Haemoglobin
A blood pigment consisting of globin as a protein component and the iron-containing prosthetic group Hematin as the active centre of oxygen binding. Each erythrocyte consists of 90% haemoglobin and thus contains about 250 million haemoglobin molecules. Each haemoglobin molecule consists of four subunits, each containing a haem group. An iron ion is bound in the centre of the heme group. This iron exerts a strong attraction, so-called affinity, on oxygen, whereby the oxygen (or one oxygen molecule in each case) is bound to the haemoglobin. Hemoglobin binds oxygen (O2) under the high partial pressure of inhalation air in the lungs and turns light red (oxyhaemoglobin), under the low partial pressure in peripheral tissues due to oxygen consumption, oxyhaemoglobin releases oxygen and turns dark red (reduced hemoglobin).
This results in the different colouring of arterial and venous blood, as well as the reddish colour of skin with good blood circulation and the bluish colour in case of poor oxygen saturation of the arterial blood. After bicarbonate, hemoglobin forms the most important buffer in the blood to keep its pH constant. Under a purple discoloration of the blood pigment, carbon monoxide (CO) displaces the oxygen from the haemoglobin as a result of higher binding strength, so that it can no longer be transported into the tissue. The resulting tissue nitrogenization causes the toxicity of carbon monoxide.
Haemoglobin is therefore an iron(II)-containing "colour defence", consisting of haem as prosthetic group and globin (approx. 94% of Hb).
The normal value of the Hb concentration in blood is 134-173 g/l (m 153, w 145), in Ery 299-357 g/l; the total amount in adults is ~750 g (per Ery 27-32 pg).
Biological functions:
a) transport of molecular oxygen (O2) by reversible formation of oxy-Hb {step binding of 4 O2 to the 4 heme without changing the valence of iron (oxygenation)}; the specific, of O2 partial pressure, temperature and hydrogen ion concentration, respectively CO2voltage dependent oxygen capacity is 1.34 ml O2/g Hb; at normal Hb content and normal O2 saturation approx. 21 ml O2/100 ml blood are carried.
b) Transport of carbon dioxide (CO2), about 20% of the blood CO2 in Carbamino-Hb (Hb-NHCOO-). -
c) buffer substance, namely by free -COOH- and -NH2-groups (30% of the total buffer capacity of the blood) as well as in connection with the formation of oxy-Hb (Hb binds hydrogen ions released from carbonic acid as a weaker acid, which are released in the lung, with formation of the stronger acid oxyhaemoglobin = HbO2). Carbon monoxide (whose affinity for Hb is > 100 times greater than that of O2) is converted into carbon monoxide haemoglobin by exposure to carbon monoxide, haemiglobin (methaemoglobin) is formed by oxidation, and sulfhaemoglobin (blood toxins) is formed by exposure to H2S.
After the release of the Hb, which takes place when the red blood cells decompose and break down, the Hb is broken down into bile dyes, iron and globin.