CHEM210 Biochemistry
Final Assessment Review
Module 7 (Questions & Solutions)
2025
©2025
, 1. Case Study – Mechanism of Glyceraldehyde-3-Phosphate
Dehydrogenase (G3PDH) in Glycolysis
While studying glycolysis in a controlled reaction system, a researcher
observes that glyceraldehyde-3-phosphate is oxidized to
1,3‑bisphosphoglycerate accompanied by the reduction of NAD⁺ to
NADH.
Question: Which description best characterizes the reaction
mechanism catalyzed by G3PDH?
A. A metal‐ion–dependent phosphorylation reaction
B. Formation of a thioester intermediate via a cysteine residue
C. A direct transfer of a phosphate group from ATP
D. A decarboxylation reaction releasing CO₂
ANS: B. Formation of a thioester intermediate via a cysteine residue
Rationale: G3PDH catalyzes the oxidation of glyceraldehyde-3-
phosphate by forming a covalent thiohemiacetal intermediate with an
active-site cysteine. This intermediate then reacts with inorganic
phosphate to produce a high‑energy thioester (1,3‐bisphosphoglycerate)
and reduce NAD⁺ to NADH.
---
2. Case Study – Allosteric Regulation in Glycolysis
A study is conducted on the regulation of phosphofructokinase‑1 (PFK‑1)
during high-energy conditions. The enzyme is found to be inhibited when
ATP concentrations are high.
Question: Which allosteric effector is responsible for converting PFK‑1
into a less active form?
A. ADP
B. Fructose 2,6‑bisphosphate
C. ATP
D. Citrate
©2025
, ANS: C. ATP
Rationale: High ATP levels serve as a negative allosteric effector for
PFK‑1, reflecting a high-energy state in the cell and thereby reducing
glycolytic flux. (Citrate may contribute to inhibition later in the cycle, but
the primary inhibitor in this context is ATP.)
---
3. Case Study – Pyruvate Kinase Regulation
In a metabolic study, pyruvate kinase is observed to increase its activity
in the presence of upstream glycolytic intermediates.
Question: Which metabolite acts as a feed-forward activator of
pyruvate kinase?
A. Lactate
B. Fructose 1,6‑bisphosphate
C. Acetyl‑CoA
D. Citrate
ANS: B. Fructose 1,6‑bisphosphate
Rationale: Fructose 1,6‑bisphosphate, a product of an early glycolytic
reaction, acts as a positive feed‑forward regulator that enhances the
catalytic activity of pyruvate kinase, ensuring a smooth flux through
glycolysis.
---
4. Case Study – Regulation of the Citric Acid Cycle: Isocitrate
Dehydrogenase
During energy-demanding conditions, a researcher measures the activity
of isocitrate dehydrogenase in isolated mitochondria.
Question: Which factor best activates isocitrate dehydrogenase in the
TCA cycle under these conditions?
A. Increased ATP
B. Elevated NADH
C. High ADP
©2025
Final Assessment Review
Module 7 (Questions & Solutions)
2025
©2025
, 1. Case Study – Mechanism of Glyceraldehyde-3-Phosphate
Dehydrogenase (G3PDH) in Glycolysis
While studying glycolysis in a controlled reaction system, a researcher
observes that glyceraldehyde-3-phosphate is oxidized to
1,3‑bisphosphoglycerate accompanied by the reduction of NAD⁺ to
NADH.
Question: Which description best characterizes the reaction
mechanism catalyzed by G3PDH?
A. A metal‐ion–dependent phosphorylation reaction
B. Formation of a thioester intermediate via a cysteine residue
C. A direct transfer of a phosphate group from ATP
D. A decarboxylation reaction releasing CO₂
ANS: B. Formation of a thioester intermediate via a cysteine residue
Rationale: G3PDH catalyzes the oxidation of glyceraldehyde-3-
phosphate by forming a covalent thiohemiacetal intermediate with an
active-site cysteine. This intermediate then reacts with inorganic
phosphate to produce a high‑energy thioester (1,3‐bisphosphoglycerate)
and reduce NAD⁺ to NADH.
---
2. Case Study – Allosteric Regulation in Glycolysis
A study is conducted on the regulation of phosphofructokinase‑1 (PFK‑1)
during high-energy conditions. The enzyme is found to be inhibited when
ATP concentrations are high.
Question: Which allosteric effector is responsible for converting PFK‑1
into a less active form?
A. ADP
B. Fructose 2,6‑bisphosphate
C. ATP
D. Citrate
©2025
, ANS: C. ATP
Rationale: High ATP levels serve as a negative allosteric effector for
PFK‑1, reflecting a high-energy state in the cell and thereby reducing
glycolytic flux. (Citrate may contribute to inhibition later in the cycle, but
the primary inhibitor in this context is ATP.)
---
3. Case Study – Pyruvate Kinase Regulation
In a metabolic study, pyruvate kinase is observed to increase its activity
in the presence of upstream glycolytic intermediates.
Question: Which metabolite acts as a feed-forward activator of
pyruvate kinase?
A. Lactate
B. Fructose 1,6‑bisphosphate
C. Acetyl‑CoA
D. Citrate
ANS: B. Fructose 1,6‑bisphosphate
Rationale: Fructose 1,6‑bisphosphate, a product of an early glycolytic
reaction, acts as a positive feed‑forward regulator that enhances the
catalytic activity of pyruvate kinase, ensuring a smooth flux through
glycolysis.
---
4. Case Study – Regulation of the Citric Acid Cycle: Isocitrate
Dehydrogenase
During energy-demanding conditions, a researcher measures the activity
of isocitrate dehydrogenase in isolated mitochondria.
Question: Which factor best activates isocitrate dehydrogenase in the
TCA cycle under these conditions?
A. Increased ATP
B. Elevated NADH
C. High ADP
©2025
