Interaction of protein with a self-associating ligand. Deviation from a hyperbolic binding curve and the appearance of apparent co-operativity in the Scatchard plot.
We derived a general formula to analyze a binding system in which a ligand self-associates, in terms of experimentally determinable quantities, i.e. r, the average number of bound ligands per protein molecule, and Lft, the total free ligand concentration, which are expressed as a ligand monomer unit. The limiting behaviors of the Scatchard plot (r/Lft vs r plot), that is, the intercepts on the r-axis and the r/Lft-axis, and the limiting slopes, are generally given. Three models that may be encountered are considered in detail. Numerical examples are also presented to illustrate how the self-association of a ligand affects the binding curves. The ligand self-association alone can cause deviation of the profile of the binding curve (r vs Lft plot) from a hyperbola, resulting in a nonlinear Scatchard plot. Therefore, analysis of the binding data without consideration of ligand self-association may lead to erroneous conclusions as to the numbers and classes of binding sites, co-operativity among the sites and binding parameter values.
Department of Biochemistry, Shiga University of Medical Science, Japan.
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Ishida
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Horiike
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Tojo
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Nozaki
D000494:Allosteric Regulation; D007700:Kinetics; D008024:Ligands / Q000378:metabolism*; D046911:Macromolecular Substances; D008433:Mathematics; D008954:Models, Biological; D011506:Proteins / Q000378:metabolism*; D013816:Thermodynamics
D008024:Ligands; D046911:Macromolecular Substances; D011506:Proteins