It is widely accepted that proteins contain numerous lysine residues. The R-grou

Question

It is widely accepted that proteins contain numerous lysine residues. The R-group is reactive at physiological pH and can form hydrogen bonding in the catalytic site. Since lysine is a polar amino acid,t is positioned so that it is near the catalytic site or outside of the protein so that it will assist in metabolic reactions. While conducting an experiment to isolate this amino acid, you direct your lab to make separate solutions of pH 1.0, plH 8.0 and pH 10.50. The amino acid in the initial reaction is Lysine [(NH (CH2) CH(NH)CO (pKa values are a-carboxyl group 2.0, a-amino group 10.0, R group 12.0.) Answer the following four questions from the passage above and select the best answer. (20 points) 1. In the initial reaction, lysine: a. Has an overall positive charge and the reaction is at the pH value of 2.0. b. Has an overall neutral charge and the reaction is at the pH value of 6.0. c. Has an overall positive charge and the reaction is at the pH value of9.0 d. Has an overall neutral charge and the reaction is at the pH value of 7.0

Explanation / Answer

In the initial reaction, it is clear that the overall molecule is neutral as there is a single positive and a single negative charge. The molecule is in a zwitterion form.

The amine group of the R and the carboxyl group have exchanged hydrogen to get to the zwitterion. The pH of the solution would therefore be --

pH = (pKa of R + pKa of acid)/2 = (12+2)/2= 7

Answer is option d.

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