Allostery and cooperativity কী?

A protein is allosteric when the binding of a ligand at one site changes the affinity of another site, through a change of conformation transmitted across the molecule. When the two sites bind the same ligand and the effect is positive, binding is cooperative: the fractional saturation $Y$ rises with ligand concentration as a sigmoid rather than a hyperbola, described empirically by the Hill equation $Y = [S]^{n_{H}}/(K^{n_{H}} + [S]^{n_{H}})$ with a Hill coefficient $n_{H} > 1$ (haemoglobin: $n_{H} \approx 2.8$ for its four sites; myoglobin, one site: $n_{H} = 1$). Haemoglobin exists in two quaternary conformations, the T state (tense, low affinity, favoured when no oxygen is bound) and the R state (relaxed, high affinity), and switches between them as a whole; protons, carbon dioxide and 2,3-bisphosphoglycerate stabilise T and so lower the affinity — the Bohr effect, by which working tissue, acid and warm, takes more oxygen from the blood.