Question

The pka of lysine is 10.8. Which of the following statements is true?

a. Most lysines in a protein will be positive at ph 10.8

b. Most lysines in a protein will be positive at pH 7.4

c. Most lysines in a protein will be negative at pH 10.8

d. Most lysines in a protein will be negative at pH 7.4

e. Most lysines in a protein will be neutral at pH 10.8

I know the answer is B. but WHY?

Answer 1

Lysine has the following structure- so we see that the top NH3 part has a Pka value of 10.5 ~10.8 and above that pH value it will lose the extra Hydrogen and thus the molecule will become neutral. But at pH 7.4, both the ammonia molecules will be in positive state and one positive charge will be neutralized by the negative charge on acetate ion..so overall charge will remain positive.