41. Hemoglobin degradation – tissue localization. Heme oxygenase. Indirect and direct bilirubin. Bile pigments. Transportation of bile pigments in the body – enterohepatic circulation of bile pigments. Types of jaundice.

Introduction to Hemoglobin

• Definition: Hemoglobin (Hb) is a protein found in red blood cells (erythrocytes) that is responsible for transporting oxygen from the lungs to tissues throughout the body.
• Structure: Hemoglobin is a tetrameric protein composed of four globin chains, each bound to a heme group.
• Function: The primary function of hemoglobin is to bind and carry oxygen from the lungs to tissues and organs, where oxygen is released to support cellular respiration. Additionally, hemoglobin plays a role in transporting carbon dioxide from tissues back to the lungs for exhalation.
• Oxygen Binding: Each heme group within hemoglobin contains an iron (Fe^2+) ion that can bind to oxygen molecules reversibly. When oxygen binds to one subunit of hemoglobin, it induces a conformational change that increases the affinity of the remaining subunits for oxygen, facilitating cooperative binding.
• Carbon Dioxide Transport: Hemoglobin also transports carbon dioxide, primarily in the form of bicarbonate ions (HCO3^-). Carbon dioxide produced by tissues diffuses into red blood cells, where it combines with water to form carbonic acid (H2CO3). Carbonic acid dissociates into bicarbonate ions and protons. Bicarbonate ions are then transported out of the red blood cells into plasma in exchange for chloride ions through the chloride shift mechanism, while protons bind to hemoglobin.
• Regulation: The affinity of hemoglobin for oxygen is regulated by factors such as pH, temperature, and the partial pressure of oxygen (PO2). This ensures that oxygen is released from hemoglobin in tissues with high metabolic demands.