Scatchard

Scatchard refers to Scatchard analysis, a method used in biochemistry to analyze the binding of a ligand to a macromolecule. The technique is based on plotting B/F versus B, where B is the amount of bound ligand and F is the free ligand concentration at equilibrium. The Scatchard equation is B/F = (n/K_d) - (B/K_d), where n is the maximal binding capacity (number of binding sites) and K_d is the dissociation constant. Thus, a Scatchard plot is linear if there is a single class of identical, independent binding sites, with slope -1/K_d and y-intercept n/K_d; the x-intercept equals n.

Procedure: measure binding at various ligand concentrations, separate bound from free, compute B and F, plot B/F against B, and interpret the results. A straight line indicates a single binding site class, while curvature suggests multiple classes or cooperative binding.

Applications: used to characterize receptor-ligand interactions, enzyme-substrate binding, and to estimate affinity and capacity of binding sites. It has been employed in pharmacology, biophysics, and related fields to derive the dissociation constant and the number of binding sites from equilibrium binding data.

Limitations: deviations from linearity indicate multiple binding site classes or cooperative effects; inaccuracies in achieving equilibrium or in separating bound from free ligand can distort results. Scatchard analysis assumes identical, independent binding sites and is less suitable for systems with allosteric interactions or complex binding mechanisms.

Historical note: the analysis is named after the biochemist Scatchard, who introduced the method to analyze ligand binding data.