How are the irreversible reactions of glycolysis bypassed in gluconeogenesis?
The correct answer and explanation is:
In gluconeogenesis, the irreversible reactions of glycolysis are bypassed through alternative enzymatic pathways. Glycolysis involves several irreversible steps, and gluconeogenesis must overcome these steps to synthesize glucose from non-carbohydrate precursors.
The three key irreversible steps in glycolysis are:
- Phosphofructokinase-1 (PFK-1): This enzyme catalyzes the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate.
- Hexokinase: This enzyme converts glucose to glucose-6-phosphate in the first step of glycolysis.
- Pyruvate kinase: This enzyme catalyzes the conversion of phosphoenolpyruvate to pyruvate.
In gluconeogenesis, these irreversible steps are bypassed by different enzymes:
- Pyruvate carboxylase: This enzyme converts pyruvate into oxaloacetate in the mitochondria. It is the first step in bypassing the pyruvate kinase step. Pyruvate carboxylase requires ATP and biotin as cofactors to function. The resulting oxaloacetate is then converted into phosphoenolpyruvate by the enzyme phosphoenolpyruvate carboxykinase (PEPCK), which is found in the cytoplasm or mitochondria.
- Fructose-1,6-bisphosphatase: This enzyme bypasses the phosphofructokinase-1 step in glycolysis. It dephosphorylates fructose-1,6-bisphosphate, converting it to fructose-6-phosphate.
- Glucose-6-phosphatase: This enzyme bypasses the hexokinase step by converting glucose-6-phosphate to free glucose. This step is important for the release of glucose into the bloodstream, particularly in the liver and kidneys.
These enzymatic steps allow for the formation of glucose from pyruvate, lactate, or other intermediates. The energy required for gluconeogenesis is provided by ATP and GTP. This process ensures that glucose is available for energy, especially during fasting or intense exercise when glucose from the diet is not available.