A 28-year-old woman with a family history of hereditary fructose intolerance presents with severe hypoglycemia, hepatomegaly, and lactic acidosis after consuming a fruit-based meal. Genetic testing reveals a deficiency in aldolase B. In vitro studies of her hepatic tissue show that the enzyme fructokinase (which catalyzes the first step of fructose metabolism) exhibits normal Vmax but a markedly reduced Km for fructose. Blood fructose levels are significantly elevated (12 mM; normal <1 mM). Based on Michaelis-Menten kinetics, which statement best explains why this patient develops severe hypoglycemia despite high fructose availability?

Explanation

## Hereditary Fructose Intolerance and Enzyme Kinetics **Key Point:** A reduced Km indicates higher affinity for substrate. At high fructose concentrations, the enzyme operates near Vmax, leading to rapid substrate consumption and severe metabolic consequences. ### Michaelis-Menten Kinetics in This Context $$V = \frac{V_{max} \times [S]}{K_m + [S]}$$ With a **reduced Km** and **normal Vmax**: - The enzyme binds fructose with higher affinity - At elevated blood fructose (12 mM), the enzyme operates at or near Vmax - Fructose-1-phosphate is produced rapidly and accumulates ### Pathophysiology of Hereditary Fructose Intolerance | Step | Mechanism | Consequence | |------|-----------|-------------| | 1. Fructose ingestion | Fructose enters hepatocytes | Blood fructose rises | | 2. Fructokinase activation | High affinity (low Km) + high [fructose] → rapid phosphorylation | Fructose-1-phosphate accumulates | | 3. ATP depletion | Fructose-1-phosphate accumulation traps phosphate; ATP regeneration cannot keep pace | Hepatic ATP drops dramatically | | 4. Inhibition of gluconeogenesis | ATP depletion inhibits gluconeogenic enzymes (especially fructose-1,6-bisphosphatase) | Blood glucose falls → hypoglycemia | | 5. Lactic acidosis | Impaired gluconeogenesis and glycogenolysis; shunting to lactate | Metabolic acidosis | | 6. Hepatomegaly | Fructose-1-phosphate accumulation + glycogen deposition | Hepatic enlargement | **High-Yield:** The **reduced Km** is the critical detail. It ensures that even at physiological fructose concentrations, the enzyme operates efficiently. When fructose is suddenly elevated (after a meal), the high affinity means rapid phosphorylation and catastrophic ATP depletion. ### Why Hypoglycemia Occurs 1. **Rapid fructose-1-phosphate formation** depletes the hepatic free phosphate pool 2. **ATP cannot be regenerated** because phosphate is sequestered in fructose-1-phosphate 3. **Gluconeogenesis is blocked** at the fructose-1,6-bisphosphatase step (ATP-dependent) 4. **Glycogenolysis is also impaired** (requires ATP) 5. **Net result**: Severe hypoglycemia despite high substrate availability **Clinical Pearl:** This is a classic example of how enzyme kinetics (Km) directly translates to clinical disease. The "efficient" enzyme (low Km) becomes pathological in the context of aldolase B deficiency, because fructose-1-phosphate cannot be cleaved and accumulates. ### Why the Correct Answer Fits Option 0 correctly identifies that: - Reduced Km → high affinity → rapid phosphorylation at elevated [fructose] - Fructose-1-phosphate accumulation → ATP depletion - ATP depletion → inhibition of gluconeogenesis → hypoglycemia This is the direct biochemical mechanism of hereditary fructose intolerance.