MCAT BIOLOGICAL & BIOCHEMICAL FOUNDATIONS OF LIVING
SYSTEMS EXAMINATION COMPLETE QUESTIONS AND
DETAILED SOLUTIONS LATEST UPDATE THIS YEAR JUST
RELEASED
1.
A researcher replaces several hydrophobic amino acids buried within a
globular protein with charged amino acids while preserving the
protein's overall length. Which consequence is most likely?
A. Increased formation of peptide bonds
B. Destabilization of the protein's hydrophobic core
C. Elimination of all hydrogen bonding
D. Conversion of the protein into a carbohydrate
Answer: B
Rationale: Hydrophobic amino acids normally stabilize the interior of
globular proteins by minimizing exposure to water. Replacing them
with charged residues can disrupt this hydrophobic core and destabilize
folding.
2.
A mutation changes one amino acid in an enzyme's active site without
changing the overall concentration of enzyme present. Which
observation most directly demonstrates altered catalytic activity?
,A. Increased enzyme molecular weight
B. Decreased substrate concentration
C. Changed reaction velocity at a fixed substrate concentration
D. Increased number of peptide bonds
Answer: C
Rationale: Enzyme activity is directly evaluated by measuring reaction
velocity. A change in active-site structure can alter substrate binding or
catalytic efficiency without changing enzyme abundance.
3.
An enzyme exhibits a lower apparent Km after a mutation, while its
Vmax remains unchanged under identical experimental conditions.
Which interpretation is most appropriate?
A. The enzyme has reduced catalytic capacity
B. The enzyme has increased apparent substrate affinity
C. The enzyme has become permanently denatured
D. The enzyme concentration has necessarily decreased
Answer: B
Rationale: A lower Km generally indicates that a lower substrate
concentration is required to reach half-maximal velocity, reflecting
increased apparent substrate affinity.
4.
,A competitive inhibitor is added to an enzyme-catalyzed reaction while
substrate concentration is progressively increased. Which kinetic
change should be expected?
A. Vmax decreases permanently
B. Km decreases substantially
C. Vmax remains unchanged while apparent Km increases
D. Both Vmax and Km decrease
Answer: C
Rationale: Competitive inhibitors compete with substrate for the active
site. Increasing substrate concentration can overcome inhibition,
leaving Vmax unchanged but increasing the apparent Km.
5.
A protein contains four polypeptide chains that associate through
noncovalent interactions to form its functional complex. Which
structural level describes this arrangement?
A. Primary structure
B. Secondary structure
C. Tertiary structure
D. Quaternary structure
Answer: D
Rationale: Quaternary structure describes the association and spatial
organization of multiple independently folded polypeptide subunits
within one functional protein complex.
, 6.
A mutation replaces a cysteine residue with alanine in a protein
containing several disulfide bonds. Which structural interaction would
most directly be disrupted?
A. Peptide bonding
B. Phosphodiester bonding
C. Disulfide bonding
D. Glycosidic bonding
Answer: C
Rationale: Two cysteine side chains can undergo oxidation to form
disulfide bonds, which contribute substantially to tertiary and
sometimes quaternary protein structure.
7.
A researcher lowers the pH surrounding an enzyme whose active site
contains several acidic amino acids. Why might catalytic activity change
substantially?
A. Protonation can alter active-site charge and substrate interactions
B. Peptide bonds immediately become phosphodiester bonds
C. All amino acids become hydrophobic at low pH
D. Enzymes lose their genetic code
Answer: A
Rationale: Changes in pH alter amino-acid side-chain protonation
states. This can change charge distribution, substrate binding, catalytic
chemistry, and protein conformation.
SYSTEMS EXAMINATION COMPLETE QUESTIONS AND
DETAILED SOLUTIONS LATEST UPDATE THIS YEAR JUST
RELEASED
1.
A researcher replaces several hydrophobic amino acids buried within a
globular protein with charged amino acids while preserving the
protein's overall length. Which consequence is most likely?
A. Increased formation of peptide bonds
B. Destabilization of the protein's hydrophobic core
C. Elimination of all hydrogen bonding
D. Conversion of the protein into a carbohydrate
Answer: B
Rationale: Hydrophobic amino acids normally stabilize the interior of
globular proteins by minimizing exposure to water. Replacing them
with charged residues can disrupt this hydrophobic core and destabilize
folding.
2.
A mutation changes one amino acid in an enzyme's active site without
changing the overall concentration of enzyme present. Which
observation most directly demonstrates altered catalytic activity?
,A. Increased enzyme molecular weight
B. Decreased substrate concentration
C. Changed reaction velocity at a fixed substrate concentration
D. Increased number of peptide bonds
Answer: C
Rationale: Enzyme activity is directly evaluated by measuring reaction
velocity. A change in active-site structure can alter substrate binding or
catalytic efficiency without changing enzyme abundance.
3.
An enzyme exhibits a lower apparent Km after a mutation, while its
Vmax remains unchanged under identical experimental conditions.
Which interpretation is most appropriate?
A. The enzyme has reduced catalytic capacity
B. The enzyme has increased apparent substrate affinity
C. The enzyme has become permanently denatured
D. The enzyme concentration has necessarily decreased
Answer: B
Rationale: A lower Km generally indicates that a lower substrate
concentration is required to reach half-maximal velocity, reflecting
increased apparent substrate affinity.
4.
,A competitive inhibitor is added to an enzyme-catalyzed reaction while
substrate concentration is progressively increased. Which kinetic
change should be expected?
A. Vmax decreases permanently
B. Km decreases substantially
C. Vmax remains unchanged while apparent Km increases
D. Both Vmax and Km decrease
Answer: C
Rationale: Competitive inhibitors compete with substrate for the active
site. Increasing substrate concentration can overcome inhibition,
leaving Vmax unchanged but increasing the apparent Km.
5.
A protein contains four polypeptide chains that associate through
noncovalent interactions to form its functional complex. Which
structural level describes this arrangement?
A. Primary structure
B. Secondary structure
C. Tertiary structure
D. Quaternary structure
Answer: D
Rationale: Quaternary structure describes the association and spatial
organization of multiple independently folded polypeptide subunits
within one functional protein complex.
, 6.
A mutation replaces a cysteine residue with alanine in a protein
containing several disulfide bonds. Which structural interaction would
most directly be disrupted?
A. Peptide bonding
B. Phosphodiester bonding
C. Disulfide bonding
D. Glycosidic bonding
Answer: C
Rationale: Two cysteine side chains can undergo oxidation to form
disulfide bonds, which contribute substantially to tertiary and
sometimes quaternary protein structure.
7.
A researcher lowers the pH surrounding an enzyme whose active site
contains several acidic amino acids. Why might catalytic activity change
substantially?
A. Protonation can alter active-site charge and substrate interactions
B. Peptide bonds immediately become phosphodiester bonds
C. All amino acids become hydrophobic at low pH
D. Enzymes lose their genetic code
Answer: A
Rationale: Changes in pH alter amino-acid side-chain protonation
states. This can change charge distribution, substrate binding, catalytic
chemistry, and protein conformation.
