A 16-year-old teenager is brought to the emergency department after having slipped on ice while walking to school. She hit her head on the side of the pavement and retained consciousness. She was brought to the closest ER within an hour of the incident. The ER physician sends her immediately to get a CT scan and also orders routine blood work. The physician understands that in cases of stress, such as in this patient, the concentration of certain hormones will be increased, while others will be decreased. Considering allosteric regulation by hormones, which of the following enzymes will most likely be inhibited in this patient?
Q2
A mother brings her newborn baby to the pediatrician after noting that his skin looks yellow. The patient's lactate dehydrogenase is elevated and haptoglobin is decreased. A smear of the child's blood is shown below. The patient is ultimately found to have decreased ability to process phosphoenolpyruvate to pyruvate. Which of the following metabolic changes is most likely to occur in this patient?
Q3
Researchers are investigating a new mouse model of glycogen regulation. They add hepatocyte enzyme extracts to radiolabeled glucose to investigate glycogen synthesis, in particular two enzymes. They notice that the first enzyme adds a radiolabeled glucose to the end of a long strand of radiolabeled glucose. The second enzyme then appears to rearrange the glycogen structure such that there appears to be shorter strands that are linked. Which of the following pairs of enzymes in humans is most similar to the enzymes being investigated by the scientists?
Q4
Maturity Onset Diabetes of the Young (MODY) type 2 is a consequence of a defective pancreatic enzyme, which normally acts as a glucose sensor, resulting in a mild hyperglycemia. The hyperglycemia is especially exacerbated during pregnancy. Which of the following pathways is controlled by this enzyme?
Q5
To maintain blood glucose levels even after glycogen stores have been depleted, the body, mainly the liver, is able to synthesize glucose in a process called gluconeogenesis. Which of the following reactions of gluconeogenesis requires an enzyme different from glycolysis?
Q6
A 22-year-old man presents to his primary care provider because of fever, diarrhea, and abdominal cramps. He has returned from Dhaka, Bangladesh recently where he was visiting his relatives. He is diagnosed with Shigella infection, and ciprofloxacin is started. He develops severe nausea and weakness 2 days later and complains of passing dark urine. The lab test results reveal a hemoglobin level of 7.9 g/dL, increased unconjugated bilirubin, increased reticulocyte count, increased lactate dehydrogenase, and increased blood urea. Which of the following is the best next step for the diagnosis of this patient’s condition?
Q7
To prepare for an endoscopy, a 27-year-old male was asked by the gastroenterologist to fast overnight for his 12 p.m. appointment the next day. Therefore, his last meal was dinner at 5 p.m. the day before the appointment. By 12 p.m. the day of the appointment, his primary source of glucose was being generated from gluconeogenesis, which occurs via the reversal of glycolysis with extra enzymes to bypass the irreversible steps in glycolysis. Which of the following irreversible steps of gluconeogenesis occurs in the mitochondria?
Q8
A 2-year-old boy presents to the emergency department with new onset seizures. After controlling the seizures with fosphenytoin loading, a history is obtained that reveals mild hypotonia and developmental delay since birth. There is also a history of a genetic biochemical disorder on the maternal side but the family does not know the name of the disease. Physical exam is unrevealing and initial lab testing shows a pH of 7.34 with a pCO2 of 31 (normal range 35-45) and a bicarbonate level of 17 mEq/L (normal range 22-28). Further bloodwork shows an accumulation of alanine and pyruvate. A deficiency in which of the following enzymes is most likely responsible for this patient's clinical syndrome?
Q9
A 16-year-old girl is brought to the emergency department unresponsive. A witness reports that she became anxious, lightheaded, and began sweating and trembling a few minutes before she lost consciousness. Her vitals are as follows: blood pressure 95/60 mm Hg, heart rate 110/min, respiratory rate 21/min, and temperature 35.5°C (95.5°F). She becomes responsive but is still somnolent. She complains of dizziness and weakness. A more detailed history reveals that she has drastically restricted her diet to lose weight for the past 18 hours, and has not eaten today. Her skin is pale, wet, and cold. The rest of the physical examination is unremarkable. Blood testing shows a plasma glucose level of 2.8 mmol/L (50.5 mg/dL). Which of the following statements is true?
Q10
A research group is investigating an allosteric modulator to improve exercise resistance and tolerance at low-oxygen conditions. The group has created cultures of myocytes derived from high-performance college athletes. The application of this compound to these cultures in a low-oxygen environment and during vigorous contraction leads to longer utilization of glucose before reaching a plateau and cell death; however, the culture medium is significantly acidified in this experiment. An activating effect on which of the following enzymes would explain these results?
Glycolysis US Medical PG Practice Questions and MCQs
Question 1: A 16-year-old teenager is brought to the emergency department after having slipped on ice while walking to school. She hit her head on the side of the pavement and retained consciousness. She was brought to the closest ER within an hour of the incident. The ER physician sends her immediately to get a CT scan and also orders routine blood work. The physician understands that in cases of stress, such as in this patient, the concentration of certain hormones will be increased, while others will be decreased. Considering allosteric regulation by hormones, which of the following enzymes will most likely be inhibited in this patient?
A. Glucose-6-phosphatase
B. Fructose 1,6-bisphosphatase
C. Pyruvate carboxylase
D. Phosphofructokinase (Correct Answer)
E. Glycogen phosphorylase
Explanation: ***Phosphofructokinase***
- In a stress state, **cortisol** and **epinephrine** levels are elevated, leading to increased **gluconeogenesis** and **glycogenolysis** to provide rapid energy.
- **Allosteric inhibition** of PFK-1 occurs through multiple mechanisms:
- **ATP** and **citrate** (high energy signals) act as direct **allosteric inhibitors** of PFK-1
- **Glucagon** (elevated during stress) indirectly inhibits PFK-1 by reducing levels of **fructose-2,6-bisphosphate**, a potent allosteric activator
- This inhibition of glycolysis spares glucose for critical organs like the brain and heart.
*Glucose-6-phosphatase*
- This enzyme catalyzes the final step of **gluconeogenesis** and **glycogenolysis**, converting G6P to free glucose.
- During stress, its activity is **stimulated** to increase blood glucose levels, not inhibited.
*Fructose 1,6-bisphosphatase*
- This enzyme plays a key role in **gluconeogenesis**, a process vital for maintaining glucose homeostasis during stress.
- Its activity would be **upregulated** to produce glucose, rather than inhibited.
*Pyruvate carboxylase*
- This enzyme initiates **gluconeogenesis** by converting pyruvate to oxaloacetate in the mitochondria.
- During stress, its activity is **stimulated** by elevated acetyl-CoA (an allosteric activator), not inhibited.
*Glycogen phosphorylase*
- This enzyme is responsible for **glycogenolysis**, the breakdown of glycogen into glucose-1-phosphate.
- Its activity is **stimulated** by stress hormones (epinephrine and glucagon) through cAMP-mediated phosphorylation, ensuring rapid glucose availability.
Question 2: A mother brings her newborn baby to the pediatrician after noting that his skin looks yellow. The patient's lactate dehydrogenase is elevated and haptoglobin is decreased. A smear of the child's blood is shown below. The patient is ultimately found to have decreased ability to process phosphoenolpyruvate to pyruvate. Which of the following metabolic changes is most likely to occur in this patient?
A. Increased ATP availability
B. Right shift of the oxyhemoglobin curve (Correct Answer)
C. Left shift of the oxyhemoglobin curve
D. Narrowing of the oxyhemoglobin curve
E. Broadening of the oxyhemoglobin curve
Explanation: ***Right shift of the oxyhemoglobin curve***
- The patient has **pyruvate kinase deficiency**, leading to decreased ATP production and a backup of glycolytic intermediates, including **2,3-bisphosphoglycerate (2,3-BPG)**.
- Increased 2,3-BPG reduces hemoglobin's affinity for oxygen, causing a **right shift** of the oxyhemoglobin dissociation curve, which facilitates oxygen release to tissues to compensate for anemia.
*Increased ATP availability*
- **Pyruvate kinase deficiency** impairs the final step of glycolysis, significantly reducing the net production of **ATP** in erythrocytes.
- This lack of ATP is a primary reason for premature red blood cell destruction and the resulting hemolytic anemia.
*Left shift of the oxyhemoglobin curve*
- A left shift indicates increased hemoglobin affinity for oxygen and would hinder oxygen delivery to tissues, which is not the compensatory mechanism seen in **pyruvate kinase deficiency**.
- A left shift is typically caused by conditions like decreased 2,3-BPG, alkalosis, or hypothermia.
*Narrowing of the oxyhemoglobin curve*
- The oxyhemoglobin dissociation curve does not typically "narrow"; rather, its position shifts right or left, or its slope might change.
- "Narrowing" is not a standard description of changes to the oxyhemoglobin dissociation curve in metabolic disorders like **pyruvate kinase deficiency**.
*Broadening of the oxyhemoglobin curve*
- Similar to "narrowing," "broadening" is not a recognized or physiologically relevant term to describe changes in the oxyhemoglobin dissociation curve.
- The crucial alteration in conditions affecting oxygen affinity is a shift in the curve's position, reflecting changes in hemoglobin's binding capacity for oxygen.
Question 3: Researchers are investigating a new mouse model of glycogen regulation. They add hepatocyte enzyme extracts to radiolabeled glucose to investigate glycogen synthesis, in particular two enzymes. They notice that the first enzyme adds a radiolabeled glucose to the end of a long strand of radiolabeled glucose. The second enzyme then appears to rearrange the glycogen structure such that there appears to be shorter strands that are linked. Which of the following pairs of enzymes in humans is most similar to the enzymes being investigated by the scientists?
A. Glycogen synthase and debranching enzyme
B. Branching enzyme and debranching enzyme
C. Glycogen phosphorylase and branching enzyme
D. Glycogen phosphorylase and glycogen synthase
E. Glycogen synthase and branching enzyme (Correct Answer)
Explanation: ***Glycogen synthase and branching enzyme***
- The first enzyme described, adding a **radiolabeled glucose to the end of a long strand of radiolabeled glucose**, perfectly matches the function of **glycogen synthase**, which elongates existing glycogen chains.
- The second enzyme, rearranging the glycogen structure to create **shorter strands that are linked**, describes the role of the **branching enzyme**, which forms α-1,6 glycosidic bonds to create branches in the glycogen molecule.
*Glycogen synthase and debranching enzyme*
- While **glycogen synthase** does elongate glycogen chains, the **debranching enzyme** (α-1,6 glucosidase) is involved in glycogenolysis by removing glucose units at branch points, not in synthesizing shorter, linked strands during glycogen synthesis.
- The description of the second enzyme's action is inconsistent with the role of a debranching enzyme, which breaks down branches rather than creating them in a synthetic process.
*Branching enzyme and debranching enzyme*
- The first enzyme is described as adding a single glucose unit to the end of a strand, which is the function of **glycogen synthase**, not the branching enzyme.
- The second enzyme's action of creating shorter, linked strands is consistent with the branching enzyme, but the initial enzyme described does not fit the role of either a branching or debranching enzyme in its initiation phase.
*Glycogen phosphorylase and branching enzyme*
- **Glycogen phosphorylase** breaks down glycogen by removing glucose-1-phosphate units from the ends of chains, which is the opposite of the described action of adding radiolabeled glucose to the end of a strand.
- While the **branching enzyme**'s action aligns with the second enzyme, the first enzyme's function is clearly not glycogen phosphorylase, as it is involved in synthesis, not degradation.
*Glycogen phosphorylase and glycogen synthase*
- **Glycogen phosphorylase** is responsible for glycogen breakdown, releasing glucose-1-phosphate, and therefore cannot be the first enzyme which is involved in adding glucose to a strand.
- While **glycogen synthase** aligns with the first enzyme, the second enzyme's action of creating shorter, linked strands is due to the **branching enzyme**, not glycogen synthase, which only extends linear chains.
Question 4: Maturity Onset Diabetes of the Young (MODY) type 2 is a consequence of a defective pancreatic enzyme, which normally acts as a glucose sensor, resulting in a mild hyperglycemia. The hyperglycemia is especially exacerbated during pregnancy. Which of the following pathways is controlled by this enzyme?
A. Fructose-6-phosphate --> fructose-1,6-bisphosphate
B. Phosphoenolpyruvate --> pyruvate
C. Glucose --> glucose-6-phosphate (Correct Answer)
D. Glucose-6-phosphate --> fructose-6-phosphate
E. Glyceraldehyde-3-phosphate --> 1,3-bisphosphoglycerate
Explanation: ***Glucose --> glucose-6-phosphate***
- This reaction is catalyzed by **glucokinase** in the pancreatic beta cells, which serves as a **glucose sensor** by controlling the rate-limiting step of glycolysis.
- MODY type 2 is caused by mutations in the **glucokinase gene (GCK)**, leading to a higher threshold for insulin secretion and mild hyperglycemia, particularly exacerbated during pregnancy.
*Fructose-6-phosphate --> fructose-1,6-bisphosphate*
- This step is catalyzed by **phosphofructokinase-1 (PFK-1)**, a key regulatory enzyme in glycolysis, but it is not the primary glucose sensor in pancreatic beta cells.
- While important for glycolysis, defects in PFK-1 are associated with glycolytic enzyme deficiencies (e.g., Tarui's disease), not MODY type 2.
*Phosphoenolpyruvate --> pyruvate*
- This final step of glycolysis is catalyzed by **pyruvate kinase**, an enzyme that is regulated but does not act as the primary glucose sensor.
- Pyruvate kinase deficiency leads to hemolytic anemia and is not associated with MODY type 2.
*Glucose-6-phosphate --> fructose-6-phosphate*
- This reversible isomerization step is catalyzed by **phosphoglucose isomerase**, and while part of glycolysis, it is not the rate-limiting step or the primary glucose sensing mechanism in pancreatic beta cells.
- Defects in this enzyme are rare and not linked to MODY type 2.
*Glyceraldehyde-3-phosphate --> 1,3-bisphosphoglycerate*
- This step is catalyzed by **glyceraldehyde-3-phosphate dehydrogenase (GAPDH)**, an important enzyme in glycolysis.
- GAPDH is involved in energy production but is not considered the glucose sensor for insulin release, and its defects are not associated with MODY type 2.
Question 5: To maintain blood glucose levels even after glycogen stores have been depleted, the body, mainly the liver, is able to synthesize glucose in a process called gluconeogenesis. Which of the following reactions of gluconeogenesis requires an enzyme different from glycolysis?
A. Fructose 1,6-bisphosphate --> Fructose-6-phosphate (Correct Answer)
B. Glyceraldehyde 3-phosphate --> 1,3-bisphosphoglycerate
C. 2-phosphoglycerate --> 3-phosphoglycerate
D. Dihydroxyacetone phosphate --> Glyceraldehyde 3-phosphate
E. Phosphoenolpyruvate --> 2-phosphoglycerate
Explanation: ***Fructose 1,6-bisphosphate --> Fructose-6-phosphate***
- This reaction in gluconeogenesis is catalyzed by **fructose 1,6-bisphosphatase**, which is distinct from **phosphofructokinase-1** that catalyzes the reverse reaction in glycolysis.
- This step is one of the three **irreversible steps** in glycolysis that must be bypassed by different enzymes in gluconeogenesis to ensure the unidirectional flow of the pathway.
*Glyceraldehyde 3-phosphate --> 1,3-bisphosphoglycerate*
- This reaction is catalyzed by **Glyceraldehyde 3-phosphate dehydrogenase** in both glycolysis and gluconeogenesis, as it is a **reversible step**.
- In gluconeogenesis, the equilibrium is shifted towards the formation of glyceraldehyde 3-phosphate due to the low concentration of products.
*2-phosphoglycerate --> 3-phosphoglycerate*
- This is a reversible isomerization reaction catalyzed by **phosphoglycerate mutase** in both glycolysis and gluconeogenesis.
- No unique enzyme is required for gluconeogenesis at this step.
*Dihydroxyacetone phosphate --> Glyceraldehyde 3-phosphate*
- This reversible interconversion between these two triose phosphates is catalyzed by **triose phosphate isomerase** in both pathways.
- These molecules are in equilibrium and can be readily converted from one to the other.
*Phosphoenolpyruvate --> 2-phosphoglycerate*
- This is a reversible reaction catalyzed by **enolase** in both glycolysis and gluconeogenesis.
- No distinct enzyme is needed for this step in gluconeogenesis.
Question 6: A 22-year-old man presents to his primary care provider because of fever, diarrhea, and abdominal cramps. He has returned from Dhaka, Bangladesh recently where he was visiting his relatives. He is diagnosed with Shigella infection, and ciprofloxacin is started. He develops severe nausea and weakness 2 days later and complains of passing dark urine. The lab test results reveal a hemoglobin level of 7.9 g/dL, increased unconjugated bilirubin, increased reticulocyte count, increased lactate dehydrogenase, and increased blood urea. Which of the following is the best next step for the diagnosis of this patient’s condition?
A. Direct antiglobulin (Coombs) test
B. ADAMTS-13 activity assay
C. Eosin-5-maleimide (EMA) binding test
D. Hemoglobin electrophoresis
E. Glucose-6-phosphate dehydrogenase (G6PD) enzyme assay (Correct Answer)
Explanation: ***Glucose-6-phosphate dehydrogenase (G6PD) enzyme assay***
- The patient's symptoms (nausea, weakness, dark urine) and lab findings (hemoglobin 7.9 g/dL, increased unconjugated bilirubin, increased reticulocyte count, increased LDH) indicate **acute hemolytic anemia**.
- **Ciprofloxacin** is an oxidant drug, and in a patient with recent travel history to Bangladesh, **glucose-6-phosphate dehydrogenase (G6PD) deficiency** should be strongly considered due to its higher prevalence in populations from South Asia, the Mediterranean, and Africa, and its role in drug-induced hemolysis.
- The **G6PD enzyme assay** (quantitative measurement) is the definitive diagnostic test for G6PD deficiency.
*Direct antiglobulin (Coombs) test*
- This test detects **autoimmune hemolytic anemia** by identifying antibodies or complement components bound to red blood cells.
- While the patient has hemolytic anemia, the context of **oxidant drug exposure (ciprofloxacin)** and travel to an endemic area makes G6PD deficiency a more likely cause than an autoimmune process.
*ADAMTS-13 activity assay*
- This assay is used to diagnose **Thrombotic Thrombocytopenic Purpura (TTP)** by measuring the activity of the ADAMTS13 enzyme.
- TTP involves microangiopathic hemolytic anemia, thrombocytopenia, and organ damage; however, the patient's presentation does not include **thrombocytopenia** or other features of TTP, making this diagnosis less likely.
*Eosin-5-maleimide (EMA) binding test*
- This test is used to diagnose **hereditary spherocytosis**, a condition in which red blood cells have a defective membrane skeleton leading to increased fragility and chronic hemolysis.
- While hereditary spherocytosis causes hemolytic anemia, the patient's **acute presentation immediately following ciprofloxacin use** makes G6PD deficiency a more pertinent diagnosis than a chronic hereditary condition.
*Hemoglobin electrophoresis*
- This test identifies abnormal hemoglobins, such as those found in **thalassemia** or **sickle cell disease**.
- These conditions cause chronic hemolytic anemia, but the acute onset of severe hemolysis directly linked to **oxidant drug exposure** is inconsistent with these inherited hemoglobinopathies.
Question 7: To prepare for an endoscopy, a 27-year-old male was asked by the gastroenterologist to fast overnight for his 12 p.m. appointment the next day. Therefore, his last meal was dinner at 5 p.m. the day before the appointment. By 12 p.m. the day of the appointment, his primary source of glucose was being generated from gluconeogenesis, which occurs via the reversal of glycolysis with extra enzymes to bypass the irreversible steps in glycolysis. Which of the following irreversible steps of gluconeogenesis occurs in the mitochondria?
A. Glucose-6-phosphate to glucose
B. Pyruvate to oxaloacetate (Correct Answer)
C. Phosphoenolypyruvate to pyruvate
D. Glucose-6-phosphate to 6-phosphogluconolactone
E. Fructose-1,6-bisphosphate to fructose-6-phosphate
Explanation: ***Pyruvate to oxaloacetate***
- This step, catalyzed by **pyruvate carboxylase**, is the initial and irreversible step of **gluconeogenesis** that occurs within the **mitochondrial matrix**.
- **Pyruvate** is converted to **oxaloacetate**, which then either is converted to malate to exit the mitochondria or remains in the mitochondria for subsequent steps of gluconeogenesis depending on the tissue.
*Glucose-6-phosphate to glucose*
- This final dephosphorylation step of gluconeogenesis, catalyzed by **glucose-6-phosphatase**, occurs in the **endoplasmic reticulum** lumen, not the mitochondria.
- It is crucial for releasing free glucose into the bloodstream.
*Phosphoenolypyruvate to pyruvate*
- This is an irreversible step in **glycolysis**, catalyzed by **pyruvate kinase**, and it is going in the *opposite direction* to what happens in gluconeogenesis.
- In gluconeogenesis, **pyruvate** is converted back to **phosphoenolpyruvate** via oxaloacetate, involving enzymes in both the mitochondria and cytoplasm.
*Glucose-6-phosphate to 6-phosphogluconolactone*
- This reaction is the first committed step of the **pentose phosphate pathway**, catalyzed by **glucose-6-phosphate dehydrogenase** and it occurs in the cytoplasm, not mitochondria.
- It is involved in producing NADPH and ribose-5-phosphate, not directly in gluconeogenesis.
*Fructose-1,6-bisphosphate to fructose-6-phosphate*
- This irreversible dephosphorylation step in gluconeogenesis, catalyzed by **fructose-1,6-bisphosphatase**, occurs in the **cytoplasm**.
- It bypasses the phosphofructokinase-1 step of glycolysis.
Question 8: A 2-year-old boy presents to the emergency department with new onset seizures. After controlling the seizures with fosphenytoin loading, a history is obtained that reveals mild hypotonia and developmental delay since birth. There is also a history of a genetic biochemical disorder on the maternal side but the family does not know the name of the disease. Physical exam is unrevealing and initial lab testing shows a pH of 7.34 with a pCO2 of 31 (normal range 35-45) and a bicarbonate level of 17 mEq/L (normal range 22-28). Further bloodwork shows an accumulation of alanine and pyruvate. A deficiency in which of the following enzymes is most likely responsible for this patient's clinical syndrome?
A. Glucose-6-phosphatase
B. Alanine transaminase
C. Glucose-6-phosphate dehydrogenase
D. Pyruvate dehydrogenase (Correct Answer)
E. Pyruvate kinase
Explanation: ***Pyruvate dehydrogenase***
- A deficiency in pyruvate dehydrogenase complex leads to the accumulation of **pyruvate** and **alanine**, as pyruvate cannot be converted into acetyl-CoA to enter the citric acid cycle.
- This accumulation results in **lactic acidosis**, presenting with symptoms like **seizures**, **hypotonia**, and developmental delay, consistent with the patient's presentation.
*Glucose-6-phosphatase*
- Deficiency in **glucose-6-phosphatase** causes **Type I glycogen storage disease (Von Gierke disease)**, characterized by **hypoglycemia**, hepatomegaly, and lactic acidosis.
- While there is lactic acidosis, the primary manifestations are related to glucose metabolism and not typically the accumulation of alanine and pyruvate to this extent.
*Alanine transaminase*
- **Alanine transaminase (ALT)** is an enzyme involved in amino acid metabolism, converting alanine and α-ketoglutarate into pyruvate and glutamate.
- A deficiency in ALT is not known to cause a distinct clinical syndrome with seizures, hypotonia, and the specific metabolic profile observed.
*Glucose-6-phosphate dehydrogenase*
- **Glucose-6-phosphate dehydrogenase (G6PD) deficiency** primarily affects **red blood cells**, leading to **hemolytic anemia** triggered by oxidative stress.
- It does not typically cause seizures, hypotonia, or the accumulation of pyruvate and alanine described in this case.
*Pyruvate kinase*
- **Pyruvate kinase deficiency** is another enzymatic defect in **glycolysis** that predominantly affects **red blood cells**, causing **hemolytic anemia**.
- While it can lead to some metabolic derangements, it is not the classic cause of central nervous system symptoms like seizures and developmental delay with the specific Lactic Acidosis described.
Question 9: A 16-year-old girl is brought to the emergency department unresponsive. A witness reports that she became anxious, lightheaded, and began sweating and trembling a few minutes before she lost consciousness. Her vitals are as follows: blood pressure 95/60 mm Hg, heart rate 110/min, respiratory rate 21/min, and temperature 35.5°C (95.5°F). She becomes responsive but is still somnolent. She complains of dizziness and weakness. A more detailed history reveals that she has drastically restricted her diet to lose weight for the past 18 hours, and has not eaten today. Her skin is pale, wet, and cold. The rest of the physical examination is unremarkable. Blood testing shows a plasma glucose level of 2.8 mmol/L (50.5 mg/dL). Which of the following statements is true?
A. Hypoglycemia in this patient is being compensated with an increased glycogenolysis rate. (Correct Answer)
B. Epinephrine-induced gluconeogenesis is the main process that allows for the compensation of a decreased glucose level.
C. There is an increase in the glycogen synthesis rate in this patient’s hepatocytes.
D. The patient’s symptoms are most likely the consequence of increased insulin secretion from the pancreatic islets.
E. The patient’s hypoglycemia inhibits glucagon release from pancreatic alpha cells.
Explanation: ***Hypoglycemia in this patient is being compensated with an increased glycogenolysis rate.***
- The patient's symptoms (anxiety, sweating, trembling, dizziness, weakness) and **low blood glucose (2.8 mmol/L)** confirm hypoglycemia. The immediate physiological response to hypoglycemia is the release of counter-regulatory hormones (glucagon, epinephrine, cortisol, growth hormone) which stimulate **glycogenolysis** (breakdown of glycogen to glucose) in the liver to maintain blood glucose, especially in the initial hours of fasting.
- Given that she has only fasted for 18 hours, her **hepatic glycogen stores** would still be recruited to provide glucose, making increased glycogenolysis a primary compensatory mechanism before gluconeogenesis becomes dominant.
*Epinephrine-induced gluconeogenesis is the main process that allows for the compensation of a decreased glucose level.*
- While epinephrine promotes **gluconeogenesis**, it is not the *main* compensatory process in the *initial* stages of fasting (0-24 hours). **Glycogenolysis** is the primary response in the first few hours.
- Gluconeogenesis becomes the predominant source of glucose after glycogen stores are significantly depleted, typically after 24 hours of fasting or longer.
*There is an increase in the glycogen synthesis rate in this patient’s hepatocytes.*
- **Glycogen synthesis (glycogenesis)** occurs when blood glucose levels are high, typically after a meal, to store excess glucose as glycogen.
- In a state of hypoglycemia, the liver's priority is to *release* glucose, meaning **glycogenolysis** is increased, and glycogen synthesis is inhibited.
*The patient’s symptoms are most likely the consequence of increased insulin secretion from the pancreatic islets.*
- **Increased insulin secretion** would *cause* hypoglycemia, not be a consequence. In response to hypoglycemia, insulin secretion is *reduced* to prevent further lowering of blood glucose.
- The symptoms described (anxiety, sweating, trembling) are characteristic of the **adrenergic response** to hypoglycemia, mediated by epinephrine and norepinephrine, which are counter-regulatory hormones.
*The patient’s hypoglycemia inhibits glucagon release from pancreatic alpha cells.*
- **Hypoglycemia** is a strong stimulant for **glucagon release** from pancreatic alpha cells. Glucagon's primary role is to raise blood glucose levels by promoting hepatic glycogenolysis and gluconeogenesis.
- Therefore, glucagon release would be *stimulated*, not inhibited, in this patient's condition.
Question 10: A research group is investigating an allosteric modulator to improve exercise resistance and tolerance at low-oxygen conditions. The group has created cultures of myocytes derived from high-performance college athletes. The application of this compound to these cultures in a low-oxygen environment and during vigorous contraction leads to longer utilization of glucose before reaching a plateau and cell death; however, the culture medium is significantly acidified in this experiment. An activating effect on which of the following enzymes would explain these results?
A. Bisphosphoglycerate mutase
B. Lactate dehydrogenase (Correct Answer)
C. Enolase
D. Malate dehydrogenase
E. Pyruvate dehydrogenase
Explanation: ***Lactate dehydrogenase***
- Enhanced **lactate dehydrogenase** activity would lead to increased conversion of **pyruvate to lactate**, regenerating **NAD+** for glycolysis to continue under **anaerobic conditions**.
- This process explains the **longer glucose utilization** and the significant **acidification of the medium** due to lactate production.
*Bisphosphoglycerate mutase*
- This enzyme is involved in the synthesis of **2,3-bisphosphoglycerate (2,3-BPG)** in red blood cells, which affects **hemoglobin's oxygen affinity**, not direct glucose utilization in myocytes under anaerobic conditions.
- While important for oxygen delivery, its activation would not primarily explain the observed **increased glucose utilization** and **lactic acid accumulation** in myocyte cultures.
*Enolase*
- **Enolase** catalyzes the conversion of **2-phosphoglycerate to phosphoenolpyruvate** in glycolysis.
- While crucial for glycolysis, its activation alone without an efficient disposal pathway for **pyruvate** (like lactate formation) would not sustain glucose metabolism and lead to such pronounced acidification under anaerobic stress.
*Malate dehydrogenase*
- **Malate dehydrogenase** is primarily involved in the **citric acid cycle** and the **malate-aspartate shuttle**, operating under **aerobic conditions** to convert malate to oxaloacetate.
- Its activation would not sustain glycolysis or lead to the observed **acidification** in a low-oxygen environment, where the citric acid cycle is inhibited.
*Pyruvate dehydrogenase*
- **Pyruvate dehydrogenase** converts **pyruvate to acetyl-CoA**, shunting carbons into the **citric acid cycle** for **aerobic respiration**.
- In a **low-oxygen environment**, this enzyme's activity would be limited due to reduced oxygen, and its activation would not explain the sustained glucose utilization or the significant **lactic acid accumulation** from anaerobic metabolism.
