Q: Which of the following equations correctly represents the calculation of the net charge of a peptide at a given pH?

$\text{Net charge} = \sum (\text{fraction protonated} \times \text{charge of group})$

Question 1

Which of the following best describes the peptide bond formation between two amino acids?

Accepted Answer

It is a condensation reaction that forms a covalent bond and releases water: $\mathrm{R_1-COOH} + \mathrm{H_2N-R_2} \rightarrow \mathrm{R_1-CONH-R_2} + \mathrm{H_2O}$.

Question 2

At physiological pH ($\approx 7.4$), which amino acid side chain is most likely to be positively charged?

Accepted Answer

Lysine

Question 3

Which of the following amino acids contains a sulfur atom in its side chain and can form disulfide bonds?

Accepted Answer

Cysteine

Question 4

Which level of protein structure is primarily stabilized by hydrogen bonds between backbone atoms, forming $\alpha$-helices and $\beta$-sheets?

Accepted Answer

Secondary structure

Question 5

A mutation replaces a glutamate residue (pK$_a$ $\approx 4.1$) with a lysine residue (pK$_a$ $\approx 10.5$) in a protein. What is the most likely effect on the protein's net charge at pH 7.4?

Accepted Answer

The net charge will become more positive.

Question 6

Which of the following best explains why glycine is unique among the standard amino acids?

Accepted Answer

Its side chain is a single hydrogen atom, making it achiral.

Question 7

Which of the following equations correctly represents the calculation of the net charge of a peptide at a given pH?

Accepted Answer

$	ext{Net charge} = \sum (	ext{fraction protonated} 	imes 	ext{charge of group})$

Question 8

Which property is shared by all nonpolar, aliphatic amino acids?

Accepted Answer

They have hydrophobic side chains that tend to cluster in the protein interior.

Question 9

Which of the following statements about the quaternary structure of proteins is correct?

Accepted Answer

It describes the arrangement of multiple polypeptide subunits in a protein complex.

Question 10

A peptide has the sequence Ala-Glu-Lys-Ser. At pH 7.4, which of the following best describes the net charge of the peptide?

Accepted Answer

Net charge is positive due to the lysine side chain.

Question 11

Which of the following amino acids is most likely to be found on the surface of a soluble, globular protein?

Accepted Answer

Aspartate

Question 12

Which of the following best describes the role of chaperone proteins in cells?

Accepted Answer

They assist in the proper folding of newly synthesized polypeptides.

Question 13

Which equation best represents the Henderson-Hasselbalch equation used to estimate the charge state of an ionizable group?

Accepted Answer

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

Question 14

Which of the following amino acids is classified as aromatic and can absorb UV light at 280 nm?

Accepted Answer

Tryptophan

Question 15

Which of the following is a correct statement about the isoelectric point ($pI$) of an amino acid?

Accepted Answer

It is the pH at which the amino acid has a net zero charge.

Question 16

Which of the following best explains why proline disrupts $\alpha$-helices in protein secondary structure?

Accepted Answer

Its side chain forms a covalent bond with the amino group, restricting backbone flexibility.

Question 17

Which of the following is NOT a component of the backbone of a polypeptide chain?

Accepted Answer

R group (side chain)

Question 18

Which of the following best describes the tertiary structure of a protein?

Accepted Answer

The overall three-dimensional arrangement of all atoms in a single polypeptide chain.

Question 19

Which of the following amino acids is most likely to be phosphorylated during cell signaling?

Accepted Answer

Serine

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