L2: Hemoglobin structure and degradation

Question 1

What is the definition of hemoglobin?

Answer 1

Tetrameric protein molecule found exclusively in the cytoplasm of Red blood cells (RBCS).

Question 2

What is the main function of hemoglobin?

Answer 2

Is to transport oxygen from lungs to the tissues & carbon dioxide and H protons from tissues to lung “To be exerted, so it maintains acid base balance”

Question 3

What is the chemical nature of hemoglobin?

Answer 3

 Conjugated protein: formed of protein part: globin & non- protein part: heme group “prosthetic group”

 Hemoprotein: contains heme as a tightly bound prosthetic group

 Chromoprotein: has a red colour in vertebrates as it contains a pigmented prosthetic heme group (or cofactor), which is the iron containing molecule that makes oxygenated blood appear red

Question 4

What is the chemical structure of hemoglobin?

Answer 4

• Hb is a tetramer consisting of 2 parts:
   Heme: 4 heme molecules
   Globin: 4 polypeptide chains

Question 5

What is the chemical structure of heme?

Answer 5

 Heme is a complex of tetrapyrrol ring called protoporphyrin IX & ferrous iron (Fe2+)

 The iron is held in the center of the heme molecule by bonds to the four nitrogens of the porphyrin ring.

 The Heme Fe2+ can form two additional bonds, one on each side of the porphyrin ring:
✓ One of these positions is coordinated to the side chain of a histidine amino acid of the globin molecule, Whereas the other position is available to bind oxygen

Question 6

What is globin?

Answer 6

Globin is a protein rich in histidine amino acid, with a quaternary structure (formed of 4 polypeptide chains)

Question 7

What does each chain of globin contain?

Answer 7

Each chain (subunit) has stretches of α-helical structure and a hydrophobic heme- binding pocket.

Question 8

What are the types of globin chains?

Answer 8

There are four types of globin chains: α, β, γ and δ (According to sequence of amino acids in the primary structure of each chain)

Question 9

What is the type of bonds between the chains of hemoglobin and what is the number of amino acids in each chain?

Answer 9

non-covalent bonds:
✓ α -chains contain 141 amino acids.
✓ β -chains contain 146 amino acids.

Question 10

What are the types of hemoglobin classified according to?

Answer 10

the primary structure of the globin polypeptide chains.

Question 11

What are the types of hemoglobin?

Answer 11

Hb A or HbA1 (Adult Hb)

Hemoglobin A2 (HbA2)

Fetal hemoglobin (Hb F)

Hemoglobin A1c (HBA1c) “doesn’t affect the function”

Question 12

What does HBA1 represent and what is composed of?

Answer 12

 Is the major form of normal Hb in adults represents about 90-95% of total Hb. “ 6 months after birth”

 It is composed of 2 α and 2 β chains.

Question 13

What does HBA2 represent and what is it composed of?

Answer 13

 Minor adult Hb, comprised 2-3% of normal adult Hb.

 Composed of 2 α and 2 δ chains

Question 14

What does HBF represent what is it composed of?

Answer 14

 Is the main Hb during fetal life and in newborns then disappear gradually where it is replaced by Hb A shortly after birth.

 It is composed of 2α and 2 γ chains.

Question 15

What does HBA1C represent and what is it composed of?

Answer 15

• Normally, it is present in conc. of 3 - 5 % of total Hb. However, in patients with D.M. it may be increased to as much as 6 -15 % of total Hb.
• Formed spontaneously by nonenzymatic reaction of hemoglobin A with Glucose (HBA1c has a glucose residues attached to β-globin chains in RBCs hemoglobin by non-enzymatic reaction).

Question 16

What does a greater affinity to O2 HBA or HBF?

Answer 16

• Hb F has greater affinity for O2 than HbA so ensure O2 transfer from maternal circulation to fetus RBCs through placenta. “May cause problems” “less than 1% in adults”

Question 17

What does the extent of glycosylation depend on in HBA1C?

Answer 17

the plasma conc. of glucose.

Question 18

What is HBA1c used for?

Answer 18

HbA1c could be used as a monitor for the control of the blood glucose level during the last 2 months for diabetic patients. “ HB lifetime is two months “

Question 19

What are hemoglobinopathies?

Answer 19

• They are members of a family of genetic disorders caused by genetic mutations in the genes producing globin protein of hemoglobin

Question 20

What are hemoglobinopathies characterized by?

Answer 20

• Production of a structurally abnormal hemoglobin molecule (Qualitative hemoglobinopathies): eg: HbS & HbM.
• Synthesis of insufficient quantities of normal hemoglobin (Quantitative hemoglobinopathies): eg: Thalasaemias.

Question 21

What is sickle cell anemia and what causes it (HB S disease)?

Answer 21

• Genetic disorder caused by mutation in 6th codon of the β-globin “In globin of hemoglobin” gene where glutamic acid (GAG) is replaced by valine (GTG).

Question 22

What is a pathogenesis of sickle cell anemia?

Answer 22

• Valine residues aggregate together by hydrophobic interactions leading to precipitation of abnormal Hb within RBCs (Hb S).

Question 23

What is the result of sickle cell anemia?

Answer 23

• RBCs which are sickle-shaped lead to fragility of their walls and high rate of hemolysis
• Such sickled cells frequently block flow of blood in narrow capillaries and block blood supply to tissue (tissue anoxia) causing pain and cell death.

Question 24

What is the definition of thalassemias?

Answer 24

A group of genetic diseases in which synthesis of either α- or β- globin chain is defective.

Question 25

What are the causes of thalassemias?

Answer 25

This due to defect or absence of one or more of genes responsible for synthesis of α or β globin chains of Hb molecule leading to malformed RBC.

“Not just a mutation in the 6th codon of the gene responsible for the synthesis of beta globin chain”

Question 26

What are the types of thalassemias?

Answer 26

• α-thalassemia: in which synthesis of α globin chain is defective or absent
• β -thalassemia: When synthesis of β globin chain is decreased or absent.“May cause death”

Question 27

What are the sites of hemoglobin degradation and when does it happen?

Answer 27

• In reticuloendothelial system: liver, spleen &bone marrow
• After approximately 120 days in the circulation: RBCs taken up by RES especially spleen and liver where HB become degraded.

Question 28

What are the steps of hemoglobin degradation?

Answer 28

1) Formation of Bilirubin
2) Uptake of bilirubin by liver
3) Conjugation of bilirubin (Formation of bilirubin diglucuronide)
4) Excretion of bilirubin into bile

Question 29

What happens when RBCs end their lifespan?

Answer 29

• The globin is degraded to amino acids (which are reutilized in the body)
• Heme releases iron (which is returned to the body’s iron stores), and the tetrapyrrole component is converted to bilirubin, which is mainly excreted into the bowel via the bile.

Question 30

Step one: formation of bilirubin

Answer 30

• Heme under the effect of heme oxygenase “in macrophages” in presence of NADPH+H become converted to Biliverdin, and iron become oxidized into ferric and released.
• Biliverdin is then reduced via biliverdin reductase enzyme and become converted into bilirubin.

“Heme to bilivirdin”
“Ferrous to ferric”

Question 31

Step two: uptake of bilirubin by liver

Answer 31

• Bilirubin is hydrophobic, so, it is transported in blood by albumin [ (unconjugated) (Indirect bilirubin (Hemobilirubin)]
• Then bilirubin uptake by the hepatocytes takes place where it dissociated from albumin.

Question 32

Step three: conjugation of bilirubin (formation of bilirubin diglucuronide)

Answer 32

• Conjugation of bilirubin to 2 molecules of glucuronic acids occurs in hepatocytes to increase its solubility.

✓ This catalyzed by bilirubin UDP glucuronyltransferase enzyme.

• The product is bilirubin diglucuronide (conjugated bilirubin) (direct bilirubin) (cholebilirubin).

Question 33

What is other name for unconjugated bilirubin what is the other name for conjugated bilirubin?

Answer 33

Unconjugated bilirubin is referred to as indirect bilirubin, while conjugated bilirubin is referred to as direct bilirubin

Question 34

Step 4: excretion of bilirubin in to bile

Answer 34

• Conjugated Bilirubin passes with the bile to the large intestine where glucuronic acid is removed leaving bilirubin. “No need anymore”
• It is converted into stercobilinogen and urobilinogen by intestinal bacteria.

✓ Then stercobilinogen is then oxidized to stercobilin which give feces it characteristic brown color

• Some urobilinogen is reabsorbed into blood via portal circulation to the liver and then re-excreted into bile (enterohepatic urobilinogen cycle).
• The remaining urobilinogen is transported via blood to the kidney

✓ Converted to yellow urobilin and excreted through urine.

✓ This gives urine its characteristic color

Question 35

What does the increase in the level of blood bilirubin (hyperbilirubinemia) indicate?

Answer 35

Increase in the level of the blood bilirubin (hyperbilirubinemia) is a case of Jaundice (yellow discoloration of skin, conjunctiva and mucous membrane)