I am trying to learn the interactions that hold the hemoglobin tetramer together, and the conformational changes induced at various allosteric sites. So far, what I know is this:

The tetramer can exist in relaxed state (R-state), or tense state (T-state).

The R-state favors oxygen binding, while the T-state favors oxygen release.

At the C-terminus (a histidine residue) and the 40th residue (aspartate) of the β-chains, and the 146th residue (histidine) of the α-chain, there are two salt bridges. When red blood cells are present in tissues (with a pH of 7.2), the formation of both bridges are favored. When both bridges are formed, the T-state is favored.

The second residues of the β-chains are also a histidine. (This is near the N-terminus). This residue participates in an ionic bond with 2,3-BPG. At the 82nd position, there is a lysine residue that also participates in this bond. (This is a little over half-way down the amino-acid sequence). By binding 2,3-BPG, the T-state is favored.

On the binding of oxygen to a heme-group, the iron atom moves to the center of the protoporphyrin ring. Attached to the iron atom is yet another histidine. This histidine is attatched to an α-helix that is attached to the terminal-carboxyl of the α-chain. By moving the α-helix, a conformational change is induced in the other subunits to shift the structure to the R-state.

What residues of which subunits are the α-carboxyl termini in close spatial relation to, and how does this movement induce the R-state? And which residues hold each dimer together, and how do they affect the R- or T-states?


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