- Gluconeogenesis is a ubiquitous multistep process in which pyruvate or a related three-carbon compound (lactate, alanine) is converted to glucose.
- Seven of the steps in gluconeogenesis are catalyzed by the same enzymes used in glycolysis; these are the reversible reactions.
- Three irreversible steps in the glycolytic pathway are bypassed by reactions catalyzed by gluconeogenic enzymes:
- Conversion of pyruvate to PEP via oxaloacetate, catalyzed by pyruvate carboxylase and PEP carboxykinase;
- Dephosphorylation of fructose 1,6-bisphosphate by FBPase-1; and
- Dephosphorylation of glucose 6-phosphate by glucose 6-phosphatase.
1. Conversion of Pyruvate to Phosphoenolpyruvate
This reaction cannot occur by reversal of the pyruvate kinase reaction of glycolysis, which has a large, negative standard free-energy change and is irreversible. Instead, the phosphorylation of pyruvate is achieved by a roundabout sequence of reactions.
Pyruvate + HCO3 + ATP ---------> oxaloacetate + ADP + Pi
Oxaloacetate + NADH + H+ ----------> L-malate + NAD
Malate + NAD+ -----------> oxaloacetate + NADH + H+
Oxaloacetate + GTP ------------> PEP + CO2 + GDP
This reaction cannot occur by reversal of the pyruvate kinase reaction of glycolysis, which has a large, negative standard free-energy change and is irreversible. Instead, the phosphorylation of pyruvate is achieved by a roundabout sequence of reactions.
- Pyruvate is first transported from the cytosol into mitochondria.
- Then pyruvate carboxylase, a mitochondrial enzyme that requires the coenzyme biotin, converts the pyruvate to oxaloacetate.
Pyruvate + HCO3 + ATP ---------> oxaloacetate + ADP + Pi
- Because the mitochondrial membrane has no transporter for oxaloacetate, before export to the cytosol the oxaloacetate formed from pyruvate must be reduced to malate by mitochondrial malate dehydrogenase, at the expense of NADH:
Oxaloacetate + NADH + H+ ----------> L-malate + NAD
- Malate leaves the mitochondrion through a specific transporter in the inner mitochondrial membrane, and in the cytosol it is reoxidized to oxaloacetate, with the production of cytosolic NADH:
Malate + NAD+ -----------> oxaloacetate + NADH + H+
- The oxaloacetate is then converted to PEP by phosphoenolpyruvate carboxykinase. This Mg2+ dependent reaction requires GTP as the phosphoryl group donor :
Oxaloacetate + GTP ------------> PEP + CO2 + GDP
2. Conversion of Fructose 1,6-Bisphosphate to Fructose 6-Phosphate
Fructose 1,6-bisphosphate + H2O ----------------> fructose 6-phosphate + Pi
3. Conversion of Glucose 6-Phosphate to Glucose
Glucose 6-phosphate + H2O --------------> glucose + Pi
- The second glycolytic reaction that cannot participate in gluconeogenesis is the phosphorylation of fructose 6-phosphate by PFK-1.
- Because this reaction is highly exergonic and therefore irreversible in intact cells, the generation of fructose 6-phosphate from fructose 1,6-bisphosphate is catalyzed by a different enzyme, Mg2+ dependent fructose 1,6-bisphosphatase (FBPase-1), which promotes the essentially irreversible hydrolysis of the C-1 phosphate.
Fructose 1,6-bisphosphate + H2O ----------------> fructose 6-phosphate + Pi
3. Conversion of Glucose 6-Phosphate to Glucose
- The third bypass is the final reaction of gluconeogenesis, the dephosphorylation of glucose-6-phosphate to yield glucose.
- The reaction catalyzed by glucose 6-phosphatase and it is a simple hydrolysis of a phosphate ester:
Glucose 6-phosphate + H2O --------------> glucose + Pi