Hemoglobin

For higher animals, simple diffusion mechanisms in body fluids are not an efficient way to meet the oxygenation needs of their tissues and cellular material. To the low area/volume ratio of these living beings, it is added the fact that O2 is a molecule that is essentially insoluble, which makes its transport even more difficult. The solution then passes through carrier proteins, associated with erythrocytes - hemoglobin, to which the following lines refer.Hemoglobin is an oligomeric protein and is generally a metalloprotein consisting of about 600 amino acids, arranged in 2 alpha chains and 2 paired beta chains in a quaternary globular structure. The four chains constitute the organic part of the molecule, and are attached to heme prosthetic groups (consisting of a porphyrin ring and a transition metal: Fe2+) which have affinity for the O2 molecules because of the electron configuration. It is the Fe2+ that assumes this function, always in its ferrous form, and the ferric form - Fe3+ - is not able to bind O2, being at the same time more unstable and prone to the formation of reactive species. Fe2+ has one O2 binding site and this bond as expected would be reversible to allow oxygen to be transported to where it is needed. Due to this binding, there is a change of color in human blood, from bright red when it is in its oxygenated form, to a more purplish tone in its venous phase. Some molecules such as CO2 and NO have a higher affinity for the heme group, "expelling" O2 molecules from erythrocytes, which explains their toxicity to the organism. 

Porphyrias are genetic diseases related to porphyrin of the heme group. Examples are acute intermittent porphyria and accumulation of uroporphyrogen I each with specific symptoms.
Concerning the coordinated transport of O2, CO2 and H+, the mechanism is as follows: 

O2 binds cooperatively to hemoglobin (this means that the bonds promote more bonds) and then the affinity of hemoglobin varies with pH. In an acidic environment, H+ and CO2 cause the release of O2 whereas in a basic medium, O2 causes the release of H+ and CO2. This is the so-called Bohr effect(reciprocal effect): CO2 + H2O <-> HCO3- + H+
The dead erythrocytes release the heme group generating: Fe3+ (which is recycled) and bilirubin (which is excreted in the liver). The latter may have a negative effect if released into the blood because it causes jaundice, or a positive antioxidant effect especially as an antioxidant of the membrane, because it collects two hydroperoxide radicals, having about 1/10 the efficiency of vitamin C.

Text written by:
Beatriz Ribeiro
Cláudia Campos
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