MCAT Biochemistry Question 13: Answer and Explanation

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Question: 13

13. In lysine, the pK_a of the side chain is about 10.5. Assuming that the pK_a of the carboxyl and amino groups are 2 and 9, respectively, the pI of lysine is closest to:

A. 5.5.
B. 6.2.
C. 7.4.
D. 9.8.

Correct Answer: D

Explanation:

Because lysine has a basic side chain, we ignore the pK_a of the carboxyl group, and average the pK_a of the side chain and the amino group; the average of 9 and 10.5 is 9.75, which is closest to choice (D).

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