Metabolic Pathways and Regulation Practice Questions
19 free Metabolic Pathways and Regulation practice questions for the USMLE Step 1. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
Which enzyme catalyzes the rate-limiting step of glycolysis and is strongly regulated by cellular energy status?
- A Hexokinase
- B Phosphofructokinase-1 (PFK-1)
- C Pyruvate kinase
- D Glyceraldehyde-3-phosphate dehydrogenase
Correct answer: Phosphofructokinase-1 (PFK-1)
PFK-1 catalyzes the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate and acts as the major regulatory checkpoint in glycolysis. Its activity is modulated by allosteric effectors like ATP, ADP/AMP and citrate, reflecting the energy status of the cell.
What effect does high intracellular ATP concentration have on PFK-1 activity?
- A Strong activation of PFK-1
- B Allosteric inhibition of PFK-1
- C No effect on PFK-1
- D Irreversible inactivation of PFK-1
Correct answer: Allosteric inhibition of PFK-1
High ATP binds to an allosteric site on PFK-1 and stabilizes the inactive (T) conformation, thereby reducing enzymatic activity and slowing glycolytic flux when energy supply is sufficient.
Which metabolite can activate PFK-1 even in the presence of high ATP, thereby promoting glycolysis?
- A Citrate from the TCA cycle
- B Fructose-2,6-bisphosphate
- C Acetyl-CoA from pyruvate
- D NADH from the electron transport chain
Correct answer: Fructose-2,6-bisphosphate
Fructose-2,6-bisphosphate is a potent allosteric activator of PFK-1; it increases the enzyme’s affinity for its substrate and diminishes the inhibitory effect of ATP, thus promoting glycolysis under conditions such as after a meal.
Which three enzymes catalyze the essentially irreversible, regulatory steps of glycolysis?
- A Hexokinase, PFK-1, Pyruvate kinase
- B Glucokinase, Aldolase, Enolase
- C Phosphoglycerate kinase, Triose phosphate isomerase, Lactate dehydrogenase
- D Glucose-6-phosphatase, Pyruvate carboxylase, PEPCK
Correct answer: Hexokinase, PFK-1, Pyruvate kinase
The irreversible, regulatory steps in glycolysis are catalyzed by hexokinase (or glucokinase), PFK-1, and pyruvate kinase — these steps have large negative ΔG and allow control of glycolytic flux.
In the liver during fasting state, which hormonal signal leads to decreased glycolysis and increased gluconeogenesis?
- A Insulin
- B Glucagon
- C High glucose
- D Thyroid hormone
Correct answer: Glucagon
During fasting, glucagon levels rise, promoting cAMP-dependent phosphorylation events that inhibit glycolytic enzymes (like pyruvate kinase) and activate gluconeogenic enzymes, shifting metabolism toward glucose production.
Which enzyme activity is inhibited by its own product, glucose-6-phosphate (feedback inhibition) to regulate glycolysis?
- A Glucokinase
- B Hexokinase
- C Phosphofructokinase-1
- D Pyruvate kinase
Correct answer: Hexokinase
Hexokinase is allosterically inhibited by its product, glucose-6-phosphate, preventing excessive phosphorylation of glucose when downstream metabolism is slowed, thus providing feedback regulation.
Which enzyme in glycolysis is subject to feedforward activation by fructose-1,6-bisphosphate (F-1,6-BP)?
- A Hexokinase
- B PFK-1
- C Pyruvate kinase
- D Phosphoglycerate kinase
Correct answer: Pyruvate kinase
Pyruvate kinase is allosterically activated by F-1,6-BP (an upstream intermediate), which promotes downstream flux — an example of feedforward regulation that helps coordinate glycolytic throughput.
Why is it important that glycolysis and gluconeogenesis are not active simultaneously in the same cell?
- A Because both pathways share enzymes and would compete
- B To prevent a 'futile cycle' that wastes ATP
- C Because gluconeogenesis consumes oxygen while glycolysis produces it
- D Because one occurs only in mitochondria and the other in cytosol
Correct answer: To prevent a 'futile cycle' that wastes ATP
If glycolysis and gluconeogenesis occurred simultaneously, substrates would cycle without net gain and consume energy (ATP) — a wasteful futile cycle. Regulation ensures only one pathway predominates under given conditions.
Which enzyme regulates the reciprocal control between glycolysis and gluconeogenesis via modification of fructose-2,6-bisphosphate levels in liver?
- A Because both pathways share enzymes and would compete
- B To prevent a wasteful, ATP-consuming futile cycle
- C Because gluconeogenesis uses oxygen that glycolysis produces
- D Because one runs in mitochondria and the other in cytosol
Correct answer: To prevent a wasteful, ATP-consuming futile cycle
The bifunctional enzyme that acts as PFK-2 and FBPase-2 regulates levels of fructose-2,6-bisphosphate — a potent activator of glycolysis and inhibitor of gluconeogenesis — thus coordinating the two pathways.
Which metabolic sensor kinase becomes activated when cellular AMP/ADP levels are high, promoting catabolic pathways and inhibiting anabolic pathways?
- A mTOR
- B Protein kinase A (PKA)
- C AMP-activated protein kinase (AMPK)
- D Glycogen synthase kinase 3 (GSK3)
Correct answer: AMP-activated protein kinase (AMPK)
AMP-activated protein kinase (AMPK) senses low-energy states via increased AMP/ADP, and activates processes that generate ATP (like fatty-acid oxidation) while inhibiting energy-consuming anabolic processes, restoring energy balance.
In the citric acid (TCA) cycle, accumulation of which intermediate can allosterically inhibit phosphofructokinase-1 and thus slow glycolysis?
- A Oxaloacetate
- B Isocitrate
- C Citrate exported to the cytosol
- D Succinyl-CoA
Correct answer: Citrate exported to the cytosol
Citrate, exported from the mitochondria when TCA cycle intermediates accumulate, acts as an allosteric inhibitor of PFK-1 — signaling that energy and biosynthetic precursors are abundant and decreasing glycolytic flux.
Which statement best defines a branched metabolic pathway?
- A A cycle that continuously regenerates its starting substrate
- B A route that converges several substrates into one product
- C An intermediate diverges to yield multiple different end-products
- D A pathway that operates exclusively under anaerobic conditions
Correct answer: An intermediate diverges to yield multiple different end-products
In a branched metabolic pathway, an intermediate metabolite can be directed into more than one downstream reaction, producing diverse end-products depending on physiological needs.
Which enzyme initiates glycolysis in the liver by phosphorylating glucose when blood glucose is high, and has a higher Km for glucose than hexokinase?
- A Glucose-6-phosphatase
- B Glucokinase
- C Phosphofructokinase-1
- D Pyruvate kinase
Correct answer: Glucokinase
Glucokinase, the hepatic isoform of hexokinase, activates only at high glucose concentrations and thereby acts as a glucose sensor, contributing to regulation of carbohydrate metabolism in the fed state.
Which enzyme in gluconeogenesis is directly inhibited by fructose-2,6-bisphosphate, thereby preventing gluconeogenesis when glycolysis is active?
- A Pyruvate carboxylase
- B Fructose-1,6-bisphosphatase (FBPase-1)
- C Glucose-6-phosphatase
- D Phosphoenolpyruvate carboxykinase (PEPCK)
Correct answer: Fructose-1,6-bisphosphatase (FBPase-1)
Fructose-2,6-bisphosphate inhibits FBPase-1, the enzyme that converts fructose-1,6-bisphosphate to fructose-6-phosphate, thereby downregulating gluconeogenesis when glycolysis is promoted.
Which of these is a catabolic pathway that generates reducing equivalents (NADH, FADH2) and feeds electrons into the electron transport chain?
- A Glycolysis in the cytosol
- B Pentose phosphate pathway
- C Urea cycle in the liver
- D Citric acid cycle (TCA cycle)
Correct answer: Citric acid cycle (TCA cycle)
The citric acid cycle oxidizes acetyl-CoA to CO₂ and in the process produces NADH and FADH₂, which serve as electron donors for the electron transport chain and thereby drive ATP production.
Which metabolic regulation mechanism refers to the end product of a pathway inhibiting an early enzyme in that same pathway?
- A Feedforward activation
- B Allosteric activation
- C Feedback inhibition
- D Covalent modification
Correct answer: Feedback inhibition
Feedback inhibition is when an end-product of a metabolic pathway binds to an enzyme near the start of the pathway, reducing its activity and preventing overproduction — a common mechanism to maintain metabolic homeostasis.
Which pathway provides reducing equivalents (NADPH) and ribose-5-phosphate for biosynthesis and antioxidant defense rather than ATP?
- A Glycolysis in the cytosol
- B Citric acid cycle in mitochondria
- C Pentose phosphate pathway (PPP)
- D Fatty acid beta-oxidation
Correct answer: Pentose phosphate pathway (PPP)
The pentose phosphate pathway generates NADPH (for biosynthesis and redox balance) and ribose-5-phosphate (for nucleotide synthesis), serving anabolic and antioxidant functions rather than directly producing ATP.
During prolonged fasting, which enzyme is activated to promote gluconeogenesis from substrates like lactate or amino acids?
- A Pyruvate kinase of glycolysis
- B Glucokinase in the liver
- C Phosphoenolpyruvate carboxykinase (PEPCK)
- D Hexokinase in muscle
Correct answer: Phosphoenolpyruvate carboxykinase (PEPCK)
PEPCK catalyzes a key step in gluconeogenesis, converting oxaloacetate to phosphoenolpyruvate, and is induced during fasting or glucagon stimulation to increase hepatic glucose production.
Which enzyme in glycolysis is regulated both by allosteric effectors and by hormonal (covalent) modification in liver tissue?
- A Hexokinase
- B Phosphofructokinase-1
- C Pyruvate kinase
- D Glyceraldehyde-3-phosphate dehydrogenase
Correct answer: Pyruvate kinase
Pyruvate kinase in the liver is allosterically activated by fructose-1,6-bisphosphate and inhibited by ATP/alanine, and is also regulated via phosphorylation by hormonal signals (e.g., glucagon) — integrating metabolic and endocrine control.