Hemoglobin

See also: Hemoglobin (homonymy)

The hemoglobin is a multimeric Protéine consisted of four Monomère S of Globuline S and four molecules of Hème, and whose principal function is the transport of the Oxygène. Hemoglobin is found primarily inside the red globules of the Sang of the Mammifère S and in that of other animals. It is an essential intermediary of the Respiration in the warm-blooded animals. The name of hemoglobin comes from the concatenation from heme and globine which is the generic term for globular proteins. One symbolizes it by " Hb". A heme molecule consists of an ion Fer complexed by a Porphyrine.

Structure

One finds in the middle of the Molécule a heterogeneous Cycle porphyritic, the heme, which contains a Ion iron. This ion iron is the site of fixing of oxygen. Since a sub-unit of hemoglobin is formed by a heme enclosed in a globular protein the name is very logical. There are many proteins containing a heme, but hemoglobin is most known. The element iron can exist in 2 forms in the molecule:

  • in ferrous form, ion Fe2+: only form activates human hemoglobin. One speaks about oxyhaemoglobine.
  • in ferric form, ion Fe3+: one speaks about Méthémoglobine.

One can speak about the capacity oxyphoric of hemoglobin. This corresponds to the quantity of dioxygene which it can fix. This value is of 1,34 ml of O2/g d' Hb.

Human hemoglobin

At the man, hemoglobin is a tetramer, made up of two globines α and two globines β bound by weak connections. The sub-units are structurally similar and have about the same size. Each globine has a molecular mass of approximately: 16000 Daltons, for a total mass of approximately: 64000 Daltons. Each globine of hemoglobin contains a heme, in such a way that the full capacity of connection of hemoglobin for dioxygene is of four molecules.

Stages of reaction:

  • Hb + O2 ↔ HbO2
  • HbO2 + O2 ↔ Hb (O2) 2
  • Hb (O2) 2 + O2 ↔ Hb (O2) 3
  • Hb (O2) 3 + O2 ↔ Hb (O2) 4

Summary of the reaction:

  • Hb + 4O2 → Hb (O2) 4

A structure of the shape désoxy of human hemoglobin is given by the PDB 1A3N.

In practice, this is not observed:

  • Hb + 4O2 ↔ Hb (O2) 4

Indeed, under the physiological conditions, two sub-units are always oxygenated by molecule of hemoglobin. One can thus write:

  • Hb (O2) 2 + 2O2 ↔ Hb (O2) 4

Connection with oxygen

In the tetrameric shape of normal hemoglobin, the connection with oxygen, or formation of oxyhaemoglobine , is a co-operative process, or allosteric, where the affinity of connection of hemoglobin for oxygen is affected by oxygen saturation of the molecule. That has as a consequence that the curve of affinity of oxygen for hemoglobin with the form of a Sigmoïde (S-shaped curve), contrary to a nonco-operative process where it has a hyperbolic form.

The affinity of hemoglobin for oxygen decrease in the presence of Carbon dioxide, with weak pH and when the temperature increases. These chemical properties are essential with alive since they allow a better release of dioxygene in active fabrics (ex: the muscle in exercise, by the production of metabolites develops an acid pH, its temperature increases affirming the dissociation of dioxygene of heme). Carbon dioxide reacts with water to form bicarbonate via the reaction:

CO2 + H2O ↔ HCO3- + H+

By this reaction one sees that blood having a high carbon dioxide rate has also a lower pH. Hemoglobin can bind to the Proton S and carbon dioxide what induces a change of conformation in the protein which facilitates the release of oxygen. The proton can bind to various places along protein, whereas carbon dioxide binds on the alpha-amino group by forming a Carbamate. Same manner, when the carbon dioxide level in blood decreases (for example close to the lungs), carbon dioxide is slackened, increasing the affinity of protein for oxygen. This control of the affinity of oxygen for its connection with the hemoglobin and the relaxation of carbon dioxide is known under the name of effect Bohr.

The connection of oxygen is also affected by molecules such as the 2,3-diphosphoglycérate, which decreases the affinity of hemoglobin for oxygen. At the people acclimatized to high-altitudes, the concentration of 2,3-diphosphoglycérate in blood is more important, which makes it possible hemoglobin to deliver larger quantities of oxygen to fabrics in atmospheres low in oxygen.

Poisons of hemoglobin

On the other hand, with the Carbon monoxide CO, hemoglobin forms a stable compound, the Carboxyhémoglobine; moreover, the affinity of hemoglobin for carbon monoxide is higher than that of oxygen, which means that CO effectively competes with oxygen during pulmonary oxidation. CO thus constitutes an antagonist of O2, thus being able to cause a tissue hypoxia: its inhalation with strong amounts involves Malaise S, Céphalée S (headaches), then Asthénie (intense weakness) and finally the Mort by Asphyxie. One treats severe poisonings with CO by immersion prolonged in box hyperbare reducing the half-life of the carboxyhaemoglobin thus.

In addition, of the oxidising agents such as nitrates contained in water or vegetables can oxidize the ferro-iron (Fe2+) of ferri-iron heme (Fe3+), making it thus unable to fix oxygen. Hemoglobin is then called Méthémoglobine. The treatment consists of the administration of methylene blue.

Finally, the ion Cyanure CN- is also an inhibiter of hemoglobineic oxidation, just as the Sulfure of hydrogen H2S.

Elimination of hemoglobin

The red globules senescent are phagocytes by the macrophages of the Rate and the Foie. During this process, the proteinic component of hemoglobin is degraded in the form of acid-amino which is recycled. The heminic component is degraded in Biliverdine then in insoluble Bilirubine (porphyrin without iron) of color Jaun E. Iron is recycled. The insoluble bilirubine is released in plasma by the Macrophage S, where it binds to the serum Albumine. It is thus conveyed in blood circulation and is collected by the Hépatocytes. It is then made soluble by a reaction of conjugation with a molecule of Glucuronic acid then is excreted by the Foie in the Bile.

The bile flows in the intestine and the soluble bilirubine is degraded by bacteria in stercobiline of color Brun E, which gives its color to the saddles. The bilirubine is also evacuated in the Urine S.

When the bilirubine cannot be excreted, its concentration increases in blood. It then is primarily eliminated by the urines, which causes dark urines and faded saddles, almost Blanc hes.

Clinical use

In medicine, several terms refer to hemoglobin.

Saturation (SaO2: defined in % and calculated by the quantity of oxyhaemoglobine divided by the total quantity of hemoglobin of blood. The value of saturation is regarded as dangerous if it is lower than 90%. The normal value is of 100% for normal atmospheric conditions. With this value one speaks about " capacity in O2 of the sang". The Cyanose is a symptom. It is about the coloring bluish of the teguments. It is in connection with an arterial oxygen desaturation whatever is the origin.

Similar molecules

The heme molecule of hemoglobin resembles the Chlorophylle, the iron atom being replaced by an atom of Magnésium, which brings its green color to him. At the Invertebrate S, the Hémocyanine plays a part similar to hemoglobin.

See too

Simple: Hemoglobin

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