Q: Which equation would you use to calculate the ratio of protonated to deprotonated forms of an amino acid side chain at a given pH?

$$pH = pK_a + \log \left( \frac{[\text{base}]}{[\text{acid}]} \right)$$

Question 1

Which of the following statements best describes the structure of an amino acid found in natural proteins?

Accepted Answer

Each amino acid contains a central tetrahedral carbon atom bonded to an amino group, a carboxylic acid group, a distinctive side chain, and a hydrogen atom.

Question 2

Which amino acid is NOT chiral and why?

Accepted Answer

Glycine, because its side chain is a hydrogen atom, making the central carbon achiral.

Question 3

Which of the following amino acids is classified as hydrophobic?

Accepted Answer

Phenylalanine

Question 4

A patient is found to have a deficiency in essential amino acids. Which of the following is TRUE regarding essential amino acids?

Accepted Answer

They must be obtained from the diet because humans cannot synthesize them.

Question 5

Which equation would you use to calculate the ratio of protonated to deprotonated forms of an amino acid side chain at a given pH?

Accepted Answer

$$pH = pK_a + \log \left( \frac{[	ext{base}]}{[	ext{acid}]} ight)$$

Question 6

At pH 8, what is the ratio of uncharged (base) to charged (acid) forms of the imidazole side chain of histidine, given $$pK_a = 6$$?

Accepted Answer

$$100$$

Question 7

Which amino acid is most likely to be found in the active site of enzymes due to its ability to be charged or uncharged at physiological pH?

Accepted Answer

Histidine

Question 8

Which of the following amino acids has a side chain that can form disulfide bonds, contributing to protein structure stabilization?

Accepted Answer

Cysteine

Question 9

A protein is found to have a high proportion of aspartic acid and glutamic acid residues. What property would you expect this protein to have at neutral pH?

Accepted Answer

It would be negatively charged.

Question 10

Which of the following is a correct statement about the zwitterion form of amino acids?

Accepted Answer

It has a positively charged amino group and a negatively charged carboxyl group.

Question 11

Which amino acid side chain is aromatic and hydrophobic?

Accepted Answer

Tryptophan

Question 12

If the pH of a solution is equal to the $$pK_a of an $$amino acid side chain, what is the ratio of base to acid forms?

Accepted Answer

$$1:1$$

Question 13

Which of the following amino acids is classified as polar but uncharged?

Accepted Answer

Serine

Question 14

A mutation replaces a hydrophobic amino acid with a charged amino acid in the core of a protein. What is the most likely effect?

Accepted Answer

The protein may misfold or lose function due to disruption of hydrophobic interactions.

Question 15

Which amino acid is most likely to act as a buffer in physiological pH due to its side chain $$pK_a$$ near 7?

Accepted Answer

Histidine

Question 16

Which of the following is a positively charged amino acid at physiological pH?

Accepted Answer

Lysine

Question 17

A researcher is studying the enzyme chymotrypsin, which operates at pH 8. Which form of histidine's imidazole side chain predominates at this pH?

Accepted Answer

The uncharged (base) form

Question 18

Which of the following best explains why histidine is often found in enzyme active sites?

Accepted Answer

Its side chain can easily switch between charged and uncharged forms near physiological pH.

Question 19

Which amino acid would you expect to be most abundant in a protein that is embedded in a cell membrane?

Accepted Answer

Leucine

Question 20

A solution contains histidine at pH 8. What percentage of the imidazole side chain is in the charged form ($$His^+$$), given $$pK_a = 6$$?

Accepted Answer

$$0.9\%$$

Biochemistry: A Short Course 2

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