Question

A protein has 2 arginines (side chain pKa=10.5), 3 aspartates (side chain pKa=3.9), 3 glutamates (side chain pKa=4.1), and 1 lysine (side chain pKa=4.1). At pH = 1, the net charge of the protein is:

+10

-6

0

+3

-10

Answer 1

Answer:
Option D is correct.

Explanation:
At a pH below pK, the side chains remain protonated.
Arginine has a positive side chain at pH = 1
2 arginines = +2

3 aspartates and 3 glutamates contain 6 carboxyl groups. They all remain protonated. So, net charge contributed by these amino acids = 0

Lysine has a positive side chain at pH = 1
1 lysine = +1

Total charge = +3

(This calculation does not consider the positive charge on the alpha amino group of the N-terminal amino acid)