What is Hemoglobin ?

Hemoglobin (/ˈhiːmɵˌɡloʊbɨn/); also spelled haemoglobin and abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates (with the exception of the fish family Channichthyidae ) as well as the tissues of some invertebrates. Hemoglobin in the blood carries oxygen from the respiratory organs (lungs or gills) to the rest of the body (i.e. the tissues). There it releases the oxygen to permit aerobic respiration to provide energy to power the functions of the organism in the process called metabolism.

In mammals, the protein makes up about 96% of the red blood cells' dry content (by weight), and around 35% of the total content (including water).Hemoglobin has an oxygen-binding capacity of 1.34 mL O2 per gram,which increases the total blood oxygen capacity seventy-fold compared to dissolved oxygen in blood. The mammalian hemoglobin molecule can bind (carry) up to four oxygen molecules.

Hemoglobin is involved in the transport of other gases: It carries some of the body's respiratory carbon dioxide (about 10% of the total) as carbaminohemoglobin, in which CO2 is bound to the globin protein. The molecule also carries the important regulatory molecule nitric oxide bound to a globin protein thiol group, releasing it at the same time as oxygen.

Hemoglobin is also found outside red blood cells and their progenitor lines. Other cells that contain hemoglobin include the A9 dopaminergic neurons in the substantia nigra, macrophages, alveolar cells, and mesangial cells in the kidney. In these tissues, hemoglobin has a non-oxygen-carrying function as an antioxidant and a regulator of iron metabolism.

Hemoglobin and hemoglobin-like molecules are also found in many invertebrates, fungi, and plants. In these organisms, hemoglobins may carry oxygen, or they may act to transport and regulate other things such as carbon dioxide, nitric oxide, hydrogen sulfide and sulfide. A variant of the molecule, called leghemoglobin, is used to scavenge oxygen away from anaerobic systems, such as the nitrogen-fixing nodules of leguminous plants, before the oxygen can poison the system.

Max Perutz, one of the founding fathers of molecular biology
In 1825 J.F. Engelhard[9] discovered that the ratio of Fe to protein is identical in the hemoglobins of several species. From the known atomic mass of iron he calculated the molecular mass of hemoglobin to n × 16000 (n = number of irons per hemoglobin, now known to be 4), the first determination of a protein's molecular mass. This "hasty conclusion" drew a lot of ridicule at the time from scientists who could not believe that any molecule could be that big. Gilbert Smithson Adair confirmed Engelhard's results in 1925 by measuring the osmotic pressure of hemoglobin solutions.

The oxygen-carrying protein hemoglobin was discovered by Hünefeld in 1840. In 1851, German physiologist Otto Funke published a series of articles in which he described growing hemoglobin crystals by successively diluting red blood cells with a solvent such as pure water, alcohol or ether, followed by slow evaporation of the solvent from the resulting protein solution. Hemoglobin's reversible oxygenation was described a few years later by Felix Hoppe-Seyler.

In 1959, Max Perutz determined the molecular structure of myoglobin (similar to hemoglobin) by X-ray crystallography.This work resulted in his sharing with John Kendrew the 1962 Nobel Prize in Chemistry.