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This last form of the Hill equation is advantageous because a plot of versus [] yields a linear plot, which is called a Hill plot. [ 7 ] [ 8 ] Because the slope of a Hill plot is equal to the Hill coefficient for the biochemical interaction, the slope is denoted by n H {\displaystyle n_{H}} .
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 sigmoidal shape of hemoglobin's oxygen-dissociation curve results from cooperative binding of oxygen to hemoglobin. An example of positive cooperativity is the binding of oxygen to hemoglobin . One oxygen molecule can bind to the ferrous iron of a heme molecule in each of the four chains of a hemoglobin molecule.
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.
Negative cooperativity occurs when binding of the first substrate decreases the affinity of the enzyme for other substrate molecules. Allosteric enzymes include mammalian tyrosyl tRNA-synthetase, which shows negative cooperativity, [37] and bacterial aspartate transcarbamoylase [38] and phosphofructokinase, [39] which show positive cooperativity.
where is the Hill coefficient which quantifies the steepness of the sigmoidal stimulus-response curve and it is therefore a sensitivity parameter. It is often used to assess the cooperativity of a system. A Hill coefficient greater than one is indicative of positive cooperativity and thus, the system exhibits ultrasensitivity. [34]
The MWC model only allows for positive cooperativity, where a single conformational switch from the T to R states results in an increase in affinity for the ligand at unligated binding sites. Ligand binding to the T state thus cannot increase the amount of the protein in the T, or low-affinity, state.
The protein of interest is isolated with a specific antibody. Interaction partners which stick to this protein are subsequently identified by Western blotting. [2] Interactions detected by this approach are considered to be real. However, this method can only verify interactions between suspected interaction partners.