Question 1

Which amino acid is most responsible for the strong UV absorbance of proteins at 280 nm, and why is this property useful in protein quantification?

Accepted Answer

Tryptophan, because its aromatic side chain absorbs strongly at 280 nm.

Question 2

A peptide has the sequence Glu-Gly-Ala-Lys. Calculate its net charge at pH 7, given the following pKa values: Glu side chain = 4.2, Lys side chain = 10.0, N-terminus = 9.0, C-terminus = 2.0.

Accepted Answer

$$+1$$

Question 3

Which of the following statements about the peptide bond is CORRECT?

Accepted Answer

The peptide bond is planar and not rotatable due to partial double-bond character.

Question 4

Which amino acid is unique in that its side chain forms a covalent bond with the backbone nitrogen, resulting in a secondary amine?

Accepted Answer

Proline

Question 5

A mutation in hemoglobin replaces a glutamic acid with valine. What is the biochemical consequence of this mutation?

Accepted Answer

It introduces a hydrophobic patch, leading to polymerization of hemoglobin and sickle cell disease.

Question 6

Which of the following amino acids can form disulfide bonds, and what is the chemical reaction involved?

Accepted Answer

Cysteine; $$2\mathrm{RSH} \rightarrow \mathrm{RS{-}SR} + 2\mathrm{H}^+ + 2e^-$$

Question 7

At a pH much higher than the pKa of an acidic amino acid side chain, what is the predominant charge on that side chain?

Accepted Answer

Negatively charged

Question 8

Which chromatography technique would be most effective for separating a mixture of proteins based primarily on their size?

Accepted Answer

Size exclusion chromatography

Question 9

A protein has a pI of 5.0. In an anion exchange column at pH 8.0, what will happen to the protein?

Accepted Answer

It will bind to the column because it is negatively charged.

Question 10

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 11

Which amino acid is most likely to be phosphorylated during cell signaling?

Accepted Answer

Serine

Question 12

What is the effect of placing a negatively charged group near the side chain of glutamic acid on its pKa?

Accepted Answer

It raises the pKa, making it less likely to lose a proton.

Question 13

Which of the following is TRUE regarding the isoelectric point (pI) of a protein?

Accepted Answer

At the pI, the protein has no net charge.

Question 14

Which of the following amino acids is NOT chiral?

Accepted Answer

Glycine

Question 15

Which of the following best explains why L-amino acids predominate in nature?

Accepted Answer

The enzymes responsible for protein synthesis are stereospecific for L-amino acids.

Question 16

Which of the following amino acids has a side chain with a pKa closest to physiological pH, making it important in enzyme catalysis?

Accepted Answer

Histidine

Question 17

A protein sample is contaminated with nucleic acids. Which absorbance ratio would indicate the presence of mostly protein?

Accepted Answer

$$A_{260}/A_{280} \approx 0.5$$

Question 18

Which of the following is a conservative amino acid substitution?

Accepted Answer

Glutamic acid replaced by aspartic acid

Question 19

Which method would be most appropriate for purifying a His-tagged recombinant protein?

Accepted Answer

Immobilized metal affinity chromatography (IMAC)

Question 20

Which of the following best describes the order of elution for proteins of different sizes in size exclusion chromatography?

Accepted Answer

Large proteins elute first, small proteins elute last.

Introduction to Proteins: Structure, Properties, and Purification

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