USMLE Step 1 Quiz: Enzymes Kinetics And Regulation

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?