rigid (spherical) as they “age”.

At ~ 120 days, “senescent “ red cells are removed from circulation in the **spleen **

Red cells are removed via phagocytosis by specific macrophages lining the spleen sinusoids (red pulp).

Red cells are completely degraded; the iron of the 4 heme groups within each hemoglobin molecule is completely recycled; the remainder of the heme moiety (protoporphyrin) becomes bilirubin.

Question 22

Polycythemia

Answer 22

if rate of RBC synthesis >destruction

RBC count will rise above normal range

Question 23

Anemia

Answer 23

rate of RBC destruction > synthesis

RBC count will drop below normal range

Question 24

Hemoglobinemia and Hemoglobinuria

Answer 24

• release of hemoglobin in plasma and release of free hemoglobin in urine.
• excess levels of free hemoglobin is nephrotoxic
• this can be the result of intravascular hemolysis of RBCs due to: hemolytic anemia, autoimmune processes, transfusion reaction or drug-induced hemolysis

Question 25

What are the three components of hemoglobin degredation?

Answer 25

After removal of senescent red cells from the circulation, hemoglobin is degraded within macrophages of the reticuloendothelial (RES) system into its 3 components:

1. Fe: goes into storage for reutilization (recycling)
2. Protoporphyrin: split and converted to bilirubin, excreted from body
3. Globin: degraded and returned to amino acid pool for possible reutilization

Question 26

heme oxygenase (oxidase)

Answer 26

an enzyme in macrophage that breaks down porphyrin ring –> CO (forms carboxyhemoglobin) + billiverdin

• Billiverdin –> billirubin –> billirubin attached to plasma albumin and sent to liver
• billirubin is removed from the circulation by liver hepatocytes and conjugated for excretion in bile

Question 27

Sickle Cell Anemia

Answer 27

• Due to missense mutation of B globin chain: amino acid substitution at position 6: glutamic acid –> valine
• results in Hgb S, which replaces HgbF at six months
• causes Hgb to be susceptible to polymerization at low O2 concentrations or with dehydration
• Reduces flexibility of RBC membrane
• “sickling” of RBC’s results in hemolytic episodes and jaundice due to hemolysis
• results in hypoxic tissue injury from microvascular occlusions = “crises”

(SC trait: A/S - 8% african americans ; SC disease: S/S - .2% of African Americans)

Question 28

Thalassemias

Answer 28

• a quantitative (synthesis) defect in resulting in inherited defects in regulation of hemoglobin A globin chain synthesis.
• normal globin structure, but produced in altered amounts: usually underproduced
• results in micocytic hypochomic anemia (small, lightly colored, decreased RBC amounts) with prominent target on cells
• Mean corpuscular volume (MCV) and Mean corpuscular hemoglobin (MCH) are decreased
• hundreds of difft. mutations on alpha (16) and beta (11) globin chains –> reduced or absent output of alpha and beta chains
• linked with past/present hetrozygote resistance to malaria

Question 29

Alpha Thalassemia (Hemoglobin S Disease and Thalassemia major)

Answer 29

• due to imbalanced rate ofproduction of alpha globin chain –> excess beta or gamma chains.
• Hemoglobin H disease: loss of 3 of 4 alpha globin genes in adult –> results in formation of hemoglobin H (with beta tetramers)
• Thalassemia Major: loss of all 4 alpha globin genes in utero–> results in hemoglobin Barts (gamma tetramers) and results in “hydrops fatalis”. baby dies of anoxia and has hepatosplenomegally: spleen and liver are overworked in trying to remove messed up RBC’s.

Question 30

Beta Thalassemia

Answer 30

• impaired production of betal globin chains, and excess of alpha chains.
• elevated Hgb. F (80-100%)–> results in hypoxia and production of excess RBC’s. All marrow starts producing RBC’s –> expansion of marrow space and bones are more prominent (esp. forehead) in order to create more RBC’s.

“Cooley’s Anemia” = B thalasemia major. homozygous disorder. requires lifelong RBC transfucsions. “hair-on-end” appearance at frontal region of lobe.

• seen mostly in mediterranean, italian and southeast populations