Question

Which of the following amino acids may or may not be protonated at neutral pH?

(a) Glutamate

(b) Lysine

(c) Histidine

(d) Arginine

The question was asked in my homework.

My query is from Biological Molecules Structures and Functions in section Structures and Functions of Biological Molecules of Cell Biology

Answer 1

Right option is (c) Histidine

To explain: The amino acids may be negatively charged, positively charged or neutral at a neutral pH. Here, Arginine and Lysine are positively charged or protonated at neutral pH, while Glutamate is negatively charged at neutral pH. However, Histidine may or may not be protonated at normal pH. The side chain of Histidine has a pKa of 6.5, which means only 10% of Histidine is protonated.