Proteins and Enzymes

ID #2378

Between the following two statements about acidic amino acids: A. Incorrect: They always have a negatively charged side group. B. Correct: At physiological pH they are negatively charged. I am confused as to why statement A would not work? Since the side group is the same as the R group, aspartic Acid and glutamic acid are negatively charged amino acids. When is it the case where they are not negative? Also, why are amino acids negatively charged at physiological pH compared to other pHs?


You are correct that aspartic acid and glutamic acid contain negatively charged side groups - at a physiological pH of about 7.4. 
The following is a little more involved than what you will need to know for MCB 150, but you can think of it as a preview for MCB 250 and other classes involving more biochemistry. When determining the charge of a side chain, you need to consider the pKa of the side chain and the pH of the solution that the molecule is in.  At lower pH, there are more H+ available, so they can form bonds with Oxygen. 
In the case of Glutamatic acid, the pKa of the side chain is about 4.3.  If you had a solution of pH 1 and added Glutamate, the side chain -COO- would gain a proton and become -COOH.  At that point, the electrons from the negative charge have been used to form a bond with H+ and the side chain is no longer negatively charged.  Aspartic acid has a different pKa for its side chain but the concept is the same.
 
So to answer your questions in brief:
If the pH of the surrounding solution is low enough, the side chains of glutamic acid and aspartic acid will no longer be negatively charged.
They are negatively charged at physiological pHs because there are less H+ available and the COOH in the side chain becomes a COO-.
Be very careful about words like always, never, must, cannot, etc. Absolute terms need to be used carefully and sparingly in biology. The statement that "They always have a negatively charged side group" requires that there could never be a condition when it is not true. But think about what it means to be an acid. It means you're a proton donor, and depending on the actual conditions, you will have either done that or not. That means possibilities, which means "always" is not a word that should be used. 

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