All questions
Question 1
A 24-year-old medical student decides to run a marathon. After 3 hours of continuous, strenuous exercise, she feels weak and dizzy. Her blood glucose is measured and found to be low. To maintain glucose homeostasis during this period of prolonged exercise, her liver is actively synthesizing glucose from non-carbohydrate precursors. One crucial substrate for this process is lactate, which is produced in her exercising muscles.
The conversion of lactate to glucose in the liver is best described by which of the following metabolic pathways?
- Cahill cycle
- Cori cycle (correct answer)
- Pentose phosphate pathway
- Glycogenolysis
Explanation: The Cori cycle describes the metabolic pathway in which lactate produced by anaerobic glycolysis in the muscles is transported to the liver and converted back to glucose, which then returns to the muscles to be used for energy. This is a key mechanism for maintaining blood glucose during prolonged exercise. The Cahill cycle involves the transport of alanine from muscle to the liver. The pentose phosphate pathway produces NADPH and ribose-5-phosphate. Glycogenolysis is the breakdown of glycogen, not the synthesis of glucose from lactate.
Question 2
A 19-year-old man presents with a lifelong history of exercise intolerance. He reports severe muscle cramps, pain, and weakness within minutes of starting any strenuous physical activity. After a recent attempt to lift heavy weights, his urine appeared dark red. A forearm ischemic exercise test shows a failure of blood lactate levels to rise. A muscle biopsy is performed.
A deficiency of which of the following enzymes is the most likely cause of this patient's symptoms?
- Muscle glycogen phosphorylase (correct answer)
- Glucose-6-phosphatase
- Liver glycogen phosphorylase
- Pyruvate kinase
Explanation: This patient has McArdle disease (glycogen storage disease type V), which is caused by a deficiency of myophosphorylase (muscle glycogen phosphorylase). This enzyme is required for the initial step of glycogenolysis in skeletal muscle. Its absence prevents muscles from accessing stored glycogen for energy during exercise, leading to cramps, myoglobinuria (dark urine), and a flat lactate curve during ischemic exercise. Glucose-6-phosphatase deficiency (von Gierke disease) affects the liver and causes fasting hypoglycemia. Liver glycogen phosphorylase deficiency (Hers disease) also causes hepatomegaly and mild hypoglycemia. Pyruvate kinase deficiency causes hemolytic anemia.
Question 3
A 6-year-old boy is evaluated for recurrent episodes of weakness, sweating, and confusion, particularly in the mornings before breakfast. Physical examination reveals a protuberant abdomen due to massive hepatomegaly. Laboratory tests during an episode show blood glucose of 40 mg/dL, elevated lactate, uric acid, and triglycerides. A glucagon stimulation test results in a rise in lactate but no significant change in blood glucose.
This patient's condition is most likely caused by a deficiency in which of the following enzymes?
- Glycogen synthase
- Acid α-glucosidase
- Glucose-6-phosphatase (correct answer)
- Debranching enzyme
Explanation: This presentation is characteristic of von Gierke disease (glycogen storage disease type I), caused by a deficiency of glucose-6-phosphatase. This enzyme is required for the final step of both gluconeogenesis and glycogenolysis. Its absence prevents the liver from releasing free glucose into the blood, leading to severe fasting hypoglycemia. The trapped glucose-6-phosphate is shunted into other pathways, causing lactic acidosis, hyperuricemia, and hyperlipidemia. The massive hepatomegaly is due to glycogen accumulation.
Question 4
A 22-year-old woman with type 1 diabetes mellitus injects herself with insulin after a meal. Insulin signaling initiates a cascade that leads to the activation of protein phosphatase-1 in hepatocytes.
The activation of this phosphatase leads to the dephosphorylation and subsequent activation of which key regulatory enzyme, promoting glucose storage?
- Glycogen phosphorylase kinase
- Glycogen synthase (correct answer)
- Fructose-1,6-bisphosphatase
- Phosphofructokinase-1
Explanation: In the fed state, insulin signaling leads to the activation of protein phosphatase-1. This phosphatase dephosphorylates and activates glycogen synthase, the rate-limiting enzyme of glycogenesis, thus promoting the synthesis of glycogen for storage. Simultaneously, protein phosphatase-1 dephosphorylates and inactivates glycogen phosphorylase kinase and glycogen phosphorylase, inhibiting glycogen breakdown. Fructose-1,6-bisphosphatase is a gluconeogenic enzyme inhibited in the fed state. PFK-1 is allosterically activated, not directly by this phosphatase.
Question 5
A 7-year-old boy presents with a history of mild hypoglycemia, hepatomegaly, and growth retardation. Laboratory analysis of a liver biopsy sample shows an accumulation of glycogen with abnormally short outer branches. His fasting blood glucose levels are slightly low, but he does not have the severe lactic acidosis or hyperuricemia seen in other glycogen storage diseases.
A deficiency in which of the following enzymes best explains this patient's findings?
- Branching enzyme
- Debranching enzyme (correct answer)
- Muscle phosphofructokinase
- Glucose-6-phosphatase
Explanation: This presentation is characteristic of Cori disease (glycogen storage disease type III), caused by a deficiency of the debranching enzyme (α-1,6-glucosidase). This enzyme is needed to break down the α-1,6 linkages at branch points in glycogen. Its deficiency leads to the accumulation of abnormally structured glycogen (limit dextrin) with short outer chains. The clinical features include hepatomegaly and mild hypoglycemia, which are less severe than in von Gierke disease (glucose-6-phosphatase deficiency) because gluconeogenesis is intact.
Question 6
In red blood cells, glycolysis serves as the sole source of ATP. A patient is found to have a genetic defect in pyruvate kinase. This deficiency leads to a reduction in the rate of ATP synthesis within their erythrocytes.
The decreased ATP production in this patient's red blood cells most directly leads to which of the following consequences?
- Increased 2,3-bisphosphoglycerate levels
- Failure of the Na+/K+-ATPase pump and hemolysis (correct answer)
- Oxidative damage from reactive oxygen species
- Decreased oxygen affinity of hemoglobin
Explanation: Pyruvate kinase deficiency leads to decreased ATP production. In red blood cells, ATP is critical for maintaining the function of membrane ion pumps, such as the Na+/K+-ATPase. Failure of this pump leads to electrolyte imbalances, loss of the normal biconcave shape, and ultimately, extravascular hemolysis, resulting in chronic hemolytic anemia. While intermediates upstream of the enzymatic block (like 2,3-BPG) do increase and decrease oxygen affinity, the most direct cause of cell lysis is the membrane pump failure.
Question 7
A biochemist is comparing the kinetic properties of glucokinase in the liver with hexokinase found in most other tissues. Both enzymes catalyze the phosphorylation of glucose to glucose-6-phosphate.
Which of the following properties is characteristic of hepatic glucokinase but not of hexokinase?
- It is inhibited by its product, glucose-6-phosphate.
- It has a low Michaelis constant (Km) for glucose.
- It has a high Michaelis constant (Km) for glucose. (correct answer)
- It functions at a maximal velocity (Vmax) even at low glucose concentrations.
Explanation: Glucokinase, found in the liver and pancreatic β-cells, has a high Km for glucose (around 10 mM). This means it is only significantly active when blood glucose levels are high, such as after a carbohydrate-rich meal, allowing the liver to effectively clear glucose from the portal circulation. In contrast, hexokinase has a low Km (~0.1 mM), meaning it is saturated at normal fasting glucose levels. Glucokinase is not inhibited by glucose-6-phosphate, unlike hexokinase.
Question 8
An agricultural worker is exposed to a pesticide containing high levels of arsenate. He presents to the emergency department with symptoms of nausea, vomiting, and confusion. Arsenate is known to interfere with glycolysis by acting as a phosphate analog.
Arsenate disrupts glycolysis by substituting for phosphate in the reaction catalyzed by which of the following enzymes, leading to no net production of ATP from the pathway?
- Phosphofructokinase-1
- Pyruvate kinase
- Glyceraldehyde-3-phosphate dehydrogenase (correct answer)
- Hexokinase
Explanation: Arsenate is structurally similar to inorganic phosphate. It can substitute for phosphate in the reaction catalyzed by glyceraldehyde-3-phosphate dehydrogenase. This forms an unstable 1-arseno-3-phosphoglycerate intermediate, which spontaneously hydrolyzes to 3-phosphoglycerate. This bypasses the substrate-level phosphorylation step normally catalyzed by phosphoglycerate kinase, where ATP is generated. As a result, glycolysis can proceed, but the 2 ATP molecules normally generated at this step are lost, resulting in a net yield of 0 ATP for the entire pathway.
Question 9
A 3-month-old infant presents with cardiomegaly, profound muscle weakness (hypotonia), and feeding difficulties. A muscle biopsy reveals large, membrane-bound vacuoles filled with glycogen, disrupting the normal cellular architecture. The patient's blood glucose levels are normal.
This disorder is caused by a deficiency of an enzyme located in which of the following cellular compartments?
- Mitochondrion
- Cytosol
- Peroxisome
- Lysosome (correct answer)
Explanation: This clinical picture is characteristic of Pompe disease (glycogen storage disease type II), a lysosomal storage disorder. It is caused by a deficiency of the lysosomal enzyme acid α-glucosidase (acid maltase). This enzyme is responsible for breaking down glycogen within the lysosome. Its absence leads to massive glycogen accumulation in lysosomes, particularly in cardiac and skeletal muscle cells, causing cardiomegaly and hypotonia. Unlike other glycogen storage diseases, it does not typically cause hypoglycemia because cytosolic glycogen metabolism is unaffected.
Question 10
During a period of fasting, the hormone glucagon is released from the pancreas. It binds to receptors on hepatocytes, initiating a signal transduction cascade that involves cyclic AMP (cAMP) and protein kinase A (PKA).
The primary purpose of this signaling pathway in the liver is to increase blood glucose levels by activating glycogenolysis and inhibiting glycogenesis. PKA achieves this by phosphorylating which two key enzymes?
- Glycogen synthase and phosphofructokinase-2
- Glycogen phosphorylase kinase and glycogen synthase (correct answer)
- Pyruvate kinase and glycogen phosphorylase
- Fructose-1,6-bisphosphatase and pyruvate carboxylase
Explanation: Glucagon signaling via cAMP and PKA aims to increase blood glucose. PKA phosphorylates glycogen phosphorylase kinase, which activates it. Activated glycogen phosphorylase kinase then phosphorylates and activates glycogen phosphorylase, the key enzyme for glycogenolysis. Simultaneously, PKA directly phosphorylates glycogen synthase, which inactivates it, thereby shutting down glycogen synthesis. This coordinated regulation ensures a net release of glucose from liver glycogen stores.
Question 11
Gluconeogenesis is the metabolic process of synthesizing glucose from non-carbohydrate precursors. It is essential during fasting. Three reactions in glycolysis are physiologically irreversible and must be bypassed by different enzymes in gluconeogenesis.
Which of the following enzymes catalyzes a bypass reaction in gluconeogenesis?
- Phosphofructokinase-1
- Pyruvate kinase
- Hexokinase
- Fructose-1,6-bisphosphatase (correct answer)
Explanation: The conversion of fructose-6-phosphate to fructose-1,6-bisphosphate by phosphofructokinase-1 (PFK-1) is an irreversible step in glycolysis. In gluconeogenesis, this step is bypassed by the enzyme fructose-1,6-bisphosphatase, which hydrolyzes fructose-1,6-bisphosphate back to fructose-6-phosphate. Pyruvate kinase and hexokinase are also irreversible glycolytic enzymes, but fructose-1,6-bisphosphatase is the correct gluconeogenic bypass enzyme among the choices.
Question 12
During prolonged starvation, the body adapts to preserve protein. Muscle tissue breaks down protein to release amino acids. Alanine, in particular, is transported from the muscle to the liver.
In the liver, alanine is converted to pyruvate. This pyruvate then serves as a primary substrate for which of the following metabolic processes to maintain blood glucose?
- Glycogenolysis
- Gluconeogenesis (correct answer)
- Citric acid cycle
- Ketogenesis
Explanation: The process described is the glucose-alanine (or Cahill) cycle. Alanine from muscle protein breakdown is transported to the liver, where it is transaminated to pyruvate. This pyruvate is a major substrate for gluconeogenesis, the synthesis of new glucose, which is critical for supplying the brain and red blood cells with fuel during starvation. Glycogenolysis is the breakdown of stored glycogen, which is depleted after about 24 hours of fasting. The citric acid cycle and ketogenesis use pyruvate-derived acetyl-CoA for energy or ketone body synthesis, not for creating new glucose.
Question 13
The bifunctional enzyme phosphofructokinase-2/fructose-2,6-bisphosphatase (PFK-2/FBPase-2) plays a crucial role in regulating glycolysis and gluconeogenesis in the liver. Its activity is controlled by hormonal signaling through phosphorylation.
In response to high levels of insulin after a meal, which of the following changes occurs to this bifunctional enzyme?
- It is phosphorylated, activating the FBPase-2 domain.
- It is dephosphorylated, activating the PFK-2 domain. (correct answer)
- It is phosphorylated, activating the PFK-2 domain.
- It is dephosphorylated, activating the FBPase-2 domain.
Explanation: In the fed state, high insulin levels activate a protein phosphatase. This phosphatase dephosphorylates the PFK-2/FBPase-2 enzyme. In its dephosphorylated state, the PFK-2 kinase domain is active, leading to the synthesis of fructose 2,6-bisphosphate. This molecule then allosterically activates PFK-1, strongly stimulating glycolysis. The opposite occurs with glucagon, which causes phosphorylation and activation of the FBPase-2 domain.
Question 14
An experiment is designed to trace the path of carbon atoms during gluconeogenesis. A culture of liver cells is supplied with pyruvate in which the carboxyl carbon is labeled with ¹⁴C. The cells are stimulated to perform gluconeogenesis.
To initiate gluconeogenesis, pyruvate is first converted into which of the following four-carbon intermediates inside the mitochondria?
- Malate
- Fumarate
- Succinyl-CoA
- Oxaloacetate (correct answer)
Explanation: The first step of gluconeogenesis is the carboxylation of the three-carbon pyruvate to the four-carbon molecule oxaloacetate. This reaction is catalyzed by the mitochondrial enzyme pyruvate carboxylase, which requires biotin and ATP. Oxaloacetate is a key intermediate that can then be converted to phosphoenolpyruvate (by PEPCK) to continue the gluconeogenic pathway. Malate is often used to shuttle oxaloacetate out of the mitochondria, but oxaloacetate is the direct product of pyruvate carboxylation.
Question 15
A 30-year-old man enters a 3-day fasting state for a religious observance. During this period, his body must rely on gluconeogenesis to maintain blood glucose levels for brain function. Adipose tissue releases fatty acids and glycerol into the circulation.
Which of the following molecules derived from triglyceride breakdown can be used as a substrate for hepatic gluconeogenesis?
- Acetyl-CoA
- Glycerol (correct answer)
- Palmitate (a 16-carbon even-chain fatty acid)
- Acetoacetate (a ketone body)
Explanation: During lipolysis, triglycerides are broken down into fatty acids and glycerol. Glycerol can be taken up by the liver, phosphorylated to glycerol-3-phosphate by glycerol kinase, and then oxidized to dihydroxyacetone phosphate (DHAP), an intermediate in glycolysis and gluconeogenesis. Even-chain fatty acids are broken down into acetyl-CoA, which cannot be used for net glucose synthesis in humans. Ketone bodies are produced from acetyl-CoA but also cannot be converted back to glucose.
Question 16
A research study is investigating the regulation of glycolysis in liver cells. The researchers observe that after a carbohydrate-rich meal, the concentration of fructose 2,6-bisphosphate increases significantly. This molecule is a potent regulator of glucose metabolism.
The primary metabolic effect of increased fructose 2,6-bisphosphate concentration is the allosteric activation of which of the following enzymes?
- Hexokinase
- Pyruvate kinase
- Phosphofructokinase-1 (PFK-1) (correct answer)
- Fructose-1,6-bisphosphatase
Explanation: Fructose 2,6-bisphosphate is the most potent allosteric activator of phosphofructokinase-1 (PFK-1), the enzyme that catalyzes the committed step of glycolysis. Its presence overcomes the inhibition of PFK-1 by ATP, strongly stimulating the glycolytic pathway in the fed state. Conversely, fructose 2,6-bisphosphate is an inhibitor of fructose-1,6-bisphosphatase, the corresponding enzyme in gluconeogenesis. This reciprocal regulation ensures that glycolysis and gluconeogenesis do not run simultaneously.
Question 17
A 5-month-old infant is brought to the pediatrician due to failure to thrive, persistent vomiting, and irritability that worsens after feeding. Physical examination reveals hepatomegaly and jaundice. Laboratory studies are significant for hypoglycemia, hyperbilirubinemia, and the presence of reducing substances in the urine. Symptoms began shortly after the infant was weaned from breast milk and introduced to fruit juices.
This patient's condition is most likely caused by a deficiency in which of the following enzymes, leading to the accumulation of a toxic intermediate that depletes intracellular phosphate?
- Fructokinase
- Galactokinase
- Aldolase B (correct answer)
- Glucokinase
Explanation: This clinical presentation is classic for hereditary fructose intolerance, an autosomal recessive disorder caused by a deficiency of aldolase B. Ingesting fructose (from fruit juice or sucrose) leads to the accumulation of fructose-1-phosphate, which traps phosphate and inhibits both glycogenolysis and gluconeogenesis, causing severe hypoglycemia. Fructokinase deficiency (essential fructosuria) is benign. Galactokinase deficiency presents with cataracts. Glucokinase is involved in glucose metabolism in the liver and pancreas.
Question 18
A premature infant develops respiratory distress syndrome and is placed on a ventilator, leading to a state of hypoxia. The subsequent anaerobic metabolism in tissues causes a buildup of a specific metabolic byproduct in the blood.
The conversion of pyruvate to this byproduct is catalyzed by lactate dehydrogenase and is crucial for regenerating which of the following molecules needed for glycolysis to continue?
- ATP
- NADH
- NAD+ (correct answer)
- FAD
Explanation: Under anaerobic conditions (hypoxia), the electron transport chain cannot function to reoxidize NADH to NAD+. Glycolysis requires a continuous supply of NAD+ for the glyceraldehyde-3-phosphate dehydrogenase step. To regenerate NAD+, lactate dehydrogenase catalyzes the reduction of pyruvate to lactate, oxidizing NADH to NAD+ in the process. This allows glycolysis to continue producing a small amount of ATP even in the absence of oxygen.
Question 19
A newborn is diagnosed with a genetic disorder resulting in a complete deficiency of pyruvate carboxylase. This enzyme is crucial for the first step of gluconeogenesis. The patient is expected to have severe metabolic derangements.
Which of the following metabolic consequences is most likely to be observed in this infant during a period of fasting?
- Inability to convert acetyl-CoA to glucose
- Severe lactic acidosis and hypoglycemia (correct answer)
- Depletion of citric acid cycle intermediates
- Increased conversion of alanine to pyruvate
Explanation: Pyruvate carboxylase converts pyruvate to oxaloacetate, which is a key step in gluconeogenesis and an anaplerotic reaction for the TCA cycle. Without this enzyme, pyruvate cannot enter the gluconeogenic pathway. During fasting, accumulating pyruvate from other sources (like alanine) is shunted towards lactate production via lactate dehydrogenase, causing severe lactic acidosis. The inability to synthesize glucose results in profound hypoglycemia. While acetyl-CoA cannot be converted to glucose in humans, this is not the primary defect. TCA cycle intermediates would be depleted, not increased. Alanine would still be converted to pyruvate, but the pyruvate would accumulate.
Question 20
A 45-year-old man with a history of chronic alcohol abuse is brought to the emergency department in a confused state. He reports not having eaten for 2 days and consuming a large amount of vodka last night. His blood glucose is 35 mg/dL. The administration of glucagon fails to raise his blood glucose level. His liver is metabolizing a large amount of ethanol.
The patient's hypoglycemia is primarily caused by an increased intracellular ratio of which of the following?
- ATP/ADP
- NAD+/NADH
- Acetyl-CoA/CoA
- NADH/NAD+ (correct answer)
Explanation: Ethanol metabolism by alcohol dehydrogenase and aldehyde dehydrogenase produces a large amount of NADH, significantly increasing the NADH/NAD+ ratio in the liver. This high ratio inhibits gluconeogenesis by shunting gluconeogenic precursors away from the pathway. Specifically, it promotes the conversion of pyruvate to lactate and oxaloacetate to malate, depleting the substrates required for glucose synthesis. The failure of glucagon to work suggests depleted glycogen stores, making gluconeogenesis the only source of glucose, which is inhibited.