Hb

Hemoglobin (Hb) is an iron-containing metalloprotein in red blood cells that carries oxygen from the lungs to tissues and returns carbon dioxide for exhalation. In adults Hb primarily exists as HbA, composed of two alpha and two beta globin chains; each chain binds a heme group with iron that reversibly binds O2.

Hb is a tetramer with heme pockets. Binding of oxygen to one heme increases affinity at other hemes (cooperativity) via conformational shifts between the deoxy (T) and oxy (R) states. 2,3-bisphosphoglycerate reduces oxygen affinity in tissues; the Bohr effect lowers Hb O2 affinity with higher CO2 or lower pH.

Besides HbA1, adults have HbA2 (alpha2 delta2) and fetal Hb (HbF, alpha2 gamma2) which declines after birth. HbS (beta6 Glu→Val) causes sickle cell disease; other variants can affect oxygen binding or stability. HbA1c is a diagnostic measure of long-term glucose control in diabetes (glycated Hb).

Hb is produced in erythroid precursors in the bone marrow; mature red blood cells lack nuclei and synthesize no new Hb. The average lifespan is about 120 days, with senescent cells cleared by the spleen.

Disorders associated with Hb include iron-deficiency anemia, thalassemias (alpha or beta chain defects), and hemoglobinopathies such as sickle cell disease; methemoglobinemia involves iron in the ferric state, impairing O2 binding. Diagnosis relies on complete blood count, hemoglobin concentration, electrophoresis, high-performance liquid chromatography, and co-oximetry.

The structure of Hb was elucidated in the mid-20th century, a milestone recognized with the Nobel Prize awarded in 1962 to Max Perutz and John Kendrew for protein structures, including hemoglobin.