## Distinguishing Feature Between Two Glycolytic Enzyme Defects ### Pyruvate Dehydrogenase (PDH) vs Phosphofructokinase (PFK) Deficiency Both are carbohydrate metabolism disorders, but they affect different metabolic junctions: | Feature | Pyruvate Dehydrogenase Deficiency | Phosphofructokinase Deficiency | |---------|-----------------------------------|--------------------------------| | **Enzyme location** | Mitochondrial matrix | Cytoplasm | | **Substrate** | Pyruvate | Fructose-1,6-bisphosphate | | **Product** | Acetyl-CoA | Glyceraldehyde-3-phosphate + DHAP | | **Metabolic block** | Pyruvate → Acetyl-CoA (TCA entry) | Glycolysis (mid-stage) | | **Lactate accumulation** | Severe (pyruvate shunts to lactate) | Mild (glycolysis blocked upstream) | | **Glucose utilization** | Can enter glycolysis but cannot exit to TCA | Blocked at glycolysis rate-limiting step | | **Exercise intolerance** | Severe (ATP generation blocked) | Moderate (some ATP from upper glycolysis) | ### Key Point: **Pyruvate dehydrogenase deficiency** specifically impairs the conversion of pyruvate to acetyl-CoA, which is the critical link between glycolysis and the TCA cycle. This is the defining discriminator. ### High-Yield: PDH deficiency causes **lactic acidosis** because pyruvate accumulates and is shunted to lactate via lactate dehydrogenase. PFK deficiency causes **glycolytic blockade** with accumulation of glucose-6-phosphate and fructose-6-phosphate upstream. ### Clinical Pearl: PDH deficiency patients present with **severe exercise intolerance, developmental delay, and lactic acidosis**. PFK deficiency (Tarui disease) presents with **myalgia and cramps on exertion** but less severe systemic manifestations. ### Mnemonic: **PDH = Post-glycolysis block** (pyruvate dehydrogenase is after glycolysis ends); **PFK = Pre-glycolysis completion** (phosphofructokinase blocks glycolysis mid-way). [cite:KD Tripathi 8e Ch 8]
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