Question

Why are the glycolytic reactions that are not irreversible able to run backwards in gluconeogenesis?

Answer 1

In Glycolysis, glucose is broken down into two pyruvate molecules. These two pyruvate molecules are feed into the citric acid cycle. In gluconeogenesis, from two pyruvate molecules, one glucose molecule is synthesized.

But gluconeogenesis is not quite reverse to that of glycolysis.

Glycolysis is an old metabolic pathway. It is common to both aerobic and anaerobic forms of living organisms, prokaryotic and eukaryotic organisms, unicellular and multicellular organisms. In Glycolysis of all of the organisms, 2 ATP molecules are produced.

Gluconeogenesis is carried out to produce Glucose, which is needed to be secreted into the bloodstream, when there is low blood glucose levels in the blood.

Gluconeogenesis is needed to bypass the energetically favorable or irreversible steps of glycolysis. The steps of the Glycolysis that are bypassed by Gluconeogenesis are 1.Glucose ---> Glucose-6-phosphate, 2.Fructose-6-phosphate ---> fructose-1,6-bisphosphate. In both of these steps, one ATP is invested in each.

3. Phospho enol pyruvate is converted into pyruvate molecules, which forms ATP. This step is subjected to regulation. Thus, the first two steps involved the phosphorylation and last step generated the end product.

The final step of glycolysis or the conversion of the PEP into pyruvate, which is energetically favorable process, so this step is first bypassed by gluconeogenesis. Gluconeogenesis has a two step pathway, which is split up between the mitochondrion and the cytosol.

To bypass this step, the pyruvate is converted into oxaloacetate by pyruvate carboxylase. , which converts the pyruvate into oxaloacetate by adding a C00- group.

Oxaloacetate cannot pass through the mitochondria, so oxaloacetate is converted into malate, which is transported out of the mitochondrion. Outside the mitochondrion, again the malate is converted back to oxaloacetate.

In gluconeogenesis, fructose,1,6-bisphosphatase enzyme catalyzes the hydrolysis of phosphate group from fructose, 1,6-bis phosphate. This results in the formation of the fructose-6-phosphate. phospho gluco isomerase converts the fructose-6-phosphate into glucose-6-phosphate.

Now by the action of the glucose-6-phosphatase enzyme glucose is obtained from the glucose-6-phosphate.

Thus, gluconeogenesis is not just the revese of glycolysis. So, in gluconeogenesis ATP is not formed, because the goal of the gluconeogenesis is not to produce the energy for the cell, but to produce glucose, which has to be circulated in the blood stream.

The PEP carboxykinase converts oxaloacetate to PEP.