Affinity Electrophoresis of Proteins for Determination of Ligand Affinity and Exploration of Binding Sites

Affinity gel electrophoresis was introduced about 50 years ago. Proteins interact with a ligand immobilized in the support. Specific interactions cause a decrease in electrophoretic mobility. The presence of a free ligand, competing with an immobilized ligand, restores electrophoretic mobility. In affinity capillary electrophoresis, the ligand is mobile, and its interaction with a specific protein changes the mobility of the protein-ligand complex. This review mostly focuses on gel affinity electrophoresis. The theoretical basis of this technique, ligand immobilization strategies, and principles for determination of ligand affinity are addressed. Factors affecting specificity and strength of interactions are discussed, in particular, the structure of the affinity matrix, pH, temperature, hydrostatic pressure, solvent, co-solvents, electric field, and other physico-chemical conditions. Capillary affinity electrophoresis principles and uses are also briefly introduced. Affinity gel electrophoresis can be used for qualitative and quantitative purposes. This includes detection of specific proteins in complex media, investigation of specific interactions, protein heterogeneity, molecular and genetic polymorphism, estimation of dissociation constants of protein-ligand complexes, and conformational stability of binding sites. Future prospects, in particular for screening of engineered mutants and potential new drugs, coupling to other analytical methods, and ultra-microtechnological developments, are addressed in light of trends and renewal of this old technique.