Perhaps a suitable term for the T state and R state conformations of hemoglobin would be "Quaternary Isomers". I have previously described them as interchangeable by subunit exchange. Subunit exchange is geometrically equivalent to concerted rotation of each subunit about its radial axis by 120º and perhaps this would be the preferred pathway of isomerisation in nature. For an α2β2 tetrahedron there are 3 different quaternary isomers. For vertebrate hemoglobins, a clockwise rotation of each subunit of the R state by 120º produces the T state with a polyanion binding site. An anticlockwise rotation of each R state subunit by 120º produces an alternate T state without an intact polyanion binding site. The alternate T state probably has a minor role physiologically. In the presence of the appropriate polyanion effector the isomerisation equilibrium would shift towards the main T state.
I have placed all 3 isomers in the Gallery.
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