man u have copy&pasted the xact xplanation given in the wikitestprep site... i dint understand their xplanation plzz explain more.. m totally blank at this question...
emmm... u cant challenge usmle if u hav such weird question on ur test day....

and usmle is putting weird questions in the test more often these days....
I am not biochemist and can't explain more than explanation from wiki. Which I don't understand either to solve similar stem.
If you have such weird question you have 3 options:
1. try to solve by recollecting which letter assigned to exact AA and calculate charge. Total time spent for this will be more than 1-2-3 minutes
2. try to eliminate "incorrect" answer by guessing. Again time consuming, because resemble option 1.
3. third option in such situation I prefer more. If you see absolutely unknown question, put your favorite letter, mark this question and move on. Spare time for other questions. In the end of block if you have enough time, try to solve it.
Last option is advice from book Kaplan Qbank: " If you don't know the answer, admit it, make your best guess, and move on to the next question"
For sure we'll challenge weird questions these days, but it doesn't mean that we have to solve them wasting valuable time. They putting such questions to slow you and make you anxious.
I hope this will help you
