Question 1

Which of the following statements best explains why glycine is not chiral, while all other common amino acids are?

Accepted Answer

Glycine's side chain is a hydrogen atom, so its alpha carbon is attached to two identical groups.

Question 2

At physiological pH (around 7.4), what is the predominant form of a generic α-amino acid?

Accepted Answer

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

Question 3

Which property of the peptide bond is primarily responsible for its planarity and stability?

Accepted Answer

Delocalization of electrons between the carbonyl oxygen and the amide nitrogen.

Question 4

A researcher wants to quantify the concentration of a protein solution using UV absorbance. Which wavelength should they use and which amino acids contribute most to this absorbance?

Accepted Answer

280 nm; tyrosine and tryptophan

Question 5

Which of the following best describes the process of peptide bond formation during protein biosynthesis?

Accepted Answer

It is a condensation reaction that is not thermodynamically favorable and is coupled to ATP hydrolysis.

Question 6

Which of the following modifications is LEAST likely to occur at the N- or C-terminus of a peptide?

Accepted Answer

Phosphorylation

Question 7

A peptide is placed in a solution with a gradually increasing pH. What happens to its overall charge?

Accepted Answer

The overall charge becomes more negative as pH increases.

Question 8

Which of the following statements about the titration curve of histidine is correct?

Accepted Answer

The pI (isoelectric point) is the pH at which the net charge on histidine is zero.

Question 9

Which of the following best describes the difference between oligopeptides and polypeptides?

Accepted Answer

Oligopeptides have 3–15 residues, while polypeptides have more than 15 residues.

Question 10

Which of the following enzymes would you use to cleave a peptide specifically at the carboxyl side of lysine or arginine residues?

Accepted Answer

Trypsin

Question 11

A mutation changes a serine residue in a protein to a phosphoserine. Which of the following is a likely consequence of this posttranslational modification?

Accepted Answer

It may alter signaling pathways by introducing a negative charge at that position.

Question 12

Which of the following $$pK_a$$ values would you expect for the α-amino group of an amino acid?

Accepted Answer

$$8.0 - 9.0$$

Question 13

Which of the following best explains why peptide bonds are stable under physiological conditions?

Accepted Answer

The activation energy for hydrolysis is high, so hydrolysis requires strong acid, high temperature, or a catalyst.

Question 14

Which of the following is true regarding the classification of amino acids?

Accepted Answer

Amino acids are classified based on the properties of their side chains (R groups).

Question 15

Which of the following is a correct statement about enantiomers of amino acids?

Accepted Answer

L-alanine and D-alanine are non-superimposable mirror images of each other.

Question 16

A peptide has the sequence Glu-Ala-Lys-Gly. At pH 7, what is the net charge on this peptide? (Assume N-terminus is free and C-terminus is free.)

Accepted Answer

$$+1$$

Question 17

Which of the following posttranslational modifications is most likely to play a role in stabilizing the structure of collagen?

Accepted Answer

4-Hydroxyproline

Question 18

Which of the following best describes a zwitterion?

Accepted Answer

A molecule with both a positive and a negative charge, but overall electrically neutral.

Question 19

Which of the following statements about the ionization of peptides is correct?

Accepted Answer

As pH decreases, the overall charge on a peptide becomes more positive.

Amino Acids and Peptides: Structure, Properties, and Biochemical Significance

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