Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red cells of the blood in mammals and other animals. Hemoglobin transports oxygen from the lungs to the rest of the body, such as to the muscles, where it releases the oxygen load. 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 group. In adult humans, the most common haemoglobin is a tetramer (contains 4 subunit proteins) called haemoglobin A, consisting of two alpha and two beta subunits non-covalently bound. This is denoted as a2b2. The subunits are structurally similar and about the same size. The heme group consists of an iron atom held in a heterocyclic ring, known as a porphyrin. This iron atom is the site of oxygen binding. The iron atom binds equally to all four nitrogens in the center of the ring, which lie in one plane. Oxygen is then able to bind to the iron centre perpendicular to the plane of the porphyrin ring. Decreased levels of hemoglobin, with or without an absolute decrease of red blood cells, leads to symptoms of anemia. Anemia has many different causes, although iron deficiency and its resultant iron deficiency anemia are the most common causes in the Western world. Mutations in the gene for the haemoglobin protein result in a group of hereditary diseases termed the hemoglobinopathies, the most common members of which are sickle-cell disease and thalassaemia.
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Mauritius 
