All questions
Question 1
Phosphofructokinase-1 (PFK-1) is a key regulatory enzyme in glycolysis, catalyzing the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate. Its activity is tightly controlled to match the cell's energy needs. In a cell with high energy charge, certain metabolic intermediates accumulate and bind to PFK-1 at a site distinct from the active site, causing a conformational change that decreases the enzyme's affinity for its substrate.
Which of the following molecules acts as a key allosteric inhibitor of PFK-1, signaling a state of high cellular energy?
- AMP
- Fructose-2,6-bisphosphate
- Citrate (correct answer)
- Inorganic phosphate
Explanation: Citrate is a key allosteric inhibitor of PFK-1. High levels of citrate indicate that the citric acid cycle is well-supplied with substrate and that cellular energy needs are being met. Citrate leaves the mitochondria and binds to an allosteric site on PFK-1, inhibiting glycolysis and preventing the unnecessary breakdown of glucose. ATP is another important allosteric inhibitor. In contrast, AMP and fructose-2,6-bisphosphate are potent allosteric activators of PFK-1.
Question 2
A 2-day-old male infant develops lethargy, vomiting, and poor feeding. He rapidly becomes comatose. Laboratory studies show a plasma ammonia level of 1500 µmol/L (normal < 50 µmol/L). The plasma citrulline level is very low, but urinary orotic acid is markedly elevated. This constellation of findings suggests a defect in the urea cycle.
A deficiency of which of the following enzymes best explains this patient's clinical and laboratory findings?
- Arginase
- Carbamoyl phosphate synthetase I
- Ornithine transcarbamylase (correct answer)
- N-acetylglutamate synthase
Explanation: This presentation is classic for ornithine transcarbamylase (OTC) deficiency, the most common urea cycle disorder. It is an X-linked recessive condition. OTC combines carbamoyl phosphate and ornithine to form citrulline. In its absence, carbamoyl phosphate accumulates in the mitochondria and leaks into the cytosol, where it is shunted into the pyrimidine synthesis pathway. This leads to a massive increase in orotic acid production and excretion. The combination of hyperammonemia, low citrulline, and high urinary orotic acid is pathognomonic for OTC deficiency.
Question 3
A 10-day-old infant is brought to the clinic for poor feeding, vomiting, and jaundice. The infant has been exclusively breastfed since birth. Physical examination reveals hepatomegaly and bilateral cataracts. A urine sample is positive for reducing substances, but a urine dipstick test for glucose is negative.
This infant's condition is most likely caused by a deficiency of which of the following enzymes?
- Lactase
- Galactokinase
- Aldose reductase
- Galactose-1-phosphate uridyltransferase (correct answer)
Explanation: This is a classic presentation of classic galactosemia, an autosomal recessive disorder caused by a deficiency of galactose-1-phosphate uridyltransferase (GALT). The presence of a reducing substance in the urine (galactose) that is not glucose is a key finding. The deficiency of GALT leads to the accumulation of galactose-1-phosphate, a toxic metabolite that causes liver damage, renal dysfunction, and brain damage. The accumulation of galactose also leads to its conversion to galactitol by aldose reductase, causing cataracts.
Question 4
A researcher is developing a new inhibitor for a specific enzyme. When the inhibitor is added to the enzyme reaction, it is found to bind only to the enzyme-substrate (ES) complex, not to the free enzyme. Kinetic analysis shows that in the presence of this inhibitor, both the Vmax and the Km of the enzyme are decreased by the same factor.
Which of the following terms best describes this type of enzyme inhibition?
- Competitive
- Noncompetitive
- Irreversible
- Uncompetitive (correct answer)
Explanation: Uncompetitive inhibition occurs when the inhibitor binds only to the enzyme-substrate (ES) complex. This binding effectively removes the ES complex from the reaction pathway, leading to a decrease in Vmax. Because the inhibitor binds the ES complex, it shifts the equilibrium toward ES formation, which manifests as an apparent increase in the enzyme's affinity for the substrate, thus decreasing the apparent Km. Lithium's effect on inositol monophosphatase is an example of uncompetitive inhibition.
Question 5
A 45-year-old man with a history of alcohol abuse presents to the emergency department with severe epigastric pain radiating to the back, nausea, and vomiting. Laboratory tests show markedly elevated serum amylase and lipase levels, confirming a diagnosis of acute pancreatitis. The pathophysiology of this condition involves the inappropriate activation of digestive proenzymes within acinar cells, leading to autodigestion of the pancreas.
The premature activation of which of the following zymogens is considered the key initiating event in this process?
- Proelastase
- Chymotrypsinogen
- Procarboxypeptidase
- Trypsinogen (correct answer)
Explanation: The central event in the pathogenesis of acute pancreatitis is the premature, intra-acinar activation of trypsinogen to trypsin. Normally, this activation occurs in the duodenum by the enzyme enteropeptidase. Once formed, trypsin is a powerful protease that can activate all other pancreatic zymogens (chymotrypsinogen, proelastase, procarboxypeptidase), leading to a cascade of enzymatic activation and pancreatic autodigestion.
Question 6
A 22-year-old man consumes a large carbohydrate-rich meal. In response to the subsequent hyperglycemia, insulin is released, promoting glucose uptake by hepatocytes. The first step of glucose metabolism in the liver is its phosphorylation to glucose-6-phosphate. The primary enzyme responsible for this step in the liver has a high Michaelis constant (Km) and is not inhibited by its product.
These kinetic properties allow the hepatic enzyme to function most effectively under which of the following conditions?
- During prolonged fasting
- When blood glucose levels are low
- In the postprandial state (correct answer)
- When cellular ATP levels are high
Explanation: The enzyme described is glucokinase, which is found in the liver and pancreatic β-cells. Glucokinase has a high Km for glucose (around 10 mM), meaning it has a low affinity and is only active when blood glucose concentrations are high, such as in the postprandial (after a meal) state. This prevents the liver from taking up glucose needed by other tissues (like the brain) during fasting. In contrast, hexokinase, found in most other tissues, has a low Km and is saturated at normal fasting glucose levels.
Question 7
A 30-year-old woman of Southeast Asian descent presents with chronic abdominal cramping, bloating, and flatulence. She notes that her symptoms are consistently worse after consuming milk or ice cream. She has found that her symptoms are alleviated by avoiding dairy products or by taking an over-the-counter supplement before a dairy-containing meal. Her condition is caused by a common age-dependent decline in the expression of a digestive enzyme.
The enzyme deficient in this patient is normally located in which of the following subcellular locations?
- Mitochondrial matrix
- Lysosome
- Intestinal brush border (correct answer)
- Pancreatic acinar cell granules
Explanation: The patient has lactose intolerance due to lactase non-persistence, a condition where the expression of the lactase enzyme declines after infancy. Lactase is a disaccharidase that breaks down lactose into glucose and galactose. It is an integral membrane protein located on the apical brush border of the intestinal enterocytes. When lactase is deficient, undigested lactose passes into the large intestine, where it is fermented by bacteria, producing gas and causing osmotic diarrhea.
Question 8
A 4-year-old boy who lives in an old house with peeling paint is brought to the pediatrician due to irritability and developmental delay. Laboratory studies reveal a microcytic anemia and basophilic stippling on peripheral blood smear. Further testing reveals elevated levels of δ-aminolevulinic acid (ALA) and protoporphyrin IX. The child is diagnosed with heavy metal poisoning. The toxic substance is known to inhibit ferrochelatase and ALA dehydratase by binding to sulfhydryl groups on the enzymes, at a site distinct from the substrate-binding site.
Which of the following kinetic changes would be expected for the affected enzymes in this patient?
- Increased Km, unchanged Vmax
- Decreased Km, decreased Vmax
- Unchanged Km, decreased Vmax (correct answer)
- Increased Km, decreased Vmax
Explanation: The patient has lead poisoning. Lead is a noncompetitive inhibitor of ferrochelatase and ALA dehydratase. Noncompetitive inhibitors bind to an allosteric site (a site other than the active site) on the enzyme, changing its conformation and reducing its catalytic efficiency. This type of inhibition cannot be overcome by increasing substrate concentration. Therefore, Vmax is decreased. Because the inhibitor does not compete with the substrate for the active site, the enzyme's affinity for the substrate (Km) remains unchanged.
Question 9
A 45-year-old farmworker is brought to the emergency department with confusion, salivation, lacrimation, urination, and muscle weakness. His colleagues report he was accidentally sprayed with a pesticide. The patient is treated with atropine and pralidoxime. The pesticide is found to have covalently modified a serine residue in the active site of acetylcholinesterase, rendering the enzyme nonfunctional.
This pesticide's effect on acetylcholinesterase is an example of which of the following?
- Reversible competitive inhibition
- Allosteric activation
- Irreversible inhibition (correct answer)
- Zymogen activation
Explanation: The patient is suffering from organophosphate poisoning. Organophosphates are irreversible inhibitors that form a stable covalent bond with a serine residue in the active site of acetylcholinesterase. This permanently inactivates the enzyme, leading to an accumulation of acetylcholine and cholinergic toxicity. The kinetic profile resembles noncompetitive inhibition (decreased Vmax) but the binding is not reversible. Pralidoxime can regenerate the enzyme if administered before the bond 'ages'.
Question 10
A medical student is studying the effects of a newly synthesized drug on an enzyme involved in gluconeogenesis. When the enzyme kinetics are plotted on a Lineweaver-Burk plot (1/[V] vs. 1/[S]), she observes that in the presence of the drug, the y-intercept increases, while the x-intercept does not change compared to the uninhibited reaction.
Based on these findings, the drug is most likely which type of inhibitor?
- Competitive
- Noncompetitive (correct answer)
- Uncompetitive
- Substrate
Explanation: On a Lineweaver-Burk plot, the y-intercept represents 1/Vmax and the x-intercept represents -1/Km. An increase in the y-intercept means that 1/Vmax has increased, which corresponds to a decrease in Vmax. An unchanged x-intercept means that -1/Km is unchanged, so Km is also unchanged. A decrease in Vmax with no change in Km is the characteristic finding of noncompetitive inhibition.
Question 11
A 55-year-old man with a long history of chronic alcoholism is admitted to the hospital with confusion, ataxia, and ophthalmoplegia. He is given an intravenous infusion of dextrose. Shortly after, his neurologic symptoms worsen acutely. The attending physician immediately administers a vitamin supplement, and the patient's condition improves. The patient's acute decompensation was caused by the increased metabolic demand for a cofactor required by the pyruvate dehydrogenase complex.
A deficiency of which of the following vitamins is responsible for this patient's condition?
- Riboflavin (B2)
- Niacin (B3)
- Thiamine (B1) (correct answer)
- Pyridoxine (B6)
Explanation: This patient presents with Wernicke encephalopathy, a condition caused by thiamine (vitamin B1) deficiency, common in chronic alcoholism. Thiamine pyrophosphate (TPP) is an essential cofactor for several key enzymes in carbohydrate metabolism, including pyruvate dehydrogenase, α-ketoglutarate dehydrogenase, and transketolase. Administering a glucose load to a thiamine-deficient patient increases the flux through glycolysis, depleting the remaining thiamine stores and precipitating or worsening the neurologic symptoms.
Question 12
A 32-year-old woman presents to her physician with a 2-month history of progressive dermatitis, alopecia, and enteritis. She reports being on a fad diet that involves consuming two raw egg whites daily. Laboratory studies are ordered to assess her nutritional status. A deficiency of a certain vitamin is suspected, which is known to be a coenzyme for enzymes that catalyze the transfer of one-carbon units in their most oxidized state.
The patient's symptoms are most likely due to a deficiency of a vitamin that is an essential cofactor for which class of enzymes?
- Dehydrogenases
- Transaminases
- Carboxylases (correct answer)
- Hydroxylases
Explanation: The patient's symptoms are characteristic of biotin (vitamin B7) deficiency. Raw egg whites contain avidin, a glycoprotein that binds tightly to biotin and prevents its absorption. Biotin serves as a prosthetic group for carboxylase enzymes, which catalyze carboxylation reactions (addition of a CO2 group). Important carboxylases include pyruvate carboxylase (gluconeogenesis), acetyl-CoA carboxylase (fatty acid synthesis), and propionyl-CoA carboxylase (metabolism of odd-chain fatty acids).
Question 13
A 40-year-old man is being treated for active tuberculosis with a multi-drug regimen that includes isoniazid. After several months of treatment, he develops a tingling sensation and numbness in his hands and feet, consistent with peripheral neuropathy. This adverse effect is known to be caused by the drug's interference with the metabolism of a specific vitamin. This vitamin, in its active form, is a crucial cofactor for the enzymes alanine aminotransferase (ALT) and aspartate aminotransferase (AST).
Isoniazid is most likely interfering with the function of which of the following vitamins?
- Folate (B9)
- Cobalamin (B12)
- Ascorbic acid (C)
- Pyridoxine (B6) (correct answer)
Explanation: Isoniazid can cause a functional deficiency of pyridoxine (vitamin B6) by increasing its excretion. The active form of B6, pyridoxal phosphate (PLP), is a critical coenzyme for transamination reactions, which are catalyzed by aminotransferases like ALT and AST. PLP is also required for decarboxylation and deamination reactions involved in the synthesis of neurotransmitters, and its deficiency can lead to peripheral neuropathy.
Question 14
A patient is diagnosed with a rare genetic disorder affecting Complex II of the electron transport chain. The disorder is caused by a mutation in the gene for succinate dehydrogenase, the enzyme that catalyzes the oxidation of succinate to fumarate. This reaction is a key step in both the citric acid cycle and the electron transport chain.
This enzyme requires a tightly bound coenzyme that functions as an electron acceptor. Which of the following is this coenzyme?
- NAD+
- FAD (correct answer)
- Coenzyme A
- Thiamine pyrophosphate
Explanation: Succinate dehydrogenase uses flavin adenine dinucleotide (FAD) as a covalently bound prosthetic group to accept electrons from succinate, oxidizing it to fumarate. The reduced coenzyme, FADH2, then transfers these electrons to the electron transport chain. Unlike NAD+, which is a soluble coenzyme, FAD is tightly bound to the enzyme.
Question 15
A 62-year-old woman with a history of heart failure is prescribed a diuretic for management of her symptoms and glaucoma. The drug works by inhibiting carbonic anhydrase in the proximal convoluted tubule of the kidney and the ciliary body of the eye. This enzyme catalyzes the rapid interconversion of carbon dioxide and water to bicarbonate and protons.
The catalytic activity of carbonic anhydrase is critically dependent on a central, coordinated atom of which of the following trace elements?
- Iron
- Copper
- Zinc (correct answer)
- Selenium
Explanation: Carbonic anhydrase is a metalloenzyme that requires a zinc ion (Zn2+) as a cofactor for its catalytic activity. The zinc ion is located in the active site and is essential for activating a water molecule, which then attacks the carbon dioxide substrate. Drugs like acetazolamide inhibit this enzyme, leading to diuretic and anti-glaucoma effects.
Question 16
During a period of fasting, the pancreatic hormone glucagon binds to receptors on hepatocytes, initiating a signaling cascade that leads to the breakdown of stored glycogen. The rate-limiting enzyme for this process is glycogen phosphorylase. This enzyme exists in an inactive 'b' form and an active 'a' form. The conversion from the inactive to the active form is rapid and allows for a quick mobilization of glucose.
The activation of glycogen phosphorylase in response to glucagon signaling occurs through which of the following mechanisms?
- Allosteric binding of glucose-6-phosphate
- Proteolytic cleavage of a proenzyme
- Covalent modification by phosphorylation (correct answer)
- Induction of gene transcription
Explanation: Glucagon signaling in the liver activates protein kinase A (PKA), which in turn activates glycogen phosphorylase kinase. This kinase then catalyzes the phosphorylation of glycogen phosphorylase b at a specific serine residue, converting it to the active glycogen phosphorylase a. This is an example of covalent modification, a common mechanism for rapidly regulating enzyme activity in response to hormonal signals.
Question 17
A 60-year-old man presents with severe, crushing substernal chest pain. An electrocardiogram shows ST-segment elevation in the anterior leads. To confirm a diagnosis of acute myocardial infarction, the physician orders blood tests for cardiac biomarkers. One of these biomarkers is an enzyme that exists as different isozymes in various tissues. The isozymes are composed of two subunits, which can be either 'M' type (for muscle) or 'B' type (for brain).
Which of the following isozymes of creatine kinase (CK) is most specific for myocardial damage?
- CK-MM
- CK-MB (correct answer)
- CK-BB
- Mitochondrial CK
Explanation: Creatine kinase (CK) exists as three main cytosolic isozymes. CK-MM is the predominant form in skeletal muscle. CK-BB is the predominant form in the brain. CK-MB is found in highest concentration in cardiac muscle, comprising about 15-20% of the total CK in the myocardium. Therefore, an elevated level of the CK-MB isozyme is highly specific for myocardial injury and was historically a key biomarker for diagnosing myocardial infarction.
Question 18
A 25-year-old man of Mediterranean descent is prescribed a sulfonamide antibiotic for a urinary tract infection. Two days later, he develops fatigue, jaundice, and dark-colored urine. A peripheral blood smear shows Heinz bodies and bite cells. This patient has a common X-linked enzyme deficiency that impairs the ability of red blood cells to handle oxidative stress.
This patient's condition is caused by a deficiency in an enzyme that directly produces which of the following essential molecules?
- ATP
- NADPH (correct answer)
- FADH2
- GTP
Explanation: The patient has glucose-6-phosphate dehydrogenase (G6PD) deficiency. G6PD is the rate-limiting enzyme of the pentose phosphate pathway. Its primary function is to produce NADPH. NADPH is essential for red blood cells as it is required by glutathione reductase to maintain a supply of reduced glutathione. Reduced glutathione is critical for detoxifying reactive oxygen species. In G6PD deficiency, oxidative drugs like sulfonamides can overwhelm the red blood cell's detoxification capacity, leading to oxidative damage, hemoglobin precipitation (Heinz bodies), and hemolysis.
Question 19
A newborn infant undergoes routine screening. Results reveal a significantly elevated level of phenylalanine in the blood. If left untreated, the infant is at high risk for severe intellectual disability, seizures, and hypopigmentation. The condition is caused by a deficiency of an enzyme that requires tetrahydrobiopterin (BH4) as a cofactor.
This disorder is caused by a deficiency in the enzyme that catalyzes the conversion of phenylalanine to which of the following amino acids?
- Alanine
- Tryptophan
- Tyrosine (correct answer)
- Aspartate
Explanation: The infant has classic phenylketonuria (PKU), an autosomal recessive disorder caused by a deficiency of the enzyme phenylalanine hydroxylase. This enzyme catalyzes the conversion of the essential amino acid phenylalanine into tyrosine. In PKU, this pathway is blocked, leading to an accumulation of phenylalanine and its metabolites (like phenylketones), which are neurotoxic. Tyrosine becomes an essential amino acid in these patients.
Question 20
An 8-month-old infant is brought to the pediatrician for evaluation. The parents report that since they started introducing fruits and juices into his diet, he has had episodes of vomiting, irritability, and poor feeding. Physical examination reveals jaundice and hepatomegaly. Laboratory studies show hypoglycemia, hyperbilirubinemia, and elevated liver transaminases. The condition is known to be caused by the trapping of phosphate within a toxic metabolite.
A deficiency in which of the following enzymes is the most likely cause of this infant's condition?
- Fructokinase
- Galactokinase
- Aldolase B (correct answer)
- Glucose-6-phosphatase
Explanation: This clinical presentation is classic for hereditary fructose intolerance, an autosomal recessive disorder caused by a deficiency of aldolase B. This enzyme is required to cleave fructose-1-phosphate into glyceraldehyde and dihydroxyacetone phosphate. When aldolase B is deficient, fructose-1-phosphate accumulates in hepatocytes, leading to the depletion of intracellular phosphate and ATP. This inhibits gluconeogenesis and glycogenolysis, causing severe hypoglycemia, and also leads to liver damage.