Hemoglobin: Gas Transport in the Body

Hemoglobin: Gas Transport in the Body

Hemoglobin is a vital protein found in red blood cells, playing a crucial role in gas transport within the body.

Function:

• It carries oxygen from the lungs to the tissues and transports carbon dioxide from the tissues back to the lungs.

• In 24 hours, blood can transport up to 600 liters of oxygen to tissues throughout the body.

Characteristics:

• Oxygen concentration in blood is 0.39 ml/100 ml.

• Oxygen concentration in arterial blood is 20 ml/100 ml.

• Diameter of hemoglobin is 5.5 nm.

• Molecular weight of hemoglobin is 64,000 daltons.

Structure:

Hemoglobin is composed of four subunits, each containing two parts:

• Heme (Prosthetic group): Consists of four protoporphyrin IX molecules at four corners and one iron atom (Fe) at the center.

• Protoporphyrin is made up of porphin with substituent groups at positions 1-8.

• Porphin consists of four pyrrole rings linked together by methylene bridges.

• Substituent groups of porphin to form porphyrin are:

• 2 propionic acid groups

• 2 vinyl groups

• 4 methyl groups

• Fe is attached to the center by the enzyme ferrochelatase.

• Globin (Protein): Consists of four polypeptide chains.

• Globin is a protein that transports gas within the cell.

• The four polypeptide chains are identical in pairs.

Hemoglobin Classification:

• HbF (Fetal Hemoglobin): Synthesized from alpha and gamma chains, present from zygote formation to the third month of pregnancy, gradually decreasing after birth.

• HbA (Adult Hemoglobin): Synthesized from alpha and beta chains, appearing from the sixth month of pregnancy and continuing after birth.

• Hb Gower I: Synthesized from zeta and epsilon chains, present during embryonic development.

• Hb Gower II: Synthesized from alpha and epsilon chains, present from the 8th week to the 3rd month of pregnancy.

Oxygen Binding:

• Hemoglobin binds to oxygen at position E7 (Histidine E7) on globin.

• Fe2+ of heme binds to globin at position F8 (Histidine F8).

• In the absence of oxygen, the Fe of heme is pulled towards the non-polar plane of globin.

• One hemoglobin molecule can carry four oxygen molecules.

Oxygen Transport Mechanism:

• Hemoglobin converts from the T form (low affinity for oxygen) to the R form (high affinity for oxygen) to easily pick up and release oxygen.

• This conversion occurs due to the rotation of two chains around an axis of 15 degrees.

Carbon Dioxide Transport:

• CO2 is transported through two pathways: direct and indirect.

• Direct transport (15-20%) occurs through the combination of CO2 with free amines.

• Indirect transport (80-85%) is catalyzed by the enzyme carbonic anhydrase.

Bohr Effect:

• The Bohr effect is formed due to the interaction of H+ and CO2, helping to regulate the oxygen binding affinity of hemoglobin.

BPG (2,3-Diphosphoglycerate):

• When altitude changes abruptly, the body synthesizes BPG.

• BPG binds to hemoglobin at the central position, stabilizing the T form of hemoglobin and regulating oxygen binding affinity.

Carbon Monoxide Poisoning:

• CO has an affinity for hemoglobin 250 times higher than O2, it can displace O2 from hemoglobin, causing carbon monoxide poisoning.

• Carbon monoxide poisoning has a high mortality rate (60%).

Met Hb:

• Met Hb is hemoglobin where Fe2+ is oxidized to Fe3+, lacking the ability to transport O2.

• Some medications like sulfonamides and cyanides can oxidize hemoglobin.

Hemoglobin-related Diseases:

• Thalassemia: Caused by gene mutations leading to a deficiency in one or more polypeptide chains of hemoglobin.

• Sickle Cell Anemia: Caused by a gene mutation that replaces glutamic acid at the 6th position of the beta chain with valine.

• Spherocytosis: Caused by a gene mutation that replaces glutamic acid with lysine at position 6.

Note:

• Hemoglobin also has enzyme properties, catalyzing redox reactions and having catalase activity.

Conclusion:

Hemoglobin is a crucial protein that plays a central role in gas transport within the body. The complex structure and function of hemoglobin enable it to effectively transport oxygen and remove CO2, ensuring the normal functioning of the body.