Question 1

Which of the following statements best describes the structure of an amino acid at physiological pH (pH 7)?

Accepted Answer

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

Question 2

Which amino acid is unique in that it does NOT have a chiral center?

Accepted Answer

Glycine

Question 3

A protein is exposed to an oxidizing environment in the endoplasmic reticulum. Which amino acid is most likely to form a covalent bond as a result?

Accepted Answer

Cysteine

Question 4

Which of the following amino acids contains an aromatic side chain and absorbs UV light at 280 nm, making it useful for protein quantification?

Accepted Answer

Tryptophan

Question 5

Given the following pKa values for glycine: $$pK_{a,1} = 2.4 ($$carboxyl group), $$pK_{a,2} = 9.8 ($$amino group), calculate the isoelectric point ($$pI) of $$glycine.

Accepted Answer

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

Question 6

Which of the following best explains why proline is often found in the cis conformation in proteins?

Accepted Answer

Its cyclic structure reduces the energy difference between cis and trans peptide bonds.

Question 7

A mutation replaces a glutamate (Glu, E) residue with lysine (Lys, K) in a protein. Which of the following changes is most likely to occur at physiological pH?

Accepted Answer

The local charge changes from negative to positive.

Question 8

Which of the following amino acids is most likely to act as a nucleophile at physiological pH due to its side chain pKa being close to 7?

Accepted Answer

Histidine

Question 9

Which property is shared by all biological macromolecules discussed in the study materials?

Accepted Answer

They have 3-dimensional structures that are critical to their function.

Question 10

Which of the following amino acids is most likely to be found in the interior of a globular protein due to its hydrophobic side chain?

Accepted Answer

Leucine

Question 11

A solution contains a mixture of phenylalanine, lysine, and aspartate at pH 7. Which amino acid will have a net positive charge?

Accepted Answer

Lysine

Question 12

Which of the following modifications increases the number of possible side chain chemistries in proteins after translation?

Accepted Answer

Post-translational modifications such as phosphorylation and acetylation

Question 13

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

Accepted Answer

They are non-superimposable mirror images (enantiomers) of each other.

Question 14

Which amino acid side chain is most likely to form hydrogen bonds with water due to the presence of an amide group?

Accepted Answer

Asparagine

Question 15

A solution of a protein is analyzed by UV absorbance at 280 nm. Which amino acid residues contribute most to the absorbance?

Accepted Answer

Tryptophan and tyrosine

Question 16

Which of the following amino acids contains a sulfur atom in its side chain but does NOT form disulfide bonds?

Accepted Answer

Methionine

Question 17

Which of the following best describes the chemical property of the side chain of arginine at physiological pH?

Accepted Answer

It is positively charged due to the guanidinium group.

Question 18

Which of the following amino acids is most likely to be involved in metal ion binding due to its negatively charged side chain at neutral pH?

Accepted Answer

Aspartate

Question 19

A peptide contains the sequence: Ser-Gly-Cys-Asp. Which residue is most likely to form a covalent bond with another peptide chain under oxidizing conditions?

Accepted Answer

Cysteine

Question 20

Which of the following statements about the titration curve of an amino acid with an ionizable side chain (e.g., glutamic acid) is correct?

Accepted Answer

It will have three distinct buffering regions corresponding to the carboxyl, amino, and side chain groups.

Structure and Properties of Amino Acids: Foundations for General Chemistry

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