Question 1

Which of the following best describes the structure of an $$ \alpha -$$amino acid at physiological pH (pH 7)?

Accepted Answer

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

Question 2

Which amino acid is classified as a nonpolar aromatic amino acid?

Accepted Answer

Phenylalanine

Question 3

What is the primary driving force for the formation of a peptide bond between two amino acids?

Accepted Answer

Removal of a water molecule (condensation reaction)

Question 4

Given the free energy change for peptide bond formation at room temperature in aqueous solution is $$ \Delta G \approx +10 \ \mathrm{kJ/mol} $$, what does this imply about the reaction?

Accepted Answer

Peptide bond formation is thermodynamically unfavored and requires energy input.

Question 5

Which enzyme would cleave a polypeptide at the carboxyl side of lysine and arginine residues?

Accepted Answer

Trypsin

Question 6

A peptide sequence is written as 'IFDTH'. Which residue is at the N-terminal end?

Accepted Answer

Isoleucine

Question 7

Which of the following statements about the geometry of the peptide bond is correct?

Accepted Answer

The peptide bond is planar due to partial double-bond character.

Question 8

Which amino acid side chain is most likely to be positively charged at physiological pH?

Accepted Answer

Arginine

Question 9

If a polypeptide is hydrolyzed in 6 M HCl, what is the expected outcome?

Accepted Answer

Complete cleavage of peptide bonds, yielding free amino acids.

Question 10

Which of the following best explains why polypeptides are metastable in aqueous solution?

Accepted Answer

They hydrolyze rapidly only under extreme conditions or in the presence of catalysts.

Question 11

Which property of amino acids allows proteins to act as polyampholytes?

Accepted Answer

Amino acid side chains have a range of pKa values and can be ionized.

Question 12

Which of the following is a consequence of the Bohr effect in hemoglobin?

Accepted Answer

Decreased pH leads to decreased affinity for oxygen.

Question 13

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

Accepted Answer

The defined number and order of amino acid residues in the polypeptide chain.

Question 14

A mutation changes a codon from $$ \mathrm{GAA}  to$$ $$ \mathrm{GAG} . $$What is the effect on the protein sequence?

Accepted Answer

No change; both codons code for glutamic acid.

Question 15

Which of the following is true regarding the directionality of polypeptide synthesis?

Accepted Answer

Polypeptides are synthesized from the N-terminal to the C-terminal.

Question 16

Which codon is used as the start signal for translation in most organisms?

Accepted Answer

AUG

Question 17

A patient has a mutation in the gene encoding hemoglobin that results in a nonconservative substitution. What is the most likely effect?

Accepted Answer

The protein may lose its normal function due to a significant change in amino acid properties.

Question 18

Which of the following best describes the relationship between DNA, RNA, and protein synthesis?

Accepted Answer

DNA is transcribed to RNA, which is then translated to protein.

Question 19

Which amino acid is most likely to be found at the active site of a protease due to its ability to participate in acid-base catalysis?

Accepted Answer

Histidine

Introduction to Proteins: Structure, Function, and Sequence

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