For the purposes of estimating charge of a protein, it is reasonable to assume t
ID: 896166 • Letter: F
Question
For the purposes of estimating charge of a protein, it is reasonable to assume that if the pH of the solution the protein is in is more than one pH unit above the pKa of a particular group, the proton of the group is off. On the other hand, if the pH is more than one unit below the pKa of a particular group, the proton is on.
Using that information and the pKa information you used above, calculate the charge on the following polypeptide at the pHs below
Amino end -Gly-Arg-Cys-Asp-Leu-Pro-Ile-Ala-Ser-Trp-Lys - Carboxyl end
a. pH = 2
b. pH = 5.5
c. pH = 9.5
d. pH = 14
Explanation / Answer
Analysing the pKa values for each amino acids, the -COOH, -NH2 and -side chain we calculate the net charge at each pH.
For the given polypeptide, the net charge on the molceule at pH would be ,
a. pH = 2
Net charge : 3
b. pH = 5.5
Net charge : 1
c. pH = 9.5
Net charge : 0
d. pH = 14
Net charge : -3