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Deoxygenated hemoglobin (blue) is found in the T state, and oxygen binding (red) triggers the transition to the R state. The animation on the right shows a close-up view of the heme group (white, in ball and stick representation) in one of the hemoglobin subunits. In the deoxygenated (T) state, the iron atom is non-planar with the rest of the heme group due to its association with a histidine side chain. Oxygen.
This is more than a geometric difference. Chemical components of the cell have to be built by enzymes, and the "handedness" (chirality) of the molecule is determined by the shape of those enzymes. A cell that builds one form will not be able to build the other form. (2) Ether linkage : When side chains are added to the glycerol, most organisms bind them together using an ester linkage (see diagram above). The side chain that is added has two oxygen.