BCH 4053 Final Exam V1 | BCH 4053
Biochemistry I | Actual Q&A with
Rationale (BCH4053 Final Exam) |
University of Central Florida
1. When a weak acid is placed in a solution where the pH is equal to its pKa, what is the ratio
of the conjugate base to the weak acid?
A. 10:1
B. 1:1
C. 1:10
D. 2:1
Answer: B
Rationale: According to the Henderson-Hasselbalch equation, pH = pKa + log([A-]/[HA]).
When pH equals pKa, the log([A-]/[HA]) term must be zero, which occurs only when the
concentration of the conjugate base equals the concentration of the acid. This point
represents the center of the buffering region for a weak acid.
2. Which of the following amino acids contains a side chain that can form a disulfide bond?
A. Cysteine
B. Methionine
C. Serine
,D. Alanine
Answer: A
Rationale: Cysteine contains a highly reactive sulfhydryl or thiol group on its side chain.
Under oxidizing conditions, two cysteine residues can react to form a covalent disulfide
bridge. This linkage is a critical component of tertiary and quaternary protein structure
stabilization.
3. In thermodynamics, if the change in Gibbs free energy (delta G) is negative, the reaction is
described as:
A. Endothermic and spontaneous
B. Endergonic and non-spontaneous
C. At equilibrium
D. Exergonic and spontaneous
Answer: D
Rationale: A negative delta G indicates that the system is releasing free energy to the
surroundings. Such reactions are termed exergonic and occur spontaneously without the
input of external energy. This thermodynamic property determines the direction of
metabolic pathways in biochemical systems.
4. The alpha-helix secondary structure of proteins is primarily stabilized by hydrogen bonds
between:
A. The carbonyl oxygen and the amino hydrogen of the polypeptide backbone
, B. Amino acid side chains
C. Hydrophobic interactions between non-polar residues
D. Peptide bonds and water molecules
Answer: A
Rationale: The alpha-helix is stabilized by intrachain hydrogen bonds between the
carbonyl oxygen of the nth residue and the amide hydrogen of the (n+4)th residue. This
pattern creates a rigid, right-handed spiral structure with 3.6 residues per turn. The side
chains project outward from the helix axis to minimize steric hindrance.
5. A competitive inhibitor affects an enzyme-catalyzed reaction by:
A. Increasing the apparent Km while leaving Vmax unchanged
B. Decreasing the Vmax and increasing the Km
C. Decreasing the Vmax and decreasing the Km
D. Decreasing the Km while leaving Vmax unchanged
Answer: A
Rationale: Competitive inhibitors compete with the substrate for binding at the active site
of the enzyme. Since the inhibitor can be outcompeted by high concentrations of substrate,
the maximum velocity (Vmax) of the reaction remains reachable. However, more substrate
is required to reach half of the Vmax, resulting in an increase in the apparent Michaelis
constant (Km).
Biochemistry I | Actual Q&A with
Rationale (BCH4053 Final Exam) |
University of Central Florida
1. When a weak acid is placed in a solution where the pH is equal to its pKa, what is the ratio
of the conjugate base to the weak acid?
A. 10:1
B. 1:1
C. 1:10
D. 2:1
Answer: B
Rationale: According to the Henderson-Hasselbalch equation, pH = pKa + log([A-]/[HA]).
When pH equals pKa, the log([A-]/[HA]) term must be zero, which occurs only when the
concentration of the conjugate base equals the concentration of the acid. This point
represents the center of the buffering region for a weak acid.
2. Which of the following amino acids contains a side chain that can form a disulfide bond?
A. Cysteine
B. Methionine
C. Serine
,D. Alanine
Answer: A
Rationale: Cysteine contains a highly reactive sulfhydryl or thiol group on its side chain.
Under oxidizing conditions, two cysteine residues can react to form a covalent disulfide
bridge. This linkage is a critical component of tertiary and quaternary protein structure
stabilization.
3. In thermodynamics, if the change in Gibbs free energy (delta G) is negative, the reaction is
described as:
A. Endothermic and spontaneous
B. Endergonic and non-spontaneous
C. At equilibrium
D. Exergonic and spontaneous
Answer: D
Rationale: A negative delta G indicates that the system is releasing free energy to the
surroundings. Such reactions are termed exergonic and occur spontaneously without the
input of external energy. This thermodynamic property determines the direction of
metabolic pathways in biochemical systems.
4. The alpha-helix secondary structure of proteins is primarily stabilized by hydrogen bonds
between:
A. The carbonyl oxygen and the amino hydrogen of the polypeptide backbone
, B. Amino acid side chains
C. Hydrophobic interactions between non-polar residues
D. Peptide bonds and water molecules
Answer: A
Rationale: The alpha-helix is stabilized by intrachain hydrogen bonds between the
carbonyl oxygen of the nth residue and the amide hydrogen of the (n+4)th residue. This
pattern creates a rigid, right-handed spiral structure with 3.6 residues per turn. The side
chains project outward from the helix axis to minimize steric hindrance.
5. A competitive inhibitor affects an enzyme-catalyzed reaction by:
A. Increasing the apparent Km while leaving Vmax unchanged
B. Decreasing the Vmax and increasing the Km
C. Decreasing the Vmax and decreasing the Km
D. Decreasing the Km while leaving Vmax unchanged
Answer: A
Rationale: Competitive inhibitors compete with the substrate for binding at the active site
of the enzyme. Since the inhibitor can be outcompeted by high concentrations of substrate,
the maximum velocity (Vmax) of the reaction remains reachable. However, more substrate
is required to reach half of the Vmax, resulting in an increase in the apparent Michaelis
constant (Km).
