Hemoglobin (Hb)

**Hemoglobin** is an iron-containing metalloprotein found within red blood cells (*erythrocytes*) that serves as the principal carrier of oxygen from the lungs throughout the body’s tissues. Each molecule consists of four subunits—two alpha and two beta chains—each containing an iron-bound heme group capable of reversibly binding one molecule of oxygen, allowing each tetrameric protein complex to carry up to four molecules at once. This cooperative binding results in efficient loading/unloading depending on local oxygen tension—a property reflected by its sigmoidal dissociation curve[5][6]. In addition, red blood cells modulate vascular tone through mechanisms involving nitric oxide metabolism and ATP release under low oxygen conditions; these processes help match tissue perfusion with metabolic demand[1][4]. Defects affecting either quantity or structure/function lead directly to clinical syndromes such as anemia, sickle cell disease, thalassemias, and various forms of hypoxemia. While not generally targeted pharmacologically like classic receptors or enzymes, its central role makes it critical both physiologically and clinically—for example as a biomarker for anemia diagnosis/monitoring—and subject indirectly to drug effects/toxicities including carbon monoxide poisoning and methemoglobinemia formation.[1][2][3]

Other names

HaemoglobinOxyhemoglobin (when bound to oxygen)Deoxyhemoglobin (when unbound)HbA (adult hemoglobin)Fetal hemoglobin (HbF)

For drugs interacting with hemoglobin— - Competitive binding at heme iron site (e.g., CO displaces O2). - Oxidation of Fe2+ to Fe3+ by nitrites/nitrates leading to methemoglobinemia. - Induction of alternative globin gene expression by hydroxyurea.