What is the isoelectric point of the following tetrapetide t

What is the isoelectric point of the following tetrapetide that contains the amino acids represented by the single letters “ELYK”, such that E is at the amino-terminal end of the peptide and K is at the carboy-terminal end.

Solution

Answer:

ELYK is Glutamine-Leucine-Tyrosine-Lysine

Answer: 9.83

Estimate the pH at which this peptide is neutral

Let’s say 7

pKa values of side chains: Lys side chain:10.53, Glutamine amino group 9.13

At pH 7, this peptide is +1 due to Lys side (pKa 10.53) chain ionized.

Decrease pH to make this charge zero, the next close pKa is 9.13(Glutamine amino group).

Isoelectric pH is the average of two pKa values directly above and below the estimate.

PI is =(10.53 + 9.13)/2 =9.83