Consider the following peptide for questions 5 9 H II H H II

Consider the following peptide for questions 5 9: H II H H II N C C N C C N C C N C OH HCH HCH HCH HCOH HCH HCH 20 HC CH HCH 1 Threonine HC CH HCH H H thwNialdwint Lysine Use the pKas from the table of amino acid from Lecture 2 slide 22 1. 8 (9.0S

Solution

Answer: 7.21

Threonine and phenylalanine are nonpolar amino acids hence they do not have ionizable side chains.

Lysine is positively charged and Aspartate is negatively charged at around pH 7

The pI value will be the average of pKa Lys side chain and pka of Asp side chain

pKa values of side chains: Lys side chain:10.53, Aspartate amino group 3.9

PI is =(10.53 + 3.9)/2 =7.21

Substitution of alanine at position 3 has no effect on pI since it is a nonpolar amino acid