Bchadvanced
Explain why gluconeogenesis needs bypass reactions rather than simply reversing glycolysis.
Answer
Some glycolytic reactions are irreversible under cellular conditions. Gluconeogenesis bypasses these reactions using pyruvate carboxylase and phosphoenolpyruvate carboxykinase for the pyruvate-to-phosphoenolpyruvate route, fructose-1,6-bisphosphatase for the phosphofructokinase step, and glucose-6-phosphatase for glucose release.
Explanation
### What a good answer covers
- Several glycolytic reactions are effectively irreversible.
- Gluconeogenesis uses alternate enzymes to bypass them.
- Pyruvate carboxylase and phosphoenolpyruvate carboxykinase bypass the pyruvate kinase step.
- Fructose-1,6-bisphosphatase and glucose-6-phosphatase bypass other irreversible steps.
### Terms
- **Pyruvate carboxylase**: Adds carbon dioxide to pyruvate to form oxaloacetate.
- **Phosphoenolpyruvate carboxykinase**: Converts oxaloacetate to phosphoenolpyruvate.
### Remember
Pathways that oppose one another can share reversible steps but need distinct enzymes for irreversible steps.
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