A 3-year-old boy from rural Maharashtra presents with recurrent episodes of severe hypoglycemia, lactic acidosis, and hepatomegaly. His parents report that symptoms worsen during fasting and improve after meals. Laboratory investigations show: blood glucose 35 mg/dL (fasting), lactate 8 mmol/L (normal <2), pyruvate 2.5 mmol/L (normal <0.1), and normal liver function tests. Genetic testing confirms homozygous loss-of-function mutation in the LDHA gene encoding lactate dehydrogenase A. Which glycolytic enzyme is MOST likely to be functionally impaired as a consequence of this mutation, leading to the observed metabolic derangement?

Explanation

## Pathophysiology of LDHA Deficiency ### Role of Lactate Dehydrogenase in Glycolysis **Key Point:** Lactate dehydrogenase (LDH) catalyzes the reversible conversion of pyruvate ↔ lactate and is essential for NAD^+^ regeneration during anaerobic glycolysis. The reaction is: $$\text{Pyruvate} + \text{NADH} + H^+ \rightleftharpoons \text{Lactate} + \text{NAD}^+$$ ### Why GAPDH Function Becomes Impaired Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) catalyzes the oxidation of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate: $$\text{G3P} + \text{NAD}^+ + P_i \rightarrow \text{1,3-BPG} + \text{NADH} + H^+$$ This is the **only NAD^+^-dependent step in glycolysis**. GAPDH requires continuous NAD^+^ regeneration. In normal conditions, LDH regenerates NAD^+^ from NADH by converting pyruvate to lactate. **High-Yield:** When LDHA is non-functional: 1. Pyruvate cannot be converted to lactate 2. NADH accumulates; NAD^+^ becomes depleted 3. GAPDH activity stalls due to lack of NAD^+^ substrate 4. Glycolysis is blocked at the GAPDH step 5. Glucose cannot be metabolized → severe hypoglycemia 6. Pyruvate accumulates (cannot be shunted to lactate) → lactic acidosis paradoxically worsens via alternative pathways ### Clinical Correlation **Clinical Pearl:** The elevated pyruvate with elevated lactate (despite LDH deficiency) occurs because: - Pyruvate is shunted into gluconeogenesis and alanine synthesis (Cori and glucose-alanine cycles) - Lactate is produced via alternative routes (e.g., from amino acid catabolism, residual LDH activity from LDHB) - The NAD^+^ depletion prevents further glycolysis, trapping the system ### Mnemonic **GAPDH-NAD Rule:** GAPDH is the **only NAD^+^-dependent glycolytic enzyme**. Any condition that depletes NAD^+^ (including LDH deficiency) impairs GAPDH first. ### Table: Glycolytic Enzyme Defects and Their Consequences | Enzyme | Substrate Accumulation | Product Deficiency | Clinical Presentation | |--------|------------------------|-------------------|----------------------| | Hexokinase | Glucose | G6P | Neonatal hypoglycemia (rare) | | Phosphofructokinase | F6P | F1,6BP | Myopathy, hemolysis, no hypoglycemia | | Aldolase | F1,6BP | DHAP + G3P | Hepatomegaly, hypoglycemia, acidosis | | GAPDH | G3P | 1,3-BPG | **Severe hypoglycemia, lactic acidosis** | | Pyruvate kinase | PEP | Pyruvate | Hemolytic anemia, mild hypoglycemia | **Warning:** Do not confuse LDH deficiency with pyruvate kinase deficiency. PK deficiency causes hemolytic anemia as the primary feature; LDH deficiency causes severe metabolic derangement.