C₂₉₅₂H₄₆₆₄N₈₁₂O₈₃₂S₈Fe₄ — Hemoglobin

Hemoglobin is an essential iron-containing metalloprotein present in the red blood cells of almost all vertebrates. Its primary function is to transport oxygen from the lungs to body tissues and carry carbon dioxide back to the lungs for exhalation. Each hemoglobin molecule is made up of four polypeptide chains — two alpha (α) and two beta (β) globin chains — each surrounding an iron-containing heme group.

The molecular formula of hemoglobin is approximately \(C_{2952}H_{4664}N_{812}O_{832}S_8Fe_4\). The four iron atoms present in the molecule are responsible for reversible oxygen binding. When oxygen binds to hemoglobin, it forms oxyhemoglobin (bright red), and when oxygen is released, it becomes deoxyhemoglobin (dark red). This oxygenation and deoxygenation process is vital for cellular respiration and energy production.

Hemoglobin has a quaternary structure composed of four subunits. Each subunit contains:

• A globin chain (protein part) — two α and two β chains in adult hemoglobin (HbA).
• A heme group (non-protein prosthetic group) — a porphyrin ring containing a central ferrous ion (Fe²⁺).

The heme group’s iron atom can form six coordination bonds: four with nitrogen atoms of the porphyrin ring, one with a nitrogen from histidine residue in the globin protein, and one with molecular oxygen (\(O_2\)).

The spatial arrangement of the four subunits allows cooperative binding — binding of oxygen to one heme group increases the affinity of the remaining heme sites for oxygen, a property known as cooperativity.

Hemoglobin’s composition allows it to serve as an oxygen carrier and a buffer that helps maintain blood pH. Its primary functions include:

• Oxygen Transport: In the lungs, oxygen binds reversibly to the Fe²⁺ ions of the heme groups, forming oxyhemoglobin. This oxygen is released in tissues with lower oxygen partial pressure.
• Carbon Dioxide Transport: Hemoglobin assists in carrying carbon dioxide from tissues to lungs, forming carbaminohemoglobin through reversible binding with amino groups.
• Buffering Role: Hemoglobin acts as a pH buffer by binding hydrogen ions (H⁺), preventing drastic pH changes in the blood (Bohr effect).

The overall oxygen transport reaction can be simplified as:

This reversible reaction ensures efficient oxygen uptake in the lungs and release in tissues.

• Hemoglobin A (HbA): The most common adult form, containing two α and two β chains.
• Hemoglobin A2 (HbA2): Contains two α and two δ chains, comprising about 2-3% of total hemoglobin in adults.
• Hemoglobin F (Fetal Hemoglobin): Composed of two α and two γ chains. It has a higher affinity for oxygen, facilitating transfer from maternal to fetal blood.
• Abnormal Hemoglobins: Variants like HbS (sickle cell hemoglobin) result from genetic mutations causing structural changes that affect oxygen transport efficiency.

• Physical State: Hemoglobin is a soluble globular protein in red blood cells.
• Color: Oxygenated hemoglobin appears bright red; deoxygenated form appears purplish-red.
• Solubility: Soluble in aqueous buffers but denatures under extreme temperatures or pH.
• Reactivity: Reacts with gases like CO (carbon monoxide), NO (nitric oxide), and CO₂.
• Affinity: The oxygen-binding affinity is influenced by factors such as pH, temperature, and CO₂ concentration (Bohr effect).

Carbon monoxide (CO) competes with oxygen for binding to the Fe²⁺ center in heme, forming carboxyhemoglobin, which can be toxic by preventing oxygen transport:

Even low concentrations of CO can severely impair oxygen delivery to tissues.

The Bohr effect describes how hemoglobin’s oxygen-binding affinity decreases under low pH or high CO₂ concentration, facilitating oxygen release in metabolically active tissues. The reaction can be summarized as:

The produced H⁺ ions bind to hemoglobin, promoting deoxygenation. Conversely, in the lungs, where CO₂ levels are low and pH is higher, oxygen binds more readily to hemoglobin.

Hemoglobin synthesis occurs in the bone marrow within immature red blood cells (erythroblasts). The process involves:

1. Formation of Heme: Synthesized from glycine and succinyl-CoA in mitochondria via several enzymatic steps, including the rate-limiting enzyme aminolevulinic acid synthase (ALAS).
2. Globin Chain Synthesis: Occurs in the cytoplasm on ribosomes, directed by genes on chromosomes 11 (β chain) and 16 (α chain).
3. Assembly: Heme and globin chains combine to form functional hemoglobin molecules before the red blood cell matures.

Normal adult hemoglobin concentration ranges between 13.5–17.5 g/dL for males and 12.0–15.5 g/dL for females. Abnormalities can lead to several disorders:

• Anemia: Caused by low hemoglobin levels or red blood cell count, resulting in reduced oxygen delivery.
• Sickle Cell Anemia: Mutation in the β-chain gene leads to HbS formation, causing red blood cells to become sickle-shaped.
• Thalassemia: Genetic disorders affecting globin chain synthesis, leading to ineffective erythropoiesis.
• Carbon Monoxide Poisoning: Occurs when CO binds to hemoglobin, forming carboxyhemoglobin and preventing oxygen transport.

Hemoglobin has applications in medical diagnostics, biotechnology, and biochemistry research:

• Used in hemoglobin-based oxygen carriers (HBOCs) as blood substitutes.
• Applied in spectroscopic studies for understanding protein-ligand interactions.
• Used in biosensors to detect gases like oxygen and carbon monoxide.
• Assists in forensic investigations through hemoglobin detection tests such as Kastle–Meyer and luminol reactions.