Hemoglobin: Structure and Function

Overview of Hemoglobin

Hemoglobin (Hb) is a protein found inside red blood cells (RBCs) that is essential for the transport of oxygen (O2) and carbon dioxide (CO2) throughout the body. Each RBC contains approximately 250 million hemoglobin molecules, accounting for about 97% of the cell's mass.

• Function: Hemoglobin binds and releases O2 and CO2 reversibly, allowing efficient gas exchange.

• Transport: Hemoglobin picks up O2 in the lungs and delivers it to tissues, while collecting CO2 from tissues to be expelled in the lungs.

Key Terms:

• Oxyhemoglobin: Hemoglobin bound to oxygen.

• Carbaminohemoglobin: Hemoglobin bound to carbon dioxide.

• Deoxyhemoglobin: Hemoglobin not bound to oxygen.

Example: If hemoglobin binds oxygen irreversibly, blood would circulate more slowly and tissues would receive less oxygen, impairing cellular respiration.

Structure of Hemoglobin

Subunit Composition and Heme Groups

Hemoglobin is a tetrameric protein composed of four subunits: two alpha (α) and two beta (β) chains. Each subunit contains a heme group with a central iron (Fe2+) atom that binds oxygen.

• Globin: The protein portion of hemoglobin, consisting of four polypeptide chains.

• Heme: The non-protein, iron-containing component that binds O2.

• Each hemoglobin molecule can bind up to four oxygen molecules, one per heme group.

• Hemoglobin can also bind CO2 at amino groups, not at the heme.

Hemoglobin Structure Diagram:

• Alpha (α) and Beta (β) chains are arranged to form the functional protein.

• Heme groups are embedded within each subunit.

Example: If each erythrocyte carries 250 million hemoglobin molecules, and each hemoglobin binds 4 oxygen molecules, then each erythrocyte can carry 1 billion oxygen molecules at a time.

Equation:

Clinical and Physiological Relevance

Adaptation to Altitude

At high altitudes, the partial pressure of oxygen is lower, resulting in less oxygen available for hemoglobin binding. The body adapts by increasing the number of erythrocytes (hematocrit) to improve oxygen transport.

• After returning from high altitude, a blood sample may show increased erythrocyte count per microliter.

• Each hemoglobin molecule still binds up to 4 oxygen molecules, but the total oxygen-carrying capacity increases due to more RBCs.

Example: Cynthia, after a trip to the Andes, would have a higher hematocrit (more erythrocytes per μL) compared to before her trip.

Anemia and Oxygen Transport

Anemia is a disorder characterized by insufficient oxygen-carrying capacity of the blood. Causes include iron deficiency, blood loss, and deformed erythrocytes. Excess erythrocytes are not a cause of anemia.

• Iron deficiency: Reduces hemoglobin synthesis.

• Blood loss: Decreases total RBC count.

• Deformed erythrocytes: Impair oxygen transport.

Example: Blood loss can lead to anemia by reducing the number of RBCs available to carry oxygen.

Tables

Hemoglobin Structure and Oxygen Binding

Causes of Anemia

Additional info:

• Volume ratio and surface area: Erythrocytes have a high surface area to volume ratio to maximize gas exchange efficiency.

• Each hemoglobin molecule requires 4 heme groups for synthesis.