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The first description of cooperative binding to a multi-site protein was developed by A.V. Hill. [4] Drawing on observations of oxygen binding to hemoglobin and the idea that cooperativity arose from the aggregation of hemoglobin molecules, each one binding one oxygen molecule, Hill suggested a phenomenological equation that has since been named after him:
The Hill coefficient, or , may describe cooperativity (or possibly other biochemical properties, depending on the context in which the Hill equation is being used). When appropriate, [clarification needed] the value of the Hill coefficient describes the cooperativity of ligand binding in the following way:
One manifestation of this is enzymes or receptors that have multiple binding sites where the affinity of the binding sites for a ligand is apparently increased, positive cooperativity, or decreased, negative cooperativity, upon the binding of a ligand to a binding site. For example, when an oxygen atom binds to one of hemoglobin's four binding ...
A multimeric protein's affinity for a ligand changes upon binding to a ligand, a process known as cooperativity. This phenomenon was first discovered by Christian Bohr's analysis of hemoglobin, whose binding affinity for molecular oxygen increases as oxygen binds its subunits. [1]
This model explains sigmoidal binding properties (i.e. positive cooperativity) as change in concentration of ligand over a small range will lead to a large increase in the proportion of molecules in the R state, and thus will lead to a high association of the ligand to the protein. It cannot explain negative cooperativity.
Allosteric enzymes include mammalian tyrosyl tRNA-synthetase, which shows negative cooperativity, [37] and bacterial aspartate transcarbamoylase [38] and phosphofructokinase, [39] which show positive cooperativity. Cooperativity is surprisingly common and can help regulate the responses of enzymes to changes in the concentrations of their ...
The equation = + + + + + + + is called the Adair equation for four binding sites. [6] ( Analogous equations can be written for other numbers of sites.) It expresses the degree of saturation at equilibrium as a function of ligand concentration and a series of dissociation constants , , , of the binding sites, assuming no interaction between them.
Hemoglobin, for comparison, has a Hill coefficient of usually 2.8–3.0. In these cases of cooperative binding hemocyanin was arranged in protein sub-complexes of 6 subunits (hexamer) each with one oxygen binding site; binding of oxygen on one unit in the complex would increase the affinity of the neighboring units. Each hexamer complex was ...