Conformational changes involving the quaternary structure of proteins are of fundamental importance for several important biological mechanisms. The R→T structural transition of haemoglobin (Hb), the protein responsible for oxygen (O2) transport in the red blood cells of vertebrates, is the hallmark example. This transition, which regulates O2 uptake in the lungs and O2 release in the tissues, is a switch in the quaternary structure of the protein from a low-affinity state (T) to a high-affinity state (R), two well-characterised structures.

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