LSCI 211 Biochemistry 1 | Amino Acids:Protein Structures Exam Bank Answered Correctly | Academic year : McGill University.

LSCI 211 Biochemistry 1 | Amino Acids:Protein Structures Exam Bank Answered Correctly | Academic year : McGill University. 1. Which type of bond is NOT considered a weak interaction in protein structure? A) Hydrogen bonds B) Hydrophobic interactions C) Ionic bonds D) Peptide bonds E) Van der Waals forces 2. What is the main driving force for protein folding in aqueous environments? A) Maximization of ionic interactions B) Hydrophobic amino acids being placed in the interior C) Formation of a rigid backbone D) The entropy of water increasing E) The maximization of disulfide bonds 3. Which of the following statements about α-helices is TRUE? A) They are stabilized by hydrogen bonds that run parallel to the helical axis. B) The R-groups project into the helical core. C) The peptide backbone adopts a fully extended conformation. D) Glycine and proline promote α-helix formation. E) They are primarily stabilized by van der Waals forces. 4. Which of the following amino acids is most likely to disrupt an α-helix? A) Alanine B) Valine C) Proline D) Leucine E) Serine 5. What is the primary reason antiparallel β-sheets are more stable than parallel β-sheets? A) They contain more amino acids per strand. B) They have stronger hydrogen bonds that are linear rather than bent. C) They have more hydrophobic interactions. D) The β-turns between strands in parallel sheets are more rigid. E) Parallel β-sheets contain more steric hindrance. 6. Which statement about intrinsically disordered proteins is FALSE? A) They have a high proportion of charged amino acids. B) They do not form stable tertiary structures. C) They often serve as signaling molecules. D) They have strong hydrophobic cores. E) They can adopt multiple conformations when binding to different partners. 7. Which feature is characteristic of peptide bonds? A) They are freely rotatable around the C-N axis. B) They exist in either the cis or trans conformation with equal probability. C) They have partial double-bond character, restricting rotation. D) They form spontaneously without an enzyme in vivo. E) They have an equal electron density between the oxygen and nitrogen atoms. 8. Which of the following amino acids would be most likely to be found in the core of a water- soluble globular protein? A) Aspartate B) Glutamine C) Arginine D) Valine E) Serine 9. In a β-turn, which two amino acids are most commonly found? A) Alanine and Valine B) Glycine and Proline C) Aspartate and Glutamate D) Serine and Threonine E) Tyrosine and Tryptophan 10. The isoelectric point (pI) of an amino acid is defined as: A) The pH at which the amino acid is fully protonated. B) The pH at which the amino acid is fully deprotonated. C) The pH at which the amino acid has a net charge of zero. D) The pH at which only the amino group is protonated. E) The pH at which only the carboxyl group is deprotonated. 11. Which amino acid has a side chain with a pKa close to physiological pH, allowing it to act as a proton donor/acceptor in enzymatic reactions? A) Lysine B) Arginine C) Histidine D) Cysteine E) Tyrosine 12. Disulfide bonds contribute to which level(s) of protein structure? A) Primary only B) Secondary only C) Tertiary and Quaternary D) Only Quaternary E) Only Tertiary 13. What property of collagen allows it to maintain a strong, yet flexible structure? A) Its extensive β-sheet content B) Its high glycine and proline content C) Its amphipathic α-helices D) Its ability to bind metal ions E) Its high hydrophobic amino acid content 14. Which of the following protein structural elements is most responsible for stabilizing the core of a β-barrel structure? A) Disulfide bonds B) Salt bridges C) Hydrogen bonding between β-strands D) Hydrophobic interactions E) Covalent cross-linking 15. What is the significance of the Ramachandran plot in protein structure analysis? A) It predicts the number of α-helices in a protein. B) It shows the sterically allowed dihedral angles for amino acid residues. C) It determines the rate of protein folding. D) It identifies post-translational modifications. E) It calculates the molecular weight of a protein. 16. Which of the following amino acids is most likely to be found at the active site of an enzyme that catalyzes a phosphorylation reaction? A) Phenylalanine B) Serine C) Leucine D) Alanine E) Valine 17. A β-sheet structure is stabilized primarily by: A) Ionic interactions between peptide bonds B) Hydrophobic interactions between adjacent strands C) Hydrogen bonding between backbone atoms D) Disulfide bridges between cysteine residues E) Electrostatic interactions between side chains 18. The primary determinant of a protein's final folded structure is: A) The hydrophobic effect B) The sequence of amino acids C) The number of hydrogen bonds formed D) The presence of disulfide bonds E) The speed of protein synthesis 19. Which structural feature differentiates fibrous proteins from globular proteins? A) The presence of extensive α-helices or β-sheets B) The lack of a tertiary structure C) Their ability to dissolve in water D) The presence of multiple subunits E) Their extensive enzyme activity 20. What is the major reason silk fibroin has high tensile strength but low elasticity? A) It contains extensive α-helices that stretch under tension. B) It contains disulfide bonds that prevent movement. C) It consists mainly of β-sheets held together by hydrogen bonds. D) It has a high content of charged amino acids. E) It undergoes constant enzymatic remodeling. 1. C - Silk fibroin consists of β-sheets stabilized by hydrogen bonds, giving it tensile strength but low elasticity. 1. Which of the following amino acids is most likely to be found in the hydrophobic core of a transmembrane protein? A) Glutamate B) Serine C) Phenylalanine D) Lysine E) Aspartate 2. Which of the following modifications is commonly seen in collagen and contributes to its structural stability? A) Phosphorylation B) Hydroxylation of proline C) Glycosylation of serine D) Ubiquitination E) Acetylation of lysine 3. Which statement about the β-sheet structure is FALSE? A) Hydrogen bonds occur between peptide backbones in adjacent strands. B) Parallel β-sheets are more stable than antiparallel β-sheets. C) Large, bulky side chains can disrupt β-sheet formation. D) β-turns often contain proline and glycine residues. E) β-sheets can form twisted or barrel-like structures. 4. Which amino acid is known to form covalent disulfide bonds, stabilizing protein tertiary and quaternary structures? A) Methionine B) Serine C) Cysteine D) Lysine E) Proline 5. The α-helix is stabilized by: A) Ionic interactions between side chains B) Hydrogen bonding between the carbonyl oxygen of residue n n n and the amide hydrogen of residue n + 4 n+4 n+4 C) Disulfide bonds between cysteine residues D) Hydrophobic interactions between side chains E) Electrostatic interactions between the N-terminus and C-terminus 6. The Ramachandran plot is useful for determining: A) Protein solubility B) The stability of disulfide bonds C) The allowable phi (ϕ) and psi (ψ) angles of the peptide backbone D) The tertiary structure of proteins E) The role of water in protein folding 7. Which of the following is an example of a structural motif commonly found in globular proteins? A) β-barrel B) Zinc finger C) Leucine zipper D) Helix-turn-helix E) All of the above 8. A mutation replacing a polar amino acid with a nonpolar amino acid in an α-helix is most likely to: A) Increase the solubility of the protein B) Increase the helical stability if the substitution occurs on the exterior of the helix C) Destabilize the helix if the substitution occurs in the interior D) Have no effect on the protein structure E) Increase the flexibility of the helix 9. What is the most important factor that determines whether an amino acid is in its protonated or deprotonated state? A) The isoelectric point of the protein B) The pH of the surrounding environment C) The hydrophobicity of the amino acid D) The secondary structure of the protein E) The presence of disulfide bonds 10. The peptide bond has partial double-bond character, which means: A) It allows for free rotation around the bond. B) It is always in the cis conformation. C) It restricts rotation around the C-N bond. D) It is more stable in β-sheets than in α-helices. E) It can form covalent cross-links with neighboring peptides. 11. Which of the following amino acids is most likely to be found in the active site of an enzyme that uses acid-base catalysis? A) Leucine B) Valine C) Histidine D) Isoleucine E) Alanine 12. Which statement about protein folding is FALSE? A) Proteins fold into their lowest energy conformation. B) Hydrophobic interactions help drive protein folding. C) The primary structure determines the final folded structure. D) Molecular chaperones are never needed for proper folding. E) Misfolded proteins can lead to diseases such as Alzheimer’s. 13. Which level of protein structure describes the arrangement of multiple polypeptide subunits? A) Primary B) Secondary C) Tertiary D) Quaternary E) Motif 14. The amino acid proline disrupts α-helices because: A) It is too large to fit inside the helix. B) It introduces steric hindrance and lacks an amide hydrogen for hydrogen bonding. C) It forms disulfide bonds with neighboring amino acids. D) It is highly hydrophobic and destabilizes the helix. E) It interacts strongly with the peptide backbone. 15. Which of the following proteins is an example of a fibrous protein? A) Hemoglobin B) Myoglobin C) Keratin D) DNA polymerase E) Insulin 16. What type of interaction primarily stabilizes the tertiary structure of globular proteins? A) Hydrophobic interactions B) Hydrogen bonding C) Ionic interactions D) Disulfide bonding E) All of the above 17. In the context of protein structure, what is a "domain"? A) A small peptide sequence within a protein B) A part of a protein that can fold and function independently C) A section of a β-sheet D) The region of the protein that interacts with DNA E) A repeating amino acid sequence found in fibrous proteins 18. Which of the following statements about myoglobin and hemoglobin is TRUE? A) Both are fibrous proteins involved in structural support. B) Myoglobin exhibits cooperative oxygen binding, while hemoglobin does not. C) Hemoglobin contains four subunits, while myoglobin is a single polypeptide chain. D) Hemoglobin is more abundant in muscle tissue than myoglobin. E) Myoglobin has a lower affinity for oxygen than hemoglobin. 19. What type of secondary structure is most commonly found in membrane-spanning regions of proteins? A) Parallel β-sheets B) Antiparallel β-sheets C) α-helices D) β-turns E) Random coil 20. Which of the following statements about protein denaturation is FALSE? A) Heat can disrupt hydrogen bonds, leading to unfolding. B) Changes in pH can alter the charge of amino acids, destabilizing the protein. C) Denaturation always leads to complete loss of protein function. D) Detergents can disrupt hydrophobic interactions in membrane proteins. E) Reducing agents can break disulfide bonds, affecting tertiary structure. Figure A shows the structure of the Bacillus subtilis, a bacteria, PerR-Zn DNA binding protein. The Per-Zn protein binds to itself, which can be seen in Figure B, each unit is colored in green and red. Question 1 Figure A reveals the presence of both α-helices and β-sheets. Given that PerR-Zn is a DNA- binding protein, what structural feature is most likely directly responsible for interacting with DNA? A) The α-helical region in the upper portion of Figure A, which likely forms a helix-turn-helix (HTH) motif. B) The β-sheet core, which provides the necessary rigidity for DNA recognition. C) The disordered loops, which contain the most flexible amino acid residues. D) The hydrophobic core, which stabilizes the protein-DNA interface. E) The C-terminal domain, which is responsible for nonspecific DNA interactions. Question 2: The PerR-Zn protein contains a zinc ion (Zn²⁺), which plays a critical structural role. Which of the following amino acids is most likely coordinating the zinc ion in the PerR-Zn structure? A) Histidine and Cysteine, due to their strong affinity for metal ions through nitrogen and sulfur interactions. B) Arginine and Glutamate, due to their ability to form electrostatic interactions with Zn²⁺. C) Serine and Threonine, as their hydroxyl (-OH) groups can stabilize metal ions. D) Phenylalanine and Tryptophan, due to their aromatic rings forming a cation-π interaction with Zn²⁺. E) Lysine and Aspartate, due to their involvement in hydrogen bonding and metal chelation. Question 3: The PerR-Zn protein has both α-helices and β-sheets, as shown in Figure A. Which secondary structure is most commonly found in DNA-binding proteins, and why? A) α-helices, because their cylindrical shape fits into the major groove of DNA, allowing sequence-specific binding. B) β-sheets, because their extended structure can wrap around the DNA backbone. C) Random coils, because their flexibility allows them to adapt to different DNA sequences. D) Parallel β-sheets, because they can form extensive hydrogen bonds with nucleotide bases. E) Disulfide-linked helices, because they provide structural rigidity required for DNA recognition. Question 4: Figure B shows the dimerized form of PerR-Zn, which is critical for its function. Which of the following interactions is least likely to be involved in stabilizing the dimer interface? A) Hydrophobic interactions, which drive the burial of nonpolar residues away from solvent exposure. B) Disulfide bonds, since bacterial transcription factors rarely contain inter-subunit covalent linkages. C) Ionic interactions, where charged residues (e.g., Arg, Glu, Lys) stabilize the interface via electrostatic attraction. D) Hydrogen bonds, which form between backbone amides or polar side chains to stabilize the dimer. E) Van der Waals forces, which provide fine-tuned structural complementarity between monomers. Question 5: Proline is often called a "helix breaker", yet it may be strategically placed in DNA-binding proteins like PerR-Zn. What is the most probable structural role of proline in such a protein? A) It introduces sharp bends in loops, which may enhance flexibility for DNA binding. B) It stabilizes α-helices by providing additional hydrogen-bonding interactions. C) It forms π-stacking interactions with DNA bases to strengthen binding. D) It acts as an electron donor, stabilizing zinc coordination. E) It prevents dimerization by disrupting β-sheet interactions. Question 6: DNA-binding proteins, such as PerR-Zn, often show conservation of specific amino acids in their binding domains. Which of the following amino acids is least likely to be enriched in the PerR-Zn DNA-binding interface? A) Lysine, because its positive charge facilitates interaction with negatively charged DNA. B) Arginine, which has a flexible guanidinium group that can form hydrogen bonds with DNA bases. C) Aspartate, which is negatively charged and would repel the DNA phosphate backbone. D) Glutamine, which participates in sequence-specific hydrogen bonding with DNA bases. E) Serine, which provides polar contacts through hydroxyl interactions. 1. Which of the following properties is unique to glycine compared to all other amino acids? A) It has an R-group that contains sulfur. B) It has no chiral center. C) It has an imidazole side chain. D) It has a guanidinium functional group. 2. Which of the following amino acids is most likely to be found in the interior of a globular protein? A) Lysine B) Aspartate C) Valine D) Glutamine 3. Which of the following amino acids is classified as an α-helix breaker? A) Alanine B) Leucine C) Glycine D) Methionine 4. Which type of bond is responsible for stabilizing the secondary structure of proteins? A) Peptide bonds B) Hydrogen bonds C) Disulfide bonds D) Ionic interactions 5. The primary driving force behind the folding of globular proteins is: A) Hydrogen bonding between polar groups. B) Hydrophobic interactions. C) Electrostatic interactions. D) Covalent cross-linking. 6. Which amino acid contains a sulfhydryl (-SH) group that can form disulfide bonds? A) Methionine B) Serine C) Cysteine D) Tyrosine 7. Which of the following statements about peptide bonds is correct? A) Peptide bonds are freely rotatable. B) Peptide bonds have partial double-bond character. C) Peptide bonds are formed through hydrolysis. D) Peptide bonds are weaker than hydrogen bonds. 8. What is the primary function of intrinsically disordered proteins? A) They catalyze metabolic reactions. B) They function in highly specific, rigid structural roles. C) They interact with multiple protein partners and remain flexible. D) They act as energy storage molecules. 9. What type of secondary structure is most commonly found in membrane- spanning proteins? A) Parallel β-sheets B) Antiparallel β-sheets C) α-helices D) β-barrels 10. In the collagen triple helix, which amino acid is found at every third position? A) Proline B) Glycine C) Lysine D) Serine 11. The α-helix is stabilized by hydrogen bonds between: A) The side chains of adjacent amino acids. B) The carbonyl oxygen of residue n n n and the amide hydrogen of residue n + 4 n+4 n+4. C) The first and last residues in the helix. D) The sulfur atoms in cysteine residues. 12. Which of the following interactions is primarily responsible for stabilizing β-sheets? A) Hydrogen bonds between backbone amide and carbonyl groups B) Hydrophobic interactions between nonpolar side chains C) Salt bridges between charged amino acids D) Covalent bonding between adjacent strands 13. Which protein structure level involves the formation of motifs such as β- barrels and helix-turn-helix structures? A) Primary B) Secondary C) Tertiary D) Quaternary 14. Which of the following amino acids is most likely to be found in the turns of a β-sheet? A) Glycine B) Alanine C) Leucine D) Valine 15. What is the primary function of disulfide bonds in protein structure? A) They help proteins interact with DNA. B) They stabilize tertiary and quaternary structures. C) They facilitate ionic interactions in enzyme active sites. D) They prevent protein degradation by proteases. 16. Why are parallel β-sheets less stable than antiparallel β-sheets? A) The hydrogen bonds in parallel β-sheets are distorted compared to those in antiparallel β- sheets. B) Parallel β-sheets contain fewer amino acids per strand. C) Parallel β-sheets have increased steric hindrance. D) The R-groups in parallel β-sheets are more hydrophilic. 17. Which statement best describes the concept of a protein domain? A) A domain is a stable, independently folded region of a protein that often has a specific function. B) A domain refers to a single α-helix within a protein. C) A domain is the same as a motif. D) A domain only exists in quaternary structures. 18. Which of the following amino acids is the most important for enzyme catalysis due to its ability to donate and accept protons? A) Arginine B) Glutamate C) Histidine D) Cysteine 19. Which of the following statements about α-keratin and silk fibroin is correct? A) α-Keratin contains β-sheets, while silk fibroin contains α-helices. B) Silk fibroin is flexible because of its extensive α-helical content. C) α-Keratin is strengthened by disulfide bonds, whereas silk fibroin is stabilized by van der Waals interactions. D) α-Keratin has a globular structure, while silk fibroin has a fibrous structure. 20. Which of the following statements about quaternary structure is FALSE? A) Quaternary structure involves multiple polypeptide subunits. B) Hemoglobin is an example of a protein with quaternary structure. C) Quaternary structure is stabilized solely by disulfide bonds. D) Subunit interactions in quaternary structures can involve hydrogen bonding and hydrophobic interactions. 1. Based on the glycine titration curve (first graph), what is the predominant form of glycine at pH = 5.97? A) Fully protonated form, H₃N⁺-CH₂-COOH B) Zwitterionic form, H₃N⁺-CH₂-COO⁻ C) Fully deprotonated form, H₂N-CH₂-COO⁻ D) A mixture of equal concentrations of the protonated and deprotonated forms 8. Based on the first glycine titration curve, which of the following points corresponds to the buffering region of glycine? A) pH between 5.97 and 9.60 B) pH below 2.34 C) pH exactly at 5.97 D) pH between 9.60 and 11.30 4. The second titration curve represents a general amino acid. Which of the following statements is true about the equivalence points on this titration curve? A) Each equivalence point represents the complete neutralization of an acidic proton. B) The first equivalence point represents the removal of the α-carboxyl proton, while the second represents the removal of the α-amino proton. C) The equivalence points occur at pH = 7, regardless of the amino acid type. D) The second equivalence point represents a mixture of protonated and deprotonated species at equal concentrations. 2. Referencing the titration curve of glutamate (sixth graph), what is the net charge of glutamate at its isoelectric point (pI = 3.22)? A) +1 B) 0 C) -1 D) -2 7. The titration curve of glutamate (sixth graph) shows three pKa values. What is the functional significance of the middle pKa (pK_R = 4.25)? A) It represents the dissociation of the α-carboxyl group. B) It represents the dissociation of the α-amino group. C) It represents the dissociation of the carboxyl group on the side chain. D) It represents the isoelectric point of glutamate. 9. Why does the titration curve of glutamate (sixth graph) differ from that of glycine (first graph)? A) Glutamate has an additional ionizable group in its side chain. B) Glycine has a more complex titration curve due to its secondary amine group. C) Glutamate does not have an α-carboxyl group, while glycine does. D) Glycine undergoes protonation and deprotonation more slowly than glutamate.