Hemoglobin: structure, functions and Binding site of Oxygen with Hemoglobin and MCQ for Exams

Hemoglobin is a conjugated protein with prosthetic group heme. Hb is found in the RBCs and carries oxygen from lungs to the tissues and the reverse process occurs when Hb carries CO2 from tissues to the lungs. The red color of the RBCs is due to the presence of hemoglobin content in the RBCs

Structure of hemoglobin

Hemoglobin is a globular oligomeric protein made up of two parts: the protein part known as globin and the non-protein pigment heme.

Heme

Heme is a compound which belongs to the class of compounds known as protoporphyrin. It is an iron containing compound. Protoporphyrin is composed of four pyrrol rings which are linked with methane and forms tetra-pyrrol ring (porphyrin). Various types of side chain groups are also attached to the pyrrol ring and can be arranged in 15 different ways; but only one isomer is biologically active called protoprphyrin-9.

The iron ion is held in the center of the protoporphyrin molecule by coordination bonds with four nitrogen of protoporphyrin ring. The iron ion has 6 coordinated bonds.

Four bonds between iron and nitrogen atom
Fifth bond is formed between nitrogen atom of histidine residue of globin polypeptide chain
Sixth bond is formed with oxygen

The oxygenated form of hemoglobin is held together by hydrogen bonds between oxygen and side chain of another histidine residue.

Globin

Globin belongs to the class of proteins known as globulins. Globin molecule contains 4 polypeptide chains: two alpha chain and two beta chains. With each polypeptide chain, each heme is attached a hemoglobin molecule thus have four heme molecule. The four polypeptide chains and four heme molecules are held together in a fixed arrangement to form a quaternary structure of hemoglobin. The structure is stabilized by:

Hydrogen bonds
Salt bridges
Van der walls forces

Functions of globin

It forms a protective hydrophobic pocket and prevents the conversion of ferrous form to ferric form and also permits reversible binding of oxygen with ferrous ion of heme.

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Functions of hemoglobin

Transport of oxygen from lungs to tissues
Transport of CO2 from tissues to the lungs
Acts as intracellular buffer and is thus involved in acid-base balance.

Binding site for oxygen, hydrogen and carbon-dioxide

Oxygen is bound to the ferrous atoms of heme to form oxyhemoglobin
Hydrogen is bound to the R-group of histidine residue in alpha and beta chains
Carbon-dioxide is bound by the alpha-amino group of N-terminal of each polypeptide chain of hemoglobin to form carb-amino hemoglobin.

Multiple Choice Questions (MCQs)

1. What prosthetic group is present in the hemoglobin?
A. Heme
B. Globin
C. Phosphoric acid
D. Lipoprotein

2. What is the reason of the red color of RBCs?
A. Presence of CO2
B. Presence of O2
C. It is present in blood
D. Both B and C

3. Fill in the blanks- Heme is an —– containing compound?
A. Al
B. Fe
C. Ar
D. None of the above

4. Which isomer of protoporphyrin is biologically active?
A. Protoprphyrin-2
B. Protoprphyrin-3
C. Protoprphyrin-6
D. Protoprphyrin-9

5. The oxygenated form of hemoglobin is together by which type of bonding?
A. H2-Bonding
B. Covalent bond
C. Metallic bond
D. Salt bridges

6. Globin belongs to which class of proteins?
A. Albulins
B. Globulins
C. Fibrinogen
D. All of the above

7. Hemoglobin is held together by which type of bond?
A. H2-bonding
B. Salt bridges
C. Van der walls forces
D. All of the above

8. Match the following-
A. Hemoglobin 1. Non-protein pigment
B. Heme 2. Contains 4 polypeptide chains
C. Globin 3. Globular oligomeric protein

9. What is the function of globin?
A. Forms protective hydrophilic pocket
B. Prevent conversion of ferrous to ferric
C. Prevent conversion of ferric to ferrous
D. Both A and B

10. What combination of polypeptide chain is present in globin?
A. 4 Alpha chain
B. 4 Beta Chain
C. 2 alpha and 2 beta chains
D. 2 alpha, 1 beta and 1 gamma chains

11. What forms carbamino hemoglobin?
A. Bond between CO2 and alpha-amino group
B. Bond between CO2 and heme
C. Both
D. None

12. Which of the following statement is NOT true?
A. Hemoglobin act as intracellular buffer
B. The proximal and distal histidine residue lies on the same side of heme ring
C. Exposure of heme ring to water results in oxidation
D. Protoporphyrin-9 is a tetrapyrrol ring

13. How many coordinated bonds are formed between iron and nitrogen?
A. 2
B. 3
C. 5
D. None of the above

14. What is the composition of protoporphyrin?
A. 4 pyrrol rink with methane
B. 5 pyrrol rink with methane
C. 4 pyrrol rink with ethane
D. 3 pyrrol rink with ethane

15. Hemoglobin has which structure of protein?
A. Primary structure
B. Secondary structure
C. Tertiary structure
D. Quaternary structure

ANSWERS-

1. Heme
2. Presence of O2
3. Fe
4. Protoprphyrin-9
5. H2-bond
6. Globulins
7. All of the above
8. A – 3 B – 1 C – 2
9. Prevents conversion of ferrous to ferric
10. 2 alpha and 2 beta chains
11. Bond between CO2 and alpha-amino group
12. The proximal and distal histidine residue lies on the same side of heme ring
13. None of the above
14. 4 pyrrol rings linked with methane
15. Quaternary structure

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REFERENCE:– 1. Pankaja Naik- Biochemistry; 4^th edition; page no:-112-115