Hemoglobiini

Hemoglobiini, also called hemoglobin, is a protein found in the red blood cells of vertebrates. It binds oxygen in the lungs or gills and releases it in tissues, while also carrying carbon dioxide from tissues back to the lungs for exhalation. This reversible binding enables cellular respiration and energy production.

Structurally, hemoglobiini is a tetramer formed from globin chains. In humans the most abundant form in adults is HbA, composed of two alpha and two beta chains. Each chain surrounds a heme prosthetic group containing iron in the ferrous (Fe2+) state, which binds one molecule of oxygen. The four heme groups cooperate, producing cooperative binding and a sigmoidal oxygen dissociation curve. The affinity for oxygen is modulated by pH (the Bohr effect), carbon dioxide, temperature, and 2,3-bisphosphoglycerate, which stabilizes the deoxygenated form and facilitates oxygen release in tissues.

Different hemoglobins exist, including HbA (adult), HbA2 (alpha2 delta2) and HbF (alpha2 gamma2, fetal). HbF has a higher oxygen affinity, which aids fetal oxygen uptake from maternal blood. After birth, HbF typically declines. 2,3-BPG in red blood cells reduces oxygen affinity of HbA to promote oxygen delivery to tissues.

Hemoglobini synthesis occurs during erythropoiesis in the bone marrow, with iron and heme synthesis taking place in mitochondria and the cytosol of developing erythroblasts. Mature hemoglobini is contained within circulating red blood cells, which have a lifespan of about 120 days and are removed by the spleen when aged. Abnormal variants, such as sickle hemoglobin (HbS) or various thalassemias, can cause disease, while methemoglobinemia can impair oxygen transport. Hemoglobini concentration is routinely measured in blood tests to assess anemia or polycythemia.