Question

Calculate the isoelectric point of the following amino acid – You must show your work! pKa of the following groups α-COOH has 2.09 α-NH3 has 9.82 R group has 3.86 Aspartate

Answer 1

The isoelectric point is the pH at which the amino acid does not transfer in an electric field.

This means at that pH (isoelectric point), the amino acid is neutral, i.e. the zwitterion form is dominant.

Calculation of isoelectric point for amino acids:

If the amino acid does not have any side chain like any ionizable group, then the isoelectric point is simply the average of the α- NH3 and α-COOH pKa values.

If the amino acid has any ionizable group side chain then all three pKa values must be considered.

• If the amino acid has acidic side chain (Ex: asp and glu), then average the side chain pKa with the α-COOH pKa.

• If the amino acid has basic side chain (Ex: his, arg, and lys), then average the side chain pKa with the α-NH3 pKa.

From question we have to calculate Aspartate isoelctric constant.

Aspartate has acidic group side chain, so the isoelectric point is average the side chain pKa with the α-COOH pKa.

So

Isoelectric point = (2.09+3.86)/2 = 2.97