Question 1

Which of the following statements best explains why glycine is not optically active, unlike other standard amino acids?

Accepted Answer

Glycine lacks a chiral center because its side chain is a hydrogen atom, making all four substituents on the alpha carbon not unique.

Question 2

At physiological pH (approximately 7.4), which form does a typical amino acid (without an ionizable side chain) predominantly exist in?

Accepted Answer

As a zwitterion, with a positively charged amino group and a negatively charged carboxylate group.

Question 3

Which amino acid is most likely to be found at the active site of an enzyme catalyzing a nucleophilic attack, and why?

Accepted Answer

Histidine, because its imidazole side chain has a pK_a near physiological pH and can act as both a proton donor and acceptor.

Question 4

A solution contains 0.1 M glycine. Given that the pK_a values for glycine are $$pK_{a1} = 2.4 ($$carboxyl) and $$pK_{a2} = 9.8 ($$amino), at what pH will glycine have no net charge (isoelectric point, $$pI$$)?

Accepted Answer

$$pI = \frac{2.4 + 9.8}{2} = 6.1$$

Question 5

Which of the following amino acids contains a sulfur atom in its side chain and is capable of forming disulfide bonds?

Accepted Answer

Cysteine

Question 6

A protein is exposed to a reducing agent such as dithiothreitol (DTT). What is the most likely effect on the protein's structure?

Accepted Answer

Disulfide bonds between cysteine residues will be broken, potentially altering the protein's tertiary or quaternary structure.

Question 7

Which amino acid is most likely to be found in the interior of a globular protein, and why?

Accepted Answer

Leucine, because its nonpolar, hydrophobic side chain favors the protein's interior.

Question 8

Which of the following best describes the difference between L- and D- amino acids in biological systems?

Accepted Answer

L-amino acids are the predominant form incorporated into proteins in living organisms.

Question 9

Which amino acid has a side chain that can act as a nucleophile due to its thiol group, and what is the approximate pK_a of this group?

Accepted Answer

Cysteine; pK_a \approx 8.5

Question 10

A protein sample absorbs strongly at 280 nm. Which amino acids are most likely responsible for this absorbance?

Accepted Answer

Tryptophan and tyrosine

Question 11

Which of the following amino acids is most likely to be phosphorylated during post-translational modification?

Accepted Answer

Serine

Question 12

Which property of proline most significantly affects protein secondary structure?

Accepted Answer

Its cyclic structure restricts backbone flexibility, often introducing kinks or turns.

Question 13

A solution contains a mixture of lysine and glutamate at pH 7. Which statement best describes their charges?

Accepted Answer

Lysine is positively charged, and glutamate is negatively charged.

Question 14

Which of the following best explains why histidine is often found at the active sites of enzymes?

Accepted Answer

Its imidazole side chain has a pK_a near physiological pH, allowing it to act as both a proton donor and acceptor.

Question 15

Which of the following amino acids is most likely to participate in hydrogen bonding as both a donor and acceptor due to its amide side chain?

Accepted Answer

Glutamine

Question 16

A mutation replaces a glutamate residue with a lysine residue in a protein. Which of the following changes is most likely to occur at physiological pH?

Accepted Answer

A negatively charged side chain is replaced with a positively charged side chain.

Question 17

Which of the following best describes the CORN mnemonic for identifying the L-enantiomer of amino acids?

Accepted Answer

When viewing the molecule with the hydrogen atom pointing away, the groups COOH, R, and NH$$_2$$ are arranged clockwise.

Question 18

Which of the following amino acids is most likely to bind metal ions due to its negatively charged side chain at physiological pH?

Accepted Answer

Aspartate

Question 19

A solution of alanine is titrated with NaOH. The first equivalence point is reached at pH 2.4, and the second at pH 9.8. What is the predominant ionic form of alanine at pH 11?

Accepted Answer

The amino group is neutral (NH$$_2$$), and the carboxyl group is deprotonated (COO$$^-).$$

Structure and Properties of Amino Acids: Biochemistry Study Notes

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