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Reassembled Hemoglobin, T conformation

from the high-altitude goose, Anser indicus (n1_1A4F_t.pdb)

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View down the barrel of chain A towards the origin. Iron atoms are brown.
View of anion binding site where beta chains meet. Basic nitrogens colored blue.
This hemoglobin binds to inositol hexaphosphate as its polyanionic effector.
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The initial view shows all the hemes and the sidechains of His45 of the alpha chains. Interactions between His45s mediate the R to T transition.

This protein has a five-member alpha-helix barrel structure with a right-handed twist as observed in enzymes. See the discussion under the dihydrodipicolinate synthase animation. Like the enzymes, there is a need for constraint around the centre. This allows one binding site to do work on another. The oxygen binding sites also exchange energy (work) with the alpha chain His45 sidechains at the protein-protein interfaces through the constrained central region. In addition, hemoglobin has a negative catalytic effect in slowing down the autoxidation of oxyhemoglobin to methemoglobin and participates in the catalysed reduction of methemoglobin by other cell constituents. Both these processes probably depend on constraint.

The unusual elongated lobes of the monomers could well be an adaptation to the very low solubility of oxygen in water. It may be that oxygen is more soluble at the protein-water interface and the lobes serve as conduits between bulk water and the oxygen binding sites.

For further information about this protein, contact the author, Dr Don Vanselow at dvanselow@hotmail.com

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