Hemoglobin also spelled haemoglobin and abbreviated Hb, is the iron-containing oxygen-transport metalloprotein in the red blood cells of the blood in vertebrates and other animals. In mammals the protein makes up about 97% of the red cell’s dry content, and around 35% of the total content (including water). Hemoglobin transports oxygen from the lungs to the rest of the body, such as to the muscles, where it releases its load of oxygen. Hemoglobin also has a variety of other gas-transport and effect-modulation duties, which vary from species to species, and may be quite diverse in invertebrates.

The name Hemoglobin is the concatenation of heme and globin, reflecting the fact that each subunit of hemoglobin is a globular protein with an embedded heme (or haem) group; each heme group contains an iron atom, and this is responsible for the binding of oxygen. The most common type of Hemoglobin in mammals contains four such subunits, each with one heme group. In humans, each heme group is able to bind one oxygen molecule, and thus, one hemoglobin molecule can bind four oxygen molecules.

Mutations in the genes for the Hemoglobin protein in humans result in a group of hereditary diseases termed the hemoglobinopathies, the best known of which is sickle-cell disease. Historically in human medicine, sickle-cell disease was the first disease to be understood in its mechanism of dysfunction, completely down to the molecular level. However, not all such globin-gene mutations result in illness. These mutations are formally recognized as hemoglobin variants rather than diseases.CapeceCapece, L., Marti, M. A., , Crespo , A Doctorovich, F. & , Estrin E D. A. , Heme protein oxygen affinity regulation exerted by proximal effects. Journal of the American Chemical Society Society1 128, 12455 , 12455-12461(2006)

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