Hemoglobin: Structure, Function, and Importance in the Human Body

What is Hemoglobin?

Hemoglobin (Hb) is a type of globular protein present in red blood cells (RBCs) that transports oxygen throughout the body. It is a tetrameric protein containing the heme prosthetic group attached to each subunit. Hemoglobin serves as a respiratory pigment, aiding in the transport of oxygen as oxyhemoglobin from the lungs to various tissues. Additionally, it transports a portion of carbon dioxide back to the lungs as carbaminohemoglobin.

Key Points:

Hemoglobin (Hb) is a respiratory pigment in RBCs.
It has a tetrameric structure with heme groups.
Transports oxygen and carbon dioxide.

Cooperativity

Oxygen binds reversibly to the heme group in a process called oxygenation. The interaction between the subunits in Hemoglobin is known as cooperativity. As Hemoglobin binds successive oxygen molecules, the oxygen affinity of the remaining subunits increases.

Hemoglobin Normal Range

The Hemoglobin level in blood is measured in grams per deciliter (g/dL). The normal range for a healthy individual is 12-20 g/dL. Typically, males have a higher Hemoglobin level (13.5-17.5 g/dL) compared to females (12-15.5 g/dL).

Where is Hemoglobin Found? Hemoglobin Location

Hemoglobin develops in the bone marrow cells and eventually becomes part of red blood cells. It occupies about one-third of the RBC volume and 90-95% of the dry weight of RBCs. Apart from RBCs, Hemoglobin is also found in other cells such as macrophages, certain neurons, alveolar cells, and mesangial cells in the kidneys.

Hemoglobin Structure

Max Perutz described the molecular structure of Hemoglobin in 1959. Hemoglobin is a tetrameric protein composed of two alpha (𝜶) and two beta (𝝱) polypeptide chains, each linked to a heme prosthetic group.

Structure Details:

𝜶 subunit: Made up of alpha polypeptide chains with 141 amino acid residues.
𝝱 subunit: Made up of beta polypeptide chains with 146 amino acid residues.
Heme group: Contains an iron atom at the centre of a porphyrin ring.

In infants, there are two alpha and two gamma chains, which get replaced by beta chains as they grow.

Hemoglobin exists in two conformations: R state (relaxed) and T state (tense). Oxygen has a higher affinity for the R state, while deoxyhemoglobin is primarily in the T state.

Hemoglobin Function

The primary function of Hemoglobin is to carry and transport oxygen to various tissues. The binding of oxygen to Hemoglobin is cooperative, meaning the binding of one oxygen molecule increases the affinity of the remaining subunits for oxygen.

Key Functions:

Transport of Oxygen: Hemoglobin binds oxygen in the lungs and releases it in tissues where the partial pressure of oxygen is low.
Transport of Carbon Dioxide: About 20-25% of CO2 is transported as carbaminohemoglobin. The rest is transported as bicarbonate ions facilitated by the enzyme carbonic anhydrase.
Transport of Nitric Oxide: Hemoglobin also transports nitric oxide, which is bound to thiol groups in the globin chains.

How Hemoglobin is Formed

Hemoglobin synthesis begins in proerythroblasts and continues into the reticulocyte phase of red blood cells. Iron is a crucial component of Hemoglobin.

Steps in Hemoglobin Formation:

Formation of Pyrrole: SuccinylCoA from the Krebs cycle binds with glycine to form a pyrrole molecule.
Formation of Protoporphyrin IX: Four pyrroles combine to form protoporphyrin IX, which then binds with iron to form a heme molecule.
Formation of Globin Chains: Each heme molecule associates with a globin chain (alpha, beta, gamma, delta).
Assembly of Hemoglobin Molecule: Four globin chains (two alpha and two beta in adults) come together to form the complete Hemoglobin molecule.

How Does Oxygen Bind to Hemoglobin?

Oxygen binds to the iron atom in the heme group of Hemoglobin. Each Hemoglobin molecule can bind four oxygen molecules. The binding is cooperative, meaning the binding of one oxygen molecule increases the affinity of the remaining sites for oxygen. This results in a sigmoid oxygen dissociation curve, which shows the relationship between partial pressure of oxygen (pO2) and oxygen saturation of Hemoglobin.

Types of Hemoglobin

Hemoglobin A (HbA): The most common form in adults, consisting of two alpha and two beta chains.
Hemoglobin A2 (HbA2): Comprises 2-3% of adult Hemoglobin, consisting of two alpha and two delta chains.
Hemoglobin F (HbF): Found in fetuses and newborns, consisting of two alpha and two gamma chains.

Diseases Related to Hemoglobin

Sickle Cell Anaemia: Caused by a point mutation in the beta globin gene, resulting in the substitution of valine for glutamic acid at the 6th position.
Thalassemia: Caused by reduced production of Hemoglobin. There are two types: alpha-thalassemia and beta-thalassemia, depending on which globin chain is affected.

Significance of Hemoglobin in Blood

Hemoglobin is crucial for oxygen transport and normal body function. It is also used as a diagnostic tool, with HbA1c levels indicating average blood glucose levels in diabetic patients.