Question 1

Which of the following statements best describes the structure of an amino acid as shown in the first image?

Accepted Answer

Each amino acid contains a central carbon atom (Cα) bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable side chain (R group).

Question 2

What type of bond forms between two amino acids during protein synthesis, as illustrated in the images showing the removal of a water molecule?

Accepted Answer

A peptide bond forms between the carboxyl group of one amino acid and the amino group of another, releasing a molecule of water.

Question 3

Given the table of pKₐ values, what is the net charge of lysine at physiological pH (pH 7)?

Accepted Answer

The net charge is +1 because the side chain amino group (pKₐ = 10.5) is protonated (+1), the backbone amino group (pKₐ = 9.0) is protonated (+1), and the backbone carboxyl group (pKₐ = 2.2) is deprotonated (−1).

Question 4

Which amino acid is achiral and why?

Accepted Answer

Glycine is achiral because its side chain is a hydrogen atom, making two groups attached to the α-carbon identical.

Question 5

Which amino acids are most likely to absorb UV light at 280 nm, as shown in the absorbance spectrum?

Accepted Answer

Tryptophan and tyrosine, due to their aromatic side chains.

Question 6

According to the Beer-Lambert Law, which equation correctly relates absorbance (A) to concentration (c), path length (b), and extinction coefficient (ε)?

Accepted Answer

$$A = \varepsilon c b$$

Question 7

If a protein solution absorbs 90% of the incident light at 280 nm, what is the absorbance (A) at this wavelength?

Accepted Answer

$$A = -\log_{10}(0.1) = 1$$

Question 8

Which of the following best explains why only L-amino acids are found in proteins synthesized by living organisms?

Accepted Answer

Enzymes responsible for protein synthesis are stereospecific and only recognize L-amino acids.

Question 9

Which amino acid side chains can form disulfide bonds, and under what conditions are these bonds most stable?

Accepted Answer

Cysteine side chains can form disulfide bonds, which are most stable in oxidizing environments outside the cell.

Question 10

A solution of glycine is titrated with NaOH. At the isoelectric point (pI), what is the predominant form of glycine?

Accepted Answer

The zwitterion, with a deprotonated carboxyl group ($$\mathrm{COO^-}) $$and a protonated amino group ($$\mathrm{NH_3$^+}).$$

Question 11

Which amino acid is most likely to be found at the active site of a serine protease, and why?

Accepted Answer

Serine, because its side chain contains a reactive hydroxyl group with a pKₐ near 8.3.

Question 12

Which modification neutralizes the positive charge on lysine residues in histones, thereby promoting gene transcription?

Accepted Answer

Acetylation of the ε-amino group of lysine.

Question 13

Which of the following amino acids is most likely to be positively charged at physiological pH (pH 7)?

Accepted Answer

Arginine, because its side chain pKₐ is 12.5.

Question 14

Which property of amino acids is responsible for their optical activity, as demonstrated by the rotation of plane-polarized light?

Accepted Answer

The presence of a chiral α-carbon with four different groups attached.

Question 15

Which amino acid is most likely to be involved in phosphorylation reactions catalyzed by kinases?

Accepted Answer

Serine, because it contains a hydroxyl group that can accept a phosphate group.

Question 16

A protein has a pI of 5.0. In a buffer at pH 7.0, what type of ion exchange chromatography would best be used to purify this protein?

Accepted Answer

Anion exchange chromatography, because the protein will be negatively charged at pH 7.0.

Question 17

Which of the following best describes the effect of a reducing agent such as DTT on a protein containing disulfide bonds?

Accepted Answer

It breaks the disulfide bonds by donating electrons, converting them to free thiol groups.

Question 18

Which amino acid is most likely to be found at the site of ubiquitination in a protein?

Accepted Answer

Lysine, because ubiquitin forms an isopeptide bond with the ε-amino group of lysine.

Amino Acids and Proteins: Structure, Properties, and Spectroscopy

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