Polypeptide Amino Acid Composition and Isoelectric Point II. Comparison between Experiment and Theory

Experimental isoelectric points and amino acid compositional data for 58 proteins were compiled and organized. The experimental isoelectric points correlated well with the acidic to basic amino acid molar ratio. This agreement proved the usefulness of a recently presented analytical expression correlating explicitly protein isoelectric point to acid-base composition. Regressed acidic and basic dissociation constants were determined to be pK_a = 4.9 and pK_b = 10.0, in fair agreement with the expected values of pK_a = 4.2 and pK_b = 11.2. Theoretical isoelectric points determined by a more complete computational procedure were on the average in as good an agreement with the experimental values as those calculated via the theoretical approximation using the regressed dissociation constants. Thus, the analytical approximation is a powerful tool for the convenient and accurate calculation of protein isoelectric point from the amino acid composition.