BCH 4053 Exam 4 V1 | BCH 4053 Biochemistry I | Actual Q&A with
Rationale (BCH4053 Exam 4) | University of Central Florida
1. Which enzyme catalyzes the primary rate-limiting and committed step of glycolysis?
A. Hexokinase
B. Phosphofructokinase-1
C. Pyruvate Kinase
D. Aldolase
Answer: B
Explanation: Phosphofructokinase-1 (PFK-1) is the most important regulatory enzyme in
the glycolytic pathway. It catalyzes the irreversible conversion of fructose-6-phosphate to
fructose-1,6-bisphosphate using ATP. This step is highly regulated by allosteric effectors
like ATP, AMP, and fructose-2,6-bisphosphate to match the cell’s energy needs.
2. In the first step of gluconeogenesis, which cofactor is required by pyruvate carboxylase to
fix CO2?
A. Thiamine pyrophosphate
B. Niacin
C. Biotin
D. Pyridoxal phosphate
Answer: C
Explanation: Pyruvate carboxylase requires biotin as a prosthetic group to carry the
activated carboxyl group. The enzyme utilizes ATP to activate bicarbonate before
transferring it to pyruvate to form oxaloacetate. This reaction occurs exclusively in the
mitochondria and is a key regulatory step in the synthesis of glucose.
3. Which of the following molecules acts as a potent allosteric activator of PFK-1 and an
inhibitor of Fructose-1,6-bisphosphatase-1?
A. Citrate
B. ATP
C. Fructose-2,6-bisphosphate
D. Glucose-6-phosphate
Answer: C
,Explanation: Fructose-2,6-bisphosphate is a signaling molecule that synchronizes
glycolysis and gluconeogenesis. It increases the affinity of PFK-1 for its substrate while
simultaneously decreasing the activity of the gluconeogenic enzyme FBPase-1. This ensures
that both pathways do not operate at high rates simultaneously, preventing a futile cycle.
4. Which complex in the Electron Transport Chain does not pump protons into the
mitochondrial intermembrane space?
A. Complex I
B. Complex III
C. Complex II
D. Complex IV
Answer: C
Explanation: Complex II, also known as succinate dehydrogenase, transfers electrons from
succinate to ubiquinone via FADH2. Unlike Complexes I, III, and IV, it does not have the
machinery to pump protons across the inner mitochondrial membrane. Consequently,
electrons entering the chain through Complex II contribute less to the proton motive force
than those entering through Complex I.
5. What is the net yield of ATP produced per molecule of glucose during anaerobic
fermentation to lactate?
A. 4 ATP
B. 2 ATP
C. 30 ATP
D. 32 ATP
Answer: B
Explanation: Anaerobic fermentation involves glycolysis followed by the reduction of
pyruvate to lactate to regenerate NAD+. Since oxygen is absent, the NADH produced in
glycolysis cannot be used in oxidative phosphorylation. Therefore, the cell only nets the 2
ATP molecules generated through substrate-level phosphorylation in the glycolytic
pathway.
6. The pyruvate dehydrogenase (PDH) complex requires which of the following cofactors for
the E1 subunit reaction?
A. FAD
B. Coenzyme A
C. NAD+
D. Thiamine pyrophosphate (TPP)
, Answer: D
Explanation: The E1 subunit of the PDH complex, pyruvate dehydrogenase, utilizes
thiamine pyrophosphate (TPP) to decarboxylate pyruvate. TPP forms a covalent
intermediate with the hydroxyethyl group after CO2 is released. This is the first of several
steps required to convert pyruvate into acetyl-CoA for entry into the Citric Acid Cycle.
7. Which intermediate of the Citric Acid Cycle is a pro-chiral molecule that is converted to a
chiral molecule by aconitase?
A. Oxaloacetate
B. Malate
C. Succinate
D. Citrate
Answer: D
Explanation: Citrate is a symmetric, pro-chiral molecule produced by the condensation of
acetyl-CoA and oxaloacetate. The enzyme aconitase distinguishes between the two arms of
citrate to specifically produce isocitrate. This stereospecificity is crucial for the subsequent
oxidative decarboxylation steps in the cycle.
8. Which enzyme in the Citric Acid Cycle catalyzes a substrate-level phosphorylation to
produce GTP (or ATP)?
A. Isocitrate dehydrogenase
B. Succinyl-CoA synthetase
C. Alpha-ketoglutarate dehydrogenase
D. Malate dehydrogenase
Answer: B
Explanation: Succinyl-CoA synthetase catalyzes the conversion of succinyl-CoA to
succinate, coupled with the phosphorylation of GDP to GTP (or ADP to ATP). This is the
only step in the Citric Acid Cycle that directly generates a high-energy nucleoside
triphosphate. The energy for this reaction comes from the thioester bond cleavage of
succinyl-CoA.
9. Which compound inhibits Complex IV by binding to the iron center of cytochrome c
oxidase?
A. Rotenone
B. Antimycin A
C. Cyanide
Rationale (BCH4053 Exam 4) | University of Central Florida
1. Which enzyme catalyzes the primary rate-limiting and committed step of glycolysis?
A. Hexokinase
B. Phosphofructokinase-1
C. Pyruvate Kinase
D. Aldolase
Answer: B
Explanation: Phosphofructokinase-1 (PFK-1) is the most important regulatory enzyme in
the glycolytic pathway. It catalyzes the irreversible conversion of fructose-6-phosphate to
fructose-1,6-bisphosphate using ATP. This step is highly regulated by allosteric effectors
like ATP, AMP, and fructose-2,6-bisphosphate to match the cell’s energy needs.
2. In the first step of gluconeogenesis, which cofactor is required by pyruvate carboxylase to
fix CO2?
A. Thiamine pyrophosphate
B. Niacin
C. Biotin
D. Pyridoxal phosphate
Answer: C
Explanation: Pyruvate carboxylase requires biotin as a prosthetic group to carry the
activated carboxyl group. The enzyme utilizes ATP to activate bicarbonate before
transferring it to pyruvate to form oxaloacetate. This reaction occurs exclusively in the
mitochondria and is a key regulatory step in the synthesis of glucose.
3. Which of the following molecules acts as a potent allosteric activator of PFK-1 and an
inhibitor of Fructose-1,6-bisphosphatase-1?
A. Citrate
B. ATP
C. Fructose-2,6-bisphosphate
D. Glucose-6-phosphate
Answer: C
,Explanation: Fructose-2,6-bisphosphate is a signaling molecule that synchronizes
glycolysis and gluconeogenesis. It increases the affinity of PFK-1 for its substrate while
simultaneously decreasing the activity of the gluconeogenic enzyme FBPase-1. This ensures
that both pathways do not operate at high rates simultaneously, preventing a futile cycle.
4. Which complex in the Electron Transport Chain does not pump protons into the
mitochondrial intermembrane space?
A. Complex I
B. Complex III
C. Complex II
D. Complex IV
Answer: C
Explanation: Complex II, also known as succinate dehydrogenase, transfers electrons from
succinate to ubiquinone via FADH2. Unlike Complexes I, III, and IV, it does not have the
machinery to pump protons across the inner mitochondrial membrane. Consequently,
electrons entering the chain through Complex II contribute less to the proton motive force
than those entering through Complex I.
5. What is the net yield of ATP produced per molecule of glucose during anaerobic
fermentation to lactate?
A. 4 ATP
B. 2 ATP
C. 30 ATP
D. 32 ATP
Answer: B
Explanation: Anaerobic fermentation involves glycolysis followed by the reduction of
pyruvate to lactate to regenerate NAD+. Since oxygen is absent, the NADH produced in
glycolysis cannot be used in oxidative phosphorylation. Therefore, the cell only nets the 2
ATP molecules generated through substrate-level phosphorylation in the glycolytic
pathway.
6. The pyruvate dehydrogenase (PDH) complex requires which of the following cofactors for
the E1 subunit reaction?
A. FAD
B. Coenzyme A
C. NAD+
D. Thiamine pyrophosphate (TPP)
, Answer: D
Explanation: The E1 subunit of the PDH complex, pyruvate dehydrogenase, utilizes
thiamine pyrophosphate (TPP) to decarboxylate pyruvate. TPP forms a covalent
intermediate with the hydroxyethyl group after CO2 is released. This is the first of several
steps required to convert pyruvate into acetyl-CoA for entry into the Citric Acid Cycle.
7. Which intermediate of the Citric Acid Cycle is a pro-chiral molecule that is converted to a
chiral molecule by aconitase?
A. Oxaloacetate
B. Malate
C. Succinate
D. Citrate
Answer: D
Explanation: Citrate is a symmetric, pro-chiral molecule produced by the condensation of
acetyl-CoA and oxaloacetate. The enzyme aconitase distinguishes between the two arms of
citrate to specifically produce isocitrate. This stereospecificity is crucial for the subsequent
oxidative decarboxylation steps in the cycle.
8. Which enzyme in the Citric Acid Cycle catalyzes a substrate-level phosphorylation to
produce GTP (or ATP)?
A. Isocitrate dehydrogenase
B. Succinyl-CoA synthetase
C. Alpha-ketoglutarate dehydrogenase
D. Malate dehydrogenase
Answer: B
Explanation: Succinyl-CoA synthetase catalyzes the conversion of succinyl-CoA to
succinate, coupled with the phosphorylation of GDP to GTP (or ADP to ATP). This is the
only step in the Citric Acid Cycle that directly generates a high-energy nucleoside
triphosphate. The energy for this reaction comes from the thioester bond cleavage of
succinyl-CoA.
9. Which compound inhibits Complex IV by binding to the iron center of cytochrome c
oxidase?
A. Rotenone
B. Antimycin A
C. Cyanide
