Question 1

Which of the following statements best explains why proteins are constructed from a set of 20 standard amino acids?

Accepted Answer

Using 20 amino acids allows for a vast diversity of protein sequences and functions.

Question 2

Which amino acid is unique among the standard set because it is not chiral?

Accepted Answer

Glycine

Question 3

At physiological pH ($$pH \approx 7.4$$), what is the net charge of glutamate?

Accepted Answer

-1

Question 4

Which amino acid side chain can form a covalent disulfide bond, and why is this important for protein structure?

Accepted Answer

Cysteine; it stabilizes protein structure by forming disulfide bonds.

Question 5

Which of the following amino acids can be phosphorylated as a post-translational modification?

Accepted Answer

Serine, Threonine, and Tyrosine

Question 6

A peptide bond is formed between which two functional groups of amino acids?

Accepted Answer

The carboxyl group of one amino acid and the amino group of another

Question 7

Which of the following best describes a zwitterion?

Accepted Answer

A molecule with both positive and negative charges but a net charge of zero

Question 8

Given the following pKa values for glycine: $$pK_1 = 2.34$ (carboxyl group), pK_2$ = 9.60 ($$amino group), calculate the isoelectric point ($$pI) $$for glycine.

Accepted Answer

$$pI = \frac{2.34 + 9.60}{2} = 5.97$$

Question 9

Which amino acid is most likely to be found on the surface of a soluble protein, interacting with water?

Accepted Answer

Serine

Question 10

At a pH below the $$pK_a of $$the carboxyl group, what is the predominant form of the carboxyl group in an amino acid?

Accepted Answer

$$COOH$$

Question 11

Which amino acid is responsible for the umami taste and is commonly used as a flavor enhancer?

Accepted Answer

Glutamate

Question 12

A solution contains histidine at $$pH = 7.4. $$Given that the imidazole side chain of histidine has a $$pK_a \approx 6.0$$, what is the predominant charge on the side chain?

Accepted Answer

Neutral (uncharged)

Question 13

Which of the following amino acids is aromatic and can be modified by phosphorylation?

Accepted Answer

Tyrosine

Question 14

If a protein contains a high proportion of non-polar, aliphatic amino acids, what is the most likely location of these residues in a globular protein?

Accepted Answer

In the core, away from water

Question 15

Which amino acid is most likely to participate in acid-base catalysis in enzyme active sites due to its side chain $$pK_a$$ near physiological pH?

Accepted Answer

Histidine

Question 16

Which of the following is a correct statement about the titration curve of a diprotic amino acid?

Accepted Answer

It has two buffering regions corresponding to the carboxyl and amino groups.

Question 17

Calculate the isoelectric point ($$pI) $$for glutamate, given $$pK_1 = 2.19$ (carboxyl group) and pK_2$ = 4.25 ($$side chain carboxyl group).

Accepted Answer

$$pI = \frac{2.19 + 4.25}{2} = 3.22$$

Question 18

Which of the following amino acids is most likely to be found at a polypeptide turn due to its small size and flexibility?

Accepted Answer

Glycine

Question 19

Which enzyme removes phosphoryl groups from proteins, reversing the action of kinases?

Accepted Answer

Phosphatase

Question 20

A patient is found to have a mutation that replaces a cysteine residue with a serine in insulin. What is the most likely effect on the protein?

Accepted Answer

Loss of disulfide bond formation, potentially destabilizing the protein structure

Amino Acids, Peptides, and Proteins: Structures, Properties, and Acid-Base Chemistry

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