= 2,3-Bisphosphoglycerate
\b[2,3-Bisphosphoglycerate] is the glycerate metabolite carrying two <phosphate> substituents at carbon positions 2 and 3. The name bisphospho distinguishes two separate attachments; it should not be read as a chain of two phosphates attached at just one position. Older biochemical papers call the metabolite 2,3-diphosphoglycerate. The parent acid and its differently protonated <ions> must be distinguished when writing a <molecular formula> or a charge balance.<2,3-Bisphosphoglyceric acid in ChEBI>[CHEBI:28907, Definition, Formula and conjugate-acid relation to CHEBI:19324]{ref}
In <red blood cells>, <2,3-Bisphosphoglycerate> is an <allosteric modulator> of <hemoglobin>. Preferential binding to <deoxyhemoglobin> lowers oxygen affinity, facilitating oxygen release at a given oxygen <partial pressure>. <Reinhold Benesch> and <Ruth E. Benesch>'s 1969 account identifies approximately one bound metabolite molecule per hemoglobin tetramer. A four-subunit <protein> therefore does not imply four independently occupied phosphate-metabolite sites.<Intracellular Organic Phosphates as Regulators of Oxygen Release by Haemoglobin by Reinhold Benesch and Ruth E. Benesch>[Publisher abstract, binding preference and stoichiometry, Nature221, pp.618–622]{ref}
A simple <Hill equation> illustrates an affinity change without assigning a universal physiological value. Write the fractional <oxygen saturation> as
$$
Y(p)=\frac{p^n}{P_{50}^n+p^n},
$$
where $p$ is oxygen <partial pressure> and $P_{50}$ is the pressure at half saturation. In a stipulated model with $n=2$, increasing $P_{50}$ from $20$ to $30\,\mathrm{mmHg}$ changes the saturation at $p=30\,\mathrm{mmHg}$ from
$$
\frac{30^2}{20^2+30^2}=\frac9{13}\simeq0.692
\quad\text{to}\quad
\frac{30^2}{30^2+30^2}=0.500.
$$
The same oxygen pressure now leaves a larger fraction of binding capacity unoccupied. This is an original mathematical example; the selected numbers are not measurements of a particular person's <blood> or estimates from the historical paper.
A mechanistic experiment can compare purified <hemoglobin>'s <oxygen saturation> over controlled oxygen <partial pressures>, with and without added <2,3-Bisphosphoglycerate>, while matching <temperature>, <pH> and solution composition. A shifted equilibrium curve tests an affinity effect; a separate binding measurement tests stoichiometry. Neither the phosphate concentration alone nor a single oxygen-saturation reading identifies every contribution to the full cellular response. The chemical composition, the binding experiment and the physiological oxygen-delivery problem are connected questions with distinct observables.
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