Question 1

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

Accepted Answer

Glycine is achiral because its side chain is a hydrogen atom, making its alpha carbon have two identical substituents.

Question 2

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

Accepted Answer

The amino group is protonated and the carboxyl group is deprotonated, forming a zwitterion.

Question 3

Which amino acid is capable of forming a covalent disulfide bond with another of its kind, thereby stabilizing protein structure?

Accepted Answer

Cysteine

Question 4

A mutation in hemoglobin replaces glutamate (Glu) with valine (Val) at position 6. Which property of these amino acids is most relevant to the resulting disease phenotype?

Accepted Answer

Glutamate is acidic and hydrophilic, while valine is nonpolar and hydrophobic.

Question 5

Which of the following amino acids absorbs UV light most strongly at $\lambda = 280\ \mathrm{nm}$?

Accepted Answer

Tryptophan

Question 6

Which of the following best describes the peptide bond in proteins?

Accepted Answer

It has partial double bond character, restricting rotation and creating planarity.

Question 7

A tripeptide is synthesized from three different amino acids. How many unique sequences are possible if each position can be any of the 20 standard amino acids?

Accepted Answer

$20^3 = 8,000$

Question 8

Which of the following amino acids is essential for humans and contains a sulfur atom in its side chain?

Accepted Answer

Methionine

Question 9

At pH 8.0, what is the approximate fraction of the alpha-amino group ($pK_a \approx 9.0$) that is protonated? Use the Henderson-Hasselbalch equation: $pH = pK_a + \log\left(\frac{[A^-]}{[HA]}\right)$.

Accepted Answer

About 90% protonated

Question 10

Which amino acid side chain has a $pK_a$ closest to physiological pH and can act as a buffer in proteins?

Accepted Answer

Histidine

Question 11

A patient is given a drug that increases renal excretion of bicarbonate ($\mathrm{HCO}_3^-$) to treat acute mountain sickness. Why does this help restore acid-base balance?

Accepted Answer

It compensates for respiratory alkalosis by lowering blood pH.

Question 12

Which of the following amino acids is most likely to be found in the interior of a transmembrane protein domain?

Accepted Answer

Leucine

Question 13

Which of the following best defines the 'primary structure' of a protein?

Accepted Answer

The sequence of amino acids linked by peptide bonds.

Question 14

Which of the following amino acids is NOT chiral?

Accepted Answer

Glycine

Question 15

Which amino acid substitution is least likely to affect the overall charge of a protein at physiological pH?

Accepted Answer

Lysine to Arginine

Question 16

Which of the following statements about peptide bonds is TRUE?

Accepted Answer

Peptide bonds are planar due to partial double bond character.

Question 17

Which of the following amino acids has a side chain that can be titrated within the physiological pH range?

Accepted Answer

Histidine ($pK_a \approx 6.0$)

Question 18

Which of the following is a correct single-letter code for the amino acid tryptophan?

Accepted Answer

W

Question 19

Which of the following best explains why the distance between alpha carbons in a polypeptide is almost always 3.8 Å?

Accepted Answer

It is determined by the length of the peptide bond and the planarity imposed by its partial double bond character.

Question 20

Which of the following amino acids is both aromatic and polar?

Accepted Answer

Tyrosine

Amino Acids: Properties, Structures, and Peptide Bonds

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