Glycolysis, Gluconeogenesis, And Glycogen Metabolism - USMLE Step 1

1. The conversion of lactate to glucose in the liver is best described by which of the following metabolic pathways?
   Answer: Cori cycle 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.
2. The decreased ATP production in this patient's red blood cells most directly leads to which of the following consequences?
   Answer: Failure of the Na+/K+-ATPase pump and hemolysis 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.
3. 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?
   Answer: Glyceraldehyde-3-phosphate dehydrogenase 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.
4. This patient's condition is most likely caused by a deficiency in which of the following enzymes?
   Answer: Glucose-6-phosphatase 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.
5. A deficiency of which of the following enzymes is the most likely cause of this patient's symptoms?
   Answer: Muscle glycogen phosphorylase 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.
6. A deficiency in which of the following enzymes best explains this patient's findings?
   Answer: Debranching enzyme 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.
7. 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?
   Answer: NAD+ 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.
8. 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?
   Answer: Glycogen phosphorylase kinase and glycogen synthase 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.
9. 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?
   Answer: Aldolase B 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.
10. The patient's hypoglycemia is primarily caused by an increased intracellular ratio of which of the following?
    Answer: NADH/NAD+ 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.
11. To initiate gluconeogenesis, pyruvate is first converted into which of the following four-carbon intermediates inside the mitochondria?
    Answer: Oxaloacetate 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.
12. This disorder is caused by a deficiency of an enzyme located in which of the following cellular compartments?
    Answer: Lysosome 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.
13. 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?
    Answer: Gluconeogenesis 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.
14. Which of the following molecules derived from triglyceride breakdown can be used as a substrate for hepatic gluconeogenesis?
    Answer: Glycerol 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.
15. Which of the following metabolic consequences is most likely to be observed in this infant during a period of fasting?
    Answer: Severe lactic acidosis and hypoglycemia 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.
16. Which of the following enzymes catalyzes a bypass reaction in gluconeogenesis?
    Answer: Fructose-1,6-bisphosphatase 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.
17. Which of the following properties is characteristic of hepatic glucokinase but not of hexokinase?
    Answer: It has a high Michaelis constant (Km) for glucose. 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.
18. In response to high levels of insulin after a meal, which of the following changes occurs to this bifunctional enzyme?
    Answer: It is dephosphorylated, activating the PFK-2 domain. 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.
19. The activation of this phosphatase leads to the dephosphorylation and subsequent activation of which key regulatory enzyme, promoting glucose storage?
    Answer: Glycogen synthase 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.
20. The primary metabolic effect of increased fructose 2,6-bisphosphate concentration is the allosteric activation of which of the following enzymes?
    Answer: Phosphofructokinase-1 (PFK-1) 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.

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