Which enzymes bypass irreversible glycolytic steps in gluconeogenesis?

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Multiple Choice

Which enzymes bypass irreversible glycolytic steps in gluconeogenesis?

When gluconeogenesis builds glucose from noncarbohydrate sources, the cell has to bypass the three highly irreversible steps of glycolysis with dedicated enzymes that drive the reverse reactions in a way that can proceed in the opposite direction. The first bypass involves converting pyruvate to oxaloacetate in the mitochondria using pyruvate carboxylase (with ATP and biotin), then moving oxaloacetate to the cytosol where PEP carboxykinase converts it to phosphoenolpyruvate (using GTP). This pair effectively skips the pyruvate kinase step that would otherwise push glycolysis forward. The next bypass addresses the step from fructose-6-phosphate to fructose-1,6-bisphosphate. The enzyme fructose-1,6-bisphosphatase hydrolyzes the phosphate to yield fructose-6-phosphate, reversing that hardest part of glycolysis in a way that is not simply the glycolytic enzyme working in reverse. Finally, glucose-6-phosphate is converted to glucose by glucose-6-phosphatase, allowing release of free glucose into the bloodstream. This step bypasses the hexokinase/glucokinase step. These bypass enzymes—pyruvate carboxylase, PEP carboxykinase, fructose-1,6-bisphosphatase, and glucose-6-phosphatase—are the specialists that enable gluconeogenesis to move uphill from pyruvate, through to glucose. The other enzymes listed participate in glycolysis itself or are involved in related pathways, and do not provide the bypass needed for gluconeogenesis.

When gluconeogenesis builds glucose from noncarbohydrate sources, the cell has to bypass the three highly irreversible steps of glycolysis with dedicated enzymes that drive the reverse reactions in a way that can proceed in the opposite direction. The first bypass involves converting pyruvate to oxaloacetate in the mitochondria using pyruvate carboxylase (with ATP and biotin), then moving oxaloacetate to the cytosol where PEP carboxykinase converts it to phosphoenolpyruvate (using GTP). This pair effectively skips the pyruvate kinase step that would otherwise push glycolysis forward.

The next bypass addresses the step from fructose-6-phosphate to fructose-1,6-bisphosphate. The enzyme fructose-1,6-bisphosphatase hydrolyzes the phosphate to yield fructose-6-phosphate, reversing that hardest part of glycolysis in a way that is not simply the glycolytic enzyme working in reverse.

Finally, glucose-6-phosphate is converted to glucose by glucose-6-phosphatase, allowing release of free glucose into the bloodstream. This step bypasses the hexokinase/glucokinase step.

These bypass enzymes—pyruvate carboxylase, PEP carboxykinase, fructose-1,6-bisphosphatase, and glucose-6-phosphatase—are the specialists that enable gluconeogenesis to move uphill from pyruvate, through to glucose. The other enzymes listed participate in glycolysis itself or are involved in related pathways, and do not provide the bypass needed for gluconeogenesis.

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