A 28-year-old woman from Delhi presents with a 2-year history of recurrent episodes of severe muscle pain and weakness triggered by sustained exercise or fasting. During an acute episode, her serum creatine kinase is 8,500 U/L (normal <200), myoglobin is elevated, and urine is dark brown. A muscle biopsy shows subsarcolemmal glycogen accumulation. Genetic testing identifies a homozygous mutation in the PFKM gene (phosphofructokinase-muscle isoform). Which of the following best explains why her symptoms are exacerbated during sustained exercise?

Explanation

## Phosphofructokinase (PFK) Deficiency — Glycogen Storage Disease Type VII ### Enzyme Function and Role in Glycolysis **Key Point:** Phosphofructokinase (PFK) catalyzes the phosphorylation of fructose-6-phosphate (F6P) to fructose-1,6-bisphosphate (F-1,6-BP) — the **rate-limiting and committed step of glycolysis**. $$\text{Fructose-6-phosphate} + \text{ATP} \xrightarrow{\text{PFK}} \text{Fructose-1,6-bisphosphate} + \text{ADP}$$ This step is: - **Irreversible** under physiological conditions - **Highly regulated** by ATP, AMP, citrate, and F-2,6-BP - **The major control point** for glycolytic flux ### Pathophysiology of PFK Deficiency ```mermaid flowchart TD A[PFKM mutation → PFK deficiency]:::outcome --> B[Glycolytic flux severely impaired]:::outcome B --> C[Reduced pyruvate production]:::outcome C --> D[Reduced ATP generation from glucose]:::outcome D --> E{Exercise begins}:::decision E -->|High ATP demand| F[Muscle must rely on:<br/>- Fatty acid oxidation<br/>- Amino acid catabolism<br/>- Creatine phosphate stores]:::action F --> G[ATP supply insufficient]:::urgent G --> H[Muscle cramps, pain, weakness]:::urgent B --> I[Glucose-6-phosphate accumulates]:::outcome I --> J[Shunted to glycogen synthesis]:::action J --> K[Subsarcolemmal glycogen accumulation]:::outcome ``` ### Why Exercise Triggers Acute Symptoms **High-Yield:** During sustained exercise: 1. **Normal muscle** relies on glycolysis for rapid ATP generation (especially in the first 1–2 minutes) 2. **PFK-deficient muscle** cannot generate ATP from glucose → ATP depletion 3. **Alternative fuels** (fatty acids, amino acids) are oxidized more slowly and cannot meet the acute ATP demand 4. **Creatine phosphate stores** are rapidly depleted 5. **Result:** Severe ATP deficit → muscle pain, cramps, weakness, and rhabdomyolysis **Clinical Pearl:** The "second wind" phenomenon is absent in PFK deficiency — patients do not experience the relief that normal individuals feel after 2–3 minutes of exercise (when aerobic metabolism kicks in). ### Clinical Features of GSD VII | Feature | Mechanism | |---------|----------| | **Muscle pain & weakness during exercise** | ATP depletion from blocked glycolysis | | **Dark urine (myoglobinuria)** | Rhabdomyolysis due to severe ATP deficit | | **Elevated CK (8,500 U/L)** | Muscle cell necrosis | | **Subsarcolemmal glycogen accumulation** | Glucose-6-phosphate shunted to glycogen synthesis | | **Symptoms with fasting** | Cannot mobilize glycogen for gluconeogenesis; no glucose oxidation | | **Normal lactate during exercise** | Glycolysis is blocked, so lactate is NOT produced (unlike normal exercise) | ### Diagnostic Clue: Absent Second Wind **Mnemonic:** **"No Second Wind in PFK"** — Normal exercisers feel relief after 2–3 min (aerobic metabolism begins); PFK-deficient patients get worse because they cannot switch to glycolytic ATP. ### Management **Key Point:** Avoid sustained exercise and fasting. Frequent carbohydrate intake (glucose, not fructose) may help, but glycolysis is still blocked. Creatine supplementation has shown some benefit in small studies. [cite:Robbins 10e Ch 7; Harrison 21e Ch 355]