Written by Merel Dagher (PEACE Advisor)
Histidine, one of the polar, charged amino acids, has three ionizable groups. Match the correct overall charge of histidine to the pH. (pKa1=1.8, pKa2=6.04, pKa3=9.33)
a. At pH 1, His has a net charge of +1.
b. At pH 3, His has a net charge of +1.
c. At pH 4, His has a net charge of +2.
d. At pH 7, His has a net charge of -1.
e. At pH 10, His has a net charge of -2.
a. At pH 1, His has a net charge of +1.
b. At pH 3, His has a net charge of +1.
c. At pH 4, His has a net charge of +2.
d. At pH 7, His has a net charge of -1.
e. At pH 10, His has a net charge of -2.
Click “Read More” to see the answer.
The correct answer is b. It’s important to remember how the pKa’s of the histidine groups and how they relate to pH.
Given this, at a pH less than 1.82, all of His will be protonated, thus having an overall net charge of +2 (therefore, a is wrong). When you have a pH greater than 1.82 and less than 6, the carboxy group will be deprotonated, giving His an overall charge of +1 (making b correct and c incorrect). At pH greater than 6 and less than 9.17, His will have both the carboxy and side chain deprotonated, giving it an overall charge of 0 (making d incorrect). Finally at a pH above 9.17, His will be completely deprotonated, giving it an overall charge of -1 (making e incorrect).