The pH dependence of insulin binding. A quantitative study.

A model is presented to study quantitative the effect of pH on a ligand-receptor interaction. Assuming that binding is only possible if all the "active groups" are in the correct ionic state, and that the ionic state of the other residues does not affect the association constant, it is possible to measure the number and pK values of the active groups. This model is applied to insulin and insulin analogs binding to its cellular receptor. Two active groups are responsible for the marked pH dependence of the reaction: a deprotonated residue of pK 7.6 at 25 degrees C (ionization heat: 1.5 kcal . mol-1) and a protonated residue of pK 8.0 at 25 degrees C (ionization heat: 12 kcal . mol-1). The first active group might be a carboxyl residue, in a hydrophobic environment, probably belonging to the receptor molecule. The second active group was further identified as the A1 alpha-amino residue of insulin by the study of insulin analogs.