BCH 4053 Exam 3 V2 | BCH 4053 Biochemistry I | Actual Q&A with
Rationale (BCH4053 Exam 3) | University of Central Florida
1. Which enzyme catalyzes the primary rate-limiting and committed step of glycolysis in
muscle tissue?
A. Phosphofructokinase-1
B. Hexokinase
C. Pyruvate Kinase
D. Aldolase
Answer: A
Explanation: Phosphofructokinase-1 (PFK-1) is the major regulatory enzyme of glycolysis
as it catalyzes the irreversible phosphorylation of fructose-6-phosphate. This step is
considered the committed step because the resulting fructose-1,6-bisphosphate is destined
for the glycolytic pathway. The enzyme is allosterically regulated by the cell’s energy status,
where high ATP levels inhibit its activity.
2. In the Pyruvate Dehydrogenase (PDH) complex, which coenzyme is responsible for the
decarboxylation of pyruvate?
A. Flavin adenine dinucleotide (FAD)
B. Lipoic acid
C. Thiamine pyrophosphate (TPP)
D. Nicotinamide adenine dinucleotide (NAD+)
Answer: C
Explanation: Thiamine pyrophosphate (TPP) is the prosthetic group of the E1 subunit
(pyruvate dehydrogenase) of the PDH complex. It functions by forming a covalent adduct
with pyruvate, facilitating the removal of CO2 and the transfer of the remaining
hydroxyethyl group. Deficiencies in thiamine can lead to impaired glucose metabolism and
conditions like Beriberi.
3. Which of the following molecules acts as a potent allosteric activator of
Phosphofructokinase-1 (PFK-1) and inhibitor of Fructose-1,6-bisphosphatase-1 (FBPase-1)?
A. ATP
B. Fructose-2,6-bisphosphate
C. Citrate
D. Glucose-6-phosphate
,Answer: B
Explanation: Fructose-2,6-bisphosphate is a key signaling molecule that mediates the
reciprocal regulation of glycolysis and gluconeogenesis. It increases the affinity of PFK-1
for its substrate while simultaneously inhibiting FBPase-1, preventing a futile cycle. Its
concentration is controlled by the bifunctional enzyme PFK-2/FBPase-2 in response to
hormonal signals like glucagon and insulin.
4. How many net molecules of ATP are produced by the anaerobic conversion of one
molecule of glucose to two molecules of lactate?
A. 2
B. 0
C. 4
D. 32
Answer: A
Explanation: Glycolysis yields a gross total of 4 ATP molecules, but 2 ATP are consumed
during the preparatory phase (hexokinase and PFK-1 steps). Therefore, the net gain is 2
ATP molecules per glucose unit processed. In anaerobic conditions, no additional ATP is
generated through oxidative phosphorylation, as the NADH produced is used to reduce
pyruvate to lactate.
5. Which citric acid cycle enzyme is membrane-bound and also functions as Complex II of the
Electron Transport Chain?
A. Citrate synthase
B. Succinate dehydrogenase
C. Isocitrate dehydrogenase
D. Malate dehydrogenase
Answer: B
Explanation: Succinate dehydrogenase is the only enzyme of the Citric Acid Cycle that is
embedded in the inner mitochondrial membrane. It catalyzes the oxidation of succinate to
fumarate while simultaneously transferring electrons to FAD and then to the ubiquinone
pool. This dual role links the TCA cycle directly to the process of oxidative phosphorylation.
6. The conversion of pyruvate to oxaloacetate in gluconeogenesis is catalyzed by which
enzyme?
A. Pyruvate dehydrogenase
B. PEP carboxykinase
C. Pyruvate carboxylase
, D. Malate dehydrogenase
Answer: C
Explanation: Pyruvate carboxylase is a mitochondrial enzyme that requires biotin as a
cofactor to fix CO2 onto pyruvate. This reaction provides the oxaloacetate necessary to
bypass the irreversible pyruvate kinase step of glycolysis. The enzyme is strictly regulated
by acetyl-CoA, which acts as an obligatory activator.
7. Which complex of the Electron Transport Chain does NOT pump protons across the inner
mitochondrial membrane?
A. Complex I
B. Complex II
C. Complex III
D. Complex IV
Answer: B
Explanation: Complex II, also known as succinate dehydrogenase, transfers electrons from
succinate to Coenzyme Q via FADH2. Unlike Complexes I, III, and IV, the free energy change
associated with these electron transfers is insufficient to drive proton pumping.
Consequently, FADH2 oxidation contributes less to the proton motive force than NADH
oxidation.
8. What is the effect of 2,4-dinitrophenol (DNP) on oxidative phosphorylation?
A. It inhibits ATP synthase directly.
B. It prevents the flow of electrons from Complex III to Complex IV.
C. It inhibits the Malate-Aspartate shuttle.
D. It uncouples electron transport from ATP synthesis by dissipating the proton gradient.
Answer: D
Explanation: DNP is a lipophilic weak acid that can carry protons across the inner
mitochondrial membrane, bypassing the ATP synthase channel. This action dissipates the
electrochemical gradient, which is required for ATP synthesis, while electron transport
continues at an accelerated rate. The energy normally used for ATP production is instead
released as heat, which can lead to hyperthermia.
9. Which of the following enzymes produces NADH in the Citric Acid Cycle?
A. Succinate dehydrogenase
B. Fumarase
C. Citrate synthase
Rationale (BCH4053 Exam 3) | University of Central Florida
1. Which enzyme catalyzes the primary rate-limiting and committed step of glycolysis in
muscle tissue?
A. Phosphofructokinase-1
B. Hexokinase
C. Pyruvate Kinase
D. Aldolase
Answer: A
Explanation: Phosphofructokinase-1 (PFK-1) is the major regulatory enzyme of glycolysis
as it catalyzes the irreversible phosphorylation of fructose-6-phosphate. This step is
considered the committed step because the resulting fructose-1,6-bisphosphate is destined
for the glycolytic pathway. The enzyme is allosterically regulated by the cell’s energy status,
where high ATP levels inhibit its activity.
2. In the Pyruvate Dehydrogenase (PDH) complex, which coenzyme is responsible for the
decarboxylation of pyruvate?
A. Flavin adenine dinucleotide (FAD)
B. Lipoic acid
C. Thiamine pyrophosphate (TPP)
D. Nicotinamide adenine dinucleotide (NAD+)
Answer: C
Explanation: Thiamine pyrophosphate (TPP) is the prosthetic group of the E1 subunit
(pyruvate dehydrogenase) of the PDH complex. It functions by forming a covalent adduct
with pyruvate, facilitating the removal of CO2 and the transfer of the remaining
hydroxyethyl group. Deficiencies in thiamine can lead to impaired glucose metabolism and
conditions like Beriberi.
3. Which of the following molecules acts as a potent allosteric activator of
Phosphofructokinase-1 (PFK-1) and inhibitor of Fructose-1,6-bisphosphatase-1 (FBPase-1)?
A. ATP
B. Fructose-2,6-bisphosphate
C. Citrate
D. Glucose-6-phosphate
,Answer: B
Explanation: Fructose-2,6-bisphosphate is a key signaling molecule that mediates the
reciprocal regulation of glycolysis and gluconeogenesis. It increases the affinity of PFK-1
for its substrate while simultaneously inhibiting FBPase-1, preventing a futile cycle. Its
concentration is controlled by the bifunctional enzyme PFK-2/FBPase-2 in response to
hormonal signals like glucagon and insulin.
4. How many net molecules of ATP are produced by the anaerobic conversion of one
molecule of glucose to two molecules of lactate?
A. 2
B. 0
C. 4
D. 32
Answer: A
Explanation: Glycolysis yields a gross total of 4 ATP molecules, but 2 ATP are consumed
during the preparatory phase (hexokinase and PFK-1 steps). Therefore, the net gain is 2
ATP molecules per glucose unit processed. In anaerobic conditions, no additional ATP is
generated through oxidative phosphorylation, as the NADH produced is used to reduce
pyruvate to lactate.
5. Which citric acid cycle enzyme is membrane-bound and also functions as Complex II of the
Electron Transport Chain?
A. Citrate synthase
B. Succinate dehydrogenase
C. Isocitrate dehydrogenase
D. Malate dehydrogenase
Answer: B
Explanation: Succinate dehydrogenase is the only enzyme of the Citric Acid Cycle that is
embedded in the inner mitochondrial membrane. It catalyzes the oxidation of succinate to
fumarate while simultaneously transferring electrons to FAD and then to the ubiquinone
pool. This dual role links the TCA cycle directly to the process of oxidative phosphorylation.
6. The conversion of pyruvate to oxaloacetate in gluconeogenesis is catalyzed by which
enzyme?
A. Pyruvate dehydrogenase
B. PEP carboxykinase
C. Pyruvate carboxylase
, D. Malate dehydrogenase
Answer: C
Explanation: Pyruvate carboxylase is a mitochondrial enzyme that requires biotin as a
cofactor to fix CO2 onto pyruvate. This reaction provides the oxaloacetate necessary to
bypass the irreversible pyruvate kinase step of glycolysis. The enzyme is strictly regulated
by acetyl-CoA, which acts as an obligatory activator.
7. Which complex of the Electron Transport Chain does NOT pump protons across the inner
mitochondrial membrane?
A. Complex I
B. Complex II
C. Complex III
D. Complex IV
Answer: B
Explanation: Complex II, also known as succinate dehydrogenase, transfers electrons from
succinate to Coenzyme Q via FADH2. Unlike Complexes I, III, and IV, the free energy change
associated with these electron transfers is insufficient to drive proton pumping.
Consequently, FADH2 oxidation contributes less to the proton motive force than NADH
oxidation.
8. What is the effect of 2,4-dinitrophenol (DNP) on oxidative phosphorylation?
A. It inhibits ATP synthase directly.
B. It prevents the flow of electrons from Complex III to Complex IV.
C. It inhibits the Malate-Aspartate shuttle.
D. It uncouples electron transport from ATP synthesis by dissipating the proton gradient.
Answer: D
Explanation: DNP is a lipophilic weak acid that can carry protons across the inner
mitochondrial membrane, bypassing the ATP synthase channel. This action dissipates the
electrochemical gradient, which is required for ATP synthesis, while electron transport
continues at an accelerated rate. The energy normally used for ATP production is instead
released as heat, which can lead to hyperthermia.
9. Which of the following enzymes produces NADH in the Citric Acid Cycle?
A. Succinate dehydrogenase
B. Fumarase
C. Citrate synthase
