Question 1

Which of the following statements best describes the general structure of a standard amino acid found in proteins?

Accepted Answer

It contains a central α-carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable R group.

Question 2

At physiological pH (pH 7), what is the predominant form of a standard amino acid?

Accepted Answer

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

Question 3

Which amino acid is NOT chiral and why?

Accepted Answer

Glycine, because its side chain is a hydrogen atom, making the α-carbon achiral

Question 4

Which of the following amino acids has a side chain with a pKa closest to physiological pH, allowing it to act as a buffer in biological systems?

Accepted Answer

Histidine ($$pK_a = 6.0)$$

Question 5

A peptide bond forms between two amino acids as a result of which type of reaction?

Accepted Answer

Condensation (dehydration) reaction, releasing $$\mathrm{H_2O}$$

Question 6

Which amino acid side chain can form disulfide bonds, and what is the significance of this bond in proteins?

Accepted Answer

Cysteine; disulfide bonds stabilize protein tertiary and quaternary structure

Question 7

Which of the following amino acids is classified as nonpolar and hydrophobic?

Accepted Answer

Leucine

Question 8

What is the net charge of the tripeptide Cys–Gly–Asn at pH 7?

Accepted Answer

0

Question 9

Which of the following amino acids absorbs the most UV light at 280 nm, and why is this property useful in biochemistry?

Accepted Answer

Tryptophan; it allows for protein quantification using UV spectroscopy

Question 10

Which of the following best explains why enzymes are stereospecific and typically only catalyze reactions involving L-amino acids?

Accepted Answer

The active site of enzymes is chiral and complementary only to the L-enantiomer

Question 11

Which of the following amino acids contains a sulfur atom in its side chain?

Accepted Answer

Methionine

Question 12

Which of the following modifications is commonly found in collagen and is essential for its structural stability?

Accepted Answer

Hydroxylation of proline to form hydroxyproline

Question 13

Which amino acid is most likely to be found at the active site of an enzyme that catalyzes acid-base reactions near neutral pH?

Accepted Answer

Histidine

Question 14

Which of the following statements about the pKa values of amino acid side chains is correct?

Accepted Answer

The pKa of the ε-amino group of lysine is approximately 10.5

Question 15

Which of the following best describes the function of Edman’s reagent in protein sequencing?

Accepted Answer

It reacts with the N-terminal amino acid to label and remove it for identification

Question 16

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

Accepted Answer

Glutamine

Question 17

Which of the following is a biologically active derivative of an amino acid that functions as a neurotransmitter?

Accepted Answer

γ-Aminobutyric acid (GABA)

Question 18

Which of the following amino acids is aromatic and can participate in π-π stacking interactions?

Accepted Answer

Phenylalanine

Question 19

Which of the following best explains why the pKa values of ionizable groups differ between free amino acids and amino acid residues in polypeptides?

Accepted Answer

The local environment within a polypeptide can alter the pKa by stabilizing or destabilizing charged forms

Amino Acids: Structure, Properties, and Biological Roles

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