SGU FTV Biochemistry Exam 1
Questions with Correct Answers and
Rationales
Question 1
Which of the following interactions is primarily responsible for the folding of a
soluble globular protein?
A) Hydrogen bonds between polar side chains
B) Hydrophobic interactions between nonpolar side chains
C) Ionic bonds between charged side chains
D) Disulfide bridges between cysteine residues
Correct Answer: B
Explanation: Hydrophobic interactions drive the folding of soluble proteins by
clustering nonpolar side chains in the interior, away from water. Hydrogen bonds
and ionic bonds stabilize the folded structure but are not the primary driving force.
Question 2
At physiological pH (7.4), the alpha-amino group of an amino acid exists primarily
in which form?
A) –NH₂
B) –NH₃⁺
C) –NH⁻
D) –N⁺H₂
Correct Answer: B
Explanation: The alpha-amino group has a pKa around 9–10, so at pH 7.4 it is
protonated and positively charged (–NH₃⁺). The alpha-carboxyl group (pKa ~2) is
deprotonated (–COO⁻), giving a zwitterion.
Question 3
,Which amino acid side chain can form a covalent cross-link within or between
polypeptide chains?
A) Methionine
B) Cysteine
C) Serine
D) Lysine
Correct Answer: B
Explanation: Cysteine contains a thiol group (–SH) that can oxidize to form
disulfide bonds (–S–S–), which are covalent cross-links that stabilize protein
structure.
Question 4
A protein that functions as an enzyme loses its activity when the pH is changed
from 7.4 to 4.0. This is most likely due to:
A) Hydrolysis of peptide bonds
B) Protonation of critical side chains altering active site geometry
C) Denaturation by chaotropic agents
D) Oxidation of methionine residues
Correct Answer: B
Explanation: Extreme pH changes protonate or deprotonate amino acid side
chains (e.g., histidine, glutamate), disrupting ionic bonds and hydrogen bonds that
maintain the active site conformation, leading to reversible or irreversible loss of
function.
Question 5
Which of the following best describes the term "zwitterion"?
A) A molecule with two positive charges
B) A molecule with no net electrical charge but containing both positive and
negative charges
C) A molecule that can act as both an acid and a base
D) A molecule that has only one ionizable group
,Correct Answer: B
Explanation: A zwitterion is a dipolar ion with equal numbers of positive and
negative charges, giving a net charge of zero. Amino acids at neutral pH exist as
zwitterions (NH₃⁺ and COO⁻).
Question 6
Which pair of amino acids contains side chains that can form hydrogen bonds with
water?
A) Leucine and valine
B) Serine and glutamine
C) Phenylalanine and tryptophan
D) Isoleucine and alanine
Correct Answer: B
Explanation: Serine has a hydroxyl group (–OH) and glutamine has an amide
group (–CONH₂); both are polar and uncharged, capable of forming hydrogen
bonds. Leucine, valine, phenylalanine, tryptophan, isoleucine, and alanine are
nonpolar and hydrophobic.
Question 7
The three-dimensional structure of a protein is ultimately determined by:
A) The pH of the cellular environment
B) The amino acid sequence (primary structure)
C) The presence of chaperone proteins only
D) Post-translational modifications exclusively
Correct Answer: B
Explanation: The amino acid sequence (primary structure) contains all the
information needed for a protein to fold into its native three-dimensional structure,
as demonstrated by Anfinsen’s experiment.
Question 8
, Which of the following amino acids is most likely to be found on the surface of a
membrane protein that spans the lipid bilayer?
A) Aspartic acid
B) Lysine
C) Leucine
D) Glutamine
Correct Answer: C
Explanation: Leucine is a nonpolar, hydrophobic amino acid. On a
membrane-spanning protein, the regions that contact the lipid tails are rich in
nonpolar residues, so leucine would be on the surface that interacts with the
hydrophobic core of the bilayer.
Question 9
Which amino acid has a side chain that contains a secondary amino group (imino
group) and introduces a kink into polypeptide chains?
A) Glycine
B) Proline
C) Tryptophan
D) Cysteine
Correct Answer: B
Explanation: Proline has a cyclic side chain that bonds to the backbone nitrogen,
forming an imino acid. This rigid structure restricts rotation and often introduces
turns or kinks in protein folds.
Question 10
In a protein dissolved in an aqueous buffer at pH 7.4, which of the following amino
acid side chains would carry a positive charge?
A) Glutamate
B) Lysine
C) Aspartate
D) Tyrosine
Questions with Correct Answers and
Rationales
Question 1
Which of the following interactions is primarily responsible for the folding of a
soluble globular protein?
A) Hydrogen bonds between polar side chains
B) Hydrophobic interactions between nonpolar side chains
C) Ionic bonds between charged side chains
D) Disulfide bridges between cysteine residues
Correct Answer: B
Explanation: Hydrophobic interactions drive the folding of soluble proteins by
clustering nonpolar side chains in the interior, away from water. Hydrogen bonds
and ionic bonds stabilize the folded structure but are not the primary driving force.
Question 2
At physiological pH (7.4), the alpha-amino group of an amino acid exists primarily
in which form?
A) –NH₂
B) –NH₃⁺
C) –NH⁻
D) –N⁺H₂
Correct Answer: B
Explanation: The alpha-amino group has a pKa around 9–10, so at pH 7.4 it is
protonated and positively charged (–NH₃⁺). The alpha-carboxyl group (pKa ~2) is
deprotonated (–COO⁻), giving a zwitterion.
Question 3
,Which amino acid side chain can form a covalent cross-link within or between
polypeptide chains?
A) Methionine
B) Cysteine
C) Serine
D) Lysine
Correct Answer: B
Explanation: Cysteine contains a thiol group (–SH) that can oxidize to form
disulfide bonds (–S–S–), which are covalent cross-links that stabilize protein
structure.
Question 4
A protein that functions as an enzyme loses its activity when the pH is changed
from 7.4 to 4.0. This is most likely due to:
A) Hydrolysis of peptide bonds
B) Protonation of critical side chains altering active site geometry
C) Denaturation by chaotropic agents
D) Oxidation of methionine residues
Correct Answer: B
Explanation: Extreme pH changes protonate or deprotonate amino acid side
chains (e.g., histidine, glutamate), disrupting ionic bonds and hydrogen bonds that
maintain the active site conformation, leading to reversible or irreversible loss of
function.
Question 5
Which of the following best describes the term "zwitterion"?
A) A molecule with two positive charges
B) A molecule with no net electrical charge but containing both positive and
negative charges
C) A molecule that can act as both an acid and a base
D) A molecule that has only one ionizable group
,Correct Answer: B
Explanation: A zwitterion is a dipolar ion with equal numbers of positive and
negative charges, giving a net charge of zero. Amino acids at neutral pH exist as
zwitterions (NH₃⁺ and COO⁻).
Question 6
Which pair of amino acids contains side chains that can form hydrogen bonds with
water?
A) Leucine and valine
B) Serine and glutamine
C) Phenylalanine and tryptophan
D) Isoleucine and alanine
Correct Answer: B
Explanation: Serine has a hydroxyl group (–OH) and glutamine has an amide
group (–CONH₂); both are polar and uncharged, capable of forming hydrogen
bonds. Leucine, valine, phenylalanine, tryptophan, isoleucine, and alanine are
nonpolar and hydrophobic.
Question 7
The three-dimensional structure of a protein is ultimately determined by:
A) The pH of the cellular environment
B) The amino acid sequence (primary structure)
C) The presence of chaperone proteins only
D) Post-translational modifications exclusively
Correct Answer: B
Explanation: The amino acid sequence (primary structure) contains all the
information needed for a protein to fold into its native three-dimensional structure,
as demonstrated by Anfinsen’s experiment.
Question 8
, Which of the following amino acids is most likely to be found on the surface of a
membrane protein that spans the lipid bilayer?
A) Aspartic acid
B) Lysine
C) Leucine
D) Glutamine
Correct Answer: C
Explanation: Leucine is a nonpolar, hydrophobic amino acid. On a
membrane-spanning protein, the regions that contact the lipid tails are rich in
nonpolar residues, so leucine would be on the surface that interacts with the
hydrophobic core of the bilayer.
Question 9
Which amino acid has a side chain that contains a secondary amino group (imino
group) and introduces a kink into polypeptide chains?
A) Glycine
B) Proline
C) Tryptophan
D) Cysteine
Correct Answer: B
Explanation: Proline has a cyclic side chain that bonds to the backbone nitrogen,
forming an imino acid. This rigid structure restricts rotation and often introduces
turns or kinks in protein folds.
Question 10
In a protein dissolved in an aqueous buffer at pH 7.4, which of the following amino
acid side chains would carry a positive charge?
A) Glutamate
B) Lysine
C) Aspartate
D) Tyrosine
