MCAT Chemistry Question #7: Answer and Rationale

7. Four different solutions of a single amino acid were titrated, and the pK values of the solute were determined. Which solution contains an amino acid that would be most likely to stabilize an anionic substrate in an enzyme pocket at physiological pH?

A) Solution 1

B) Solution 2

C) Solution 3

D) Solution 4

Answer Analysis

The correct answer is D.

Rationale: This question includes a data table and assesses your knowledge of structure, function, and reactivity of amino acids. Four different solutions of a single amino acid were titrated, and the pK values were determined and are displayed in the table. You must recognize a data pattern in the table, make comparisons, and use those comparisons to make a prediction. An anionic substrate will have a negative charge; therefore, to stabilize an anionic substrate, a positive charge is needed. Recall that basic amino acids will bear a positive charge at physiological pH (the normal pH of the human body is 7.35-7.45) and will have a high pKa value. Thus, you can determine that only Solution 4 will have a positive charge at physiological pH.

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