Question 1

Which of the following amino acids is classified as negatively charged (acidic) at physiological pH?

Accepted Answer

Aspartic acid

Question 2

At physiological pH ($$pH \approx 7.4$$), which group is deprotonated in glutamic acid, resulting in its negative charge?

Accepted Answer

Side chain carboxyl group

Question 3

Which of the following is the correct set of amino acids that are positively charged (basic) at physiological pH?

Accepted Answer

Lysine, Arginine, Histidine

Question 4

A patient has a mutation that replaces a lysine residue with aspartic acid in a protein. What is the most likely effect on the protein's net charge at physiological pH?

Accepted Answer

The net charge will become more negative.

Question 5

Which of the following best explains why lysine is classified as a base at physiological pH?

Accepted Answer

It accepts a proton on its side chain amine group.

Question 6

Given the peptide sequence H-E-K, what is the net charge of the peptide at physiological pH ($$pH = 7.4$$)? (Assume N-terminus and C-terminus are not modified.)

Accepted Answer

$$+1$$

Question 7

Which of the following amino acids has a side chain that can act as a buffer near physiological pH due to its $$pK_a$$ being close to 7?

Accepted Answer

Histidine

Question 8

If the pH of a solution is decreased from 7.4 to 2, what will happen to the charge on the side chain of aspartic acid?

Accepted Answer

It will become neutral.

Question 9

Which of the following is the correct order of the one-letter codes for the charged amino acids at physiological pH?

Accepted Answer

D, E, K, R, H

Question 10

A researcher is designing a peptide that must bind to a negatively charged DNA backbone. Which amino acid should be enriched in the peptide to maximize electrostatic attraction?

Accepted Answer

Lysine

Question 11

Which of the following statements about the behavior of charged amino acids at different pH values is correct?

Accepted Answer

At physiological pH, acidic amino acids are negatively charged and basic amino acids are positively charged.

Question 12

Which of the following best describes the side chain of arginine at physiological pH?

Accepted Answer

It is positively charged due to a guanidinium group.

Question 13

Which of the following amino acids would you expect to be most abundant in the active site of an enzyme that catalyzes the hydrolysis of a negatively charged substrate?

Accepted Answer

Lysine

Question 14

Which of the following equations correctly represents the relationship between acid and base forms of an amino acid side chain at a given pH?

Accepted Answer

$$pH = pK_a + \log_{10} \left( \frac{[A^-]}{[HA]} \right)$$

Question 15

Which of the following amino acids is most likely to lose a proton from its side chain at physiological pH?

Accepted Answer

Aspartic acid

Question 16

A mutation changes a glutamic acid residue to lysine in a protein. Which of the following best describes the change in the side chain's chemical properties?

Accepted Answer

From acidic to basic

Question 17

Which of the following best explains why histidine can act as both a proton donor and acceptor at physiological pH?

Accepted Answer

Its side chain $$pK_a is $$close to physiological pH.

Question 18

Which of the following amino acids would be least likely to participate in ionic interactions at physiological pH?

Accepted Answer

Serine

Question 19

Which of the following is a correct statement about the classification of amino acids as acids or bases?

Accepted Answer

Amino acids are classified as acids or bases based on their side chain behavior at physiological pH.

Question 20

Which of the following best describes the effect of increasing pH on the charge of lysine's side chain?

Accepted Answer

It loses its positive charge and becomes neutral.

Charged Amino Acids: Acidic and Basic Side Chains in Proteins

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