Oxygen transport in muscles and blood

Oxygen transport in muscles and blood

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The T and R forms of hemoglobin

Oxygen loading and release in the tissue are accompanied by a change in the three-dimensional conformation of hemoglobin. The tetramer of two α and two β chains exists as an equilibrium between two conformations, the R or relaxed (engl. relaxed) and the tensioned T-shape (engl. tense). Normal, non-oxygenated Hb is almost exclusively in the T conformation, while oxygenated Hb is 99.9% in the R conformation.

Tab. 1
The T and R forms of hemoglobin
Oxygenated HbDeoxygenated Hb
Conformation99.9% R-form99.9% T-shape
Affinity for oxygenhighlow
Affinity to protons (Bohr effect)lowhigh
Affinity for 2,3-diphosphoglyceratelowhigh

The transition from one conformation to the other is accompanied by a change in the salt bridges and a relative movement of the α and β chains. There is little contact between the two α chains or β chains, whereas the contacts between α1and β1 or α2 andβ2 are responsible for the change in conformation. B-helix, H-helix and the GH transition area are contact points that are hardly affected by the conformational change (packing contacts), while the C-helix, G-helix, and the FG transition region experience drastic changes as ligands on the heme change (slidingcontacts). In the T-shape there are 106 atoms out of 32 amino acid residues atpacking contact and 107 atoms of 27 amino acid residues on sliding contact involved.

The binding of protons shifts the R-T equilibrium to the side of the T conformation (Bohr effect).