Unit 4: Amino Acids and Proteins - Practice Quiz

1 What is the common structural feature of most amino acids?

2 What is the variable part of an amino acid called?

3 At approximately neutral pH, many amino acids exist mainly as:

4 Which group gives amino acids their acidic property?

5 Which amino acids must be obtained from the diet because the body cannot make enough of them?

6 Which type of amino acid has a side chain that does not readily interact with water?

7 Which amino acids have side chains that can carry a negative charge at physiological pH?

8 Which amino acids have side chains that can carry a positive charge at physiological pH?

9 What does a titration curve of an amino acid show?

10 What is the isoelectric point of an amino acid?

11 A buffer solution resists changes in:

12 During titration with a strong acid, an amino acid generally:

13 Proteins are polymers made from repeating units called:

14 What type of bond joins amino acids in a protein chain?

15 Which protein function involves speeding up biochemical reactions?

16 Which protein is primarily involved in transporting oxygen in red blood cells?

17 What does the primary structure of a protein describe?

18 Which structures are common examples of secondary protein structure?

19 What does the tertiary structure of a protein describe?

20 Quaternary structure is present when a protein:

21 At physiological pH, the amino group of a typical amino acid is mainly present as which form?

22 Which feature allows most amino acids, except glycine, to exist as optical isomers?

23 Why does alanine generally have a higher melting point than many small nonpolar organic compounds of similar molecular mass?

24 Which amino acid would most likely be found on the surface of a globular protein exposed to water?

25 Which amino acid is classified as conditionally essential because its requirement can increase during rapid growth or illness?

26 A mutation replaces a surface serine residue with leucine. What is the most likely effect on local protein solubility?

27 Which pair contains only amino acids with acidic side chains at physiological pH?

28 Why is proline often considered structurally distinct from other standard amino acids?

29 During titration of glycine, at the isoelectric point, the predominant species has which overall charge?

30 For an amino acid without an ionizable side chain, which expression is used to estimate its isoelectric point?

31 A buffer is most effective when the pH is close to the relevant because at that point:

32 An amino acid has an ionizable acidic side chain. Which pair of values is generally averaged to estimate its isoelectric point?

33 Why can a single amino acid substitution substantially affect protein function?

34 An enzyme loses activity after its active-site conformation is disrupted, although its peptide bonds remain intact. Which explanation is most appropriate?

35 A protein contains two polypeptide chains associated through noncovalent interactions. This feature belongs primarily to which structural level?

36 Which interaction most directly stabilizes an alpha helix within a single polypeptide chain?

37 Breaking all disulfide bonds in a protein would most directly disrupt which structural feature?

38 A protein unfolds when exposed to high concentrations of urea but regains activity after urea removal. What does this observation suggest?

39 Which treatment is most likely to denature a protein by disrupting ionic interactions and hydrogen bonding without necessarily breaking peptide bonds?

40 Why can heating cause an irreversible loss of protein activity even when the peptide chain is not hydrolyzed?

41 An amino acid has a nonionizable side chain and pKa values of 2.3 for its carboxyl group and 9.6 for its amino group. At pH 6.0, which ionic form predominates and what is its net charge?

42 Why does substitution of glycine for alanine often have a greater structural effect in an α-helix than the reverse substitution?

43 A peptide contains an amino acid whose side-chain carboxyl group has a pKa of 4.1. At pH 7.4, which change is most likely if this residue is replaced by lysine?

44 Which classification best explains why replacing leucine with valine is often less disruptive to a protein core than replacing leucine with aspartate?

45 Which amino acid is classified as conditionally essential because its synthesis may become insufficient during rapid growth or illness, despite being synthesized in healthy adults?

46 A mutation replaces cysteine with serine in a secreted protein containing several disulfide bonds. Which consequence is most directly expected?

47 An amino acid has pKa values of 2.2, 9.7, and 4.0 for its α-carboxyl, α-amino, and side-chain carboxyl groups, respectively. What is its approximate isoelectric point?

48 For histidine, the pKa values of the α-carboxyl, imidazolium side chain, and α-amino groups are approximately 1.8, 6.0, and 9.2. Which species predominates near pH 7.6?

49 For histidine, the pKa values of the α-carboxyl, imidazolium side chain, and α-amino groups are approximately 1.8, 6.0, and 9.2. Which species predominates near pH 7.6?

50 At the pH equal to the pKa of an amino acid's α-amino group, what fraction of that group is protonated under ideal Henderson–Hasselbalch conditions?

51 An amino acid has pKa values of 2.0 and 10.0 and no ionizable side chain. At pH 6.0, which statement about its buffering capacity is most accurate?

52 A protein binds oxygen cooperatively, and ligand binding at one subunit increases the affinity of the remaining subunits. Which molecular feature most directly accounts for this behavior?

53 An enzyme loses catalytic activity after a mutation replaces a conserved histidine near its active site with alanine, although the protein remains soluble. Which conclusion is most justified?

54 Why can a single amino acid substitution cause severe disease even when it changes only one residue in a protein?

55 Which interaction is classified as a primary-structure feature rather than a secondary-, tertiary-, or quaternary-structure feature?

56 A peptide bond is planar primarily because of which property?

57 A protein contains α-helices stabilized by backbone hydrogen bonds, but its three-dimensional shape is disrupted when buried hydrophobic residues are exposed to solvent. Which structural levels are being distinguished?

58 A protein unfolds in 8 M urea but regains activity after urea removal if its reducing environment is carefully controlled. What does this demonstrate?

59 A protein remains unfolded after removal of a denaturant because incorrect disulfide bonds form during refolding. Which treatment would most directly improve renaturation?

60 A protein loses enzymatic activity after heating but retains its peptide bonds. Which interpretation is most appropriate?