Gluconeogenesis

1. 1

   Pyruvate + HCO3− + ATP → oxaloacetate + ADP + Pi

   Mitochondrial pyruvate carboxylase 6.4.1.1 ST-0016 Irreversible ATP Biotin (B7) Mg2+ Pi Bicarbonate/CO2
   In Pyruvate HCO3- ATP

   Out Oxaloacetate ADP Pi
   › Notes

   Mitochondrial pyruvate carboxylase requires covalently bound biotin, ATP, Mg2+, and bicarbonate as the CO2 source. This is the first bypass of pyruvate kinase and is effectively irreversible; it is allosterically activated by acetyl-CoA, linking fatty-acid oxidation to glucose production.

2. 2

   Oxaloacetate + NADH + H+ ↔ malate + NAD+

   Mitochondrial malate dehydrogenase 1.1.1.37 ST-0017 Reversible NAD+ NADH
   In Oxaloacetate NADH H+

   Out Malate NAD+
   › Notes

   Mitochondrial malate dehydrogenase reduces oxaloacetate to malate using NADH when oxaloacetate must be exported and cytosolic NADH is needed. Oxaloacetate cannot directly cross the inner mitochondrial membrane; malate is transported to the cytosol and reoxidized by cytosolic malate dehydrogenase in the next step. When gluconeogenic carbon enters as lactate, cytosolic lactate dehydrogenase supplies NADH, and mitochondrial PEP formation/export can be used instead.

3. 3

   Malate + NAD+ ↔ oxaloacetate + NADH + H+

   Cytosolic malate dehydrogenase 1.1.1.37 ST-0018 Reversible NAD+ NADH
   In Malate NAD+

   Out Oxaloacetate NADH H+
   › Notes

   Cytosolic malate dehydrogenase regenerates oxaloacetate and produces cytosolic NADH. The malate transport step itself is carrier-mediated rather than enzymatic; the two dehydrogenase reactions constitute the redox shuttle.

4. 4

   Oxaloacetate + GTP → phosphoenolpyruvate + GDP + CO2

   Phosphoenolpyruvate carboxykinase (PEPCK) 4.1.1.32 ST-0019 Irreversible GTP Mg2+ Mn2+
   In Oxaloacetate GTP

   Out Phosphoenolpyruvate GDP CO2
   › Notes

   Cytosolic or mitochondrial phosphoenolpyruvate carboxykinase (PEPCK) requires GTP and Mg2+/Mn2+. Decarboxylation drives phosphorylation to PEP; together with step 1, it bypasses pyruvate kinase and is effectively irreversible. Mitochondrially formed PEP can be exported when the mitochondrial isoenzyme is used.

5. 5

   Phosphoenolpyruvate + H2O ↔ 2-phosphoglycerate

   Enolase 4.2.1.11 ST-0020 Reversible Mg2+ H2O
   In Phosphoenolpyruvate H2O

   Out 2-Phosphoglycerate
   › Notes

   Enolase (Mg2+-dependent) catalyzes the reverse of glycolytic dehydration. This reaction is reversible.

6. 6

   2-Phosphoglycerate ↔ 3-phosphoglycerate

   Phosphoglycerate mutase 5.4.2.11 ST-0021 Reversible
   In 2-Phosphoglycerate

   Out 3-Phosphoglycerate
   › Notes

   Phosphoglycerate mutase reversibly transfers the phosphate from carbon 2 to carbon 3; the catalytic 2,3-bisphosphoglycerate-dependent mechanism is used by the corresponding isoenzyme.

7. 7

   3-Phosphoglycerate + ATP → 1,3-bisphosphoglycerate + ADP

   Phosphoglycerate kinase 2.7.2.3 ST-0022 Irreversible/directional ATP Mg2+
   In 3-Phosphoglycerate ATP

   Out 1,3-Bisphosphoglycerate ADP
   › Notes

   Phosphoglycerate kinase requires ATP and Mg2+. This is the reverse of glycolytic substrate-level phosphorylation and occurs twice for each glucose produced.

8. 8

   1,3-Bisphosphoglycerate + NADH + H+ → glyceraldehyde-3-phosphate + Pi + NAD+

   Glyceraldehyde-3-phosphate dehydrogenase 1.2.1.12 ST-0023 Irreversible/directional NAD+ NADH Pi
   In 1,3-Bisphosphoglycerate NADH H+

   Out Glyceraldehyde-3-phosphate Pi NAD+
   › Notes

   Glyceraldehyde-3-phosphate dehydrogenase uses NADH and is the reverse of glycolytic step 6. It occurs twice per glucose; the required cytosolic NADH may derive from lactate oxidation or from the malate shuttle.

9. 9

   Glyceraldehyde-3-phosphate ↔ dihydroxyacetone phosphate

   Triose phosphate isomerase 5.3.1.1 ST-0024 Reversible
   In Glyceraldehyde-3-phosphate

   Out Dihydroxyacetone phosphate
   › Notes

   Triose phosphate isomerase reversibly interconverts the trioses. One glyceraldehyde-3-phosphate remains unchanged, while the other is isomerized for condensation.

10. 10

    Glyceraldehyde-3-phosphate + dihydroxyacetone phosphate → fructose-1,6-bisphosphate

    Fructose-bisphosphate aldolase (Aldolase A/B/C) 4.1.2.13 ST-0025 Reversible
    In Glyceraldehyde-3-phosphate Dihydroxyacetone phosphate

    Out Fructose-1,6-bisphosphate
    › Notes

    Fructose-bisphosphate aldolase catalyzes reversible aldol condensation. No ATP is used.

11. 11

    Fructose-1,6-bisphosphate + H2O → fructose-6-phosphate + Pi

    Fructose-1,6-bisphosphatase (FBPase-1) 3.1.3.11 ST-0026 Irreversible rate-limiting H2O Pi
    In Fructose-1,6-bisphosphate H2O

    Out Fructose-6-phosphate Pi
    › Notes

    Fructose-1,6-bisphosphatase (FBPase-1) hydrolyzes the C1 phosphate. This is the key rate-limiting, bypass, and irreversible gluconeogenic reaction opposing PFK-1; it is not the same enzyme as the bifunctional PFK-2/FBPase-2 regulatory protein.

12. 12

    Fructose-6-phosphate ↔ glucose-6-phosphate

    Phosphoglucose isomerase 5.3.1.9 ST-0027 Reversible
    In Fructose-6-phosphate

    Out Glucose-6-phosphate
    › Notes

    Phosphoglucose isomerase reversibly forms the aldose phosphate. No nucleotide cofactor is required.

13. 13

    Glucose-6-phosphate + H2O → glucose + Pi

    Glucose-6-phosphatase (G6PC + G6PT translocase) 3.1.3.9 ST-0028 Irreversible H2O Pi
    In Glucose-6-phosphate H2O

    Out Glucose Pi
    › Notes

    The endoplasmic-reticulum glucose-6-phosphatase system hydrolyzes glucose-6-phosphate after transport into the ER lumen by glucose-6-phosphate translocase; the glucose and phosphate are returned to the cytosol through transporters. Hydrolysis requires water and is the final bypass of hexokinase/glucokinase, is irreversible, and permits hepatic or renal release of free glucose.

Showing all 13 steps.

Overview

Gluconeogenesis synthesizes glucose from noncarbohydrate precursors, principally lactate, glucogenic amino-acid carbon skeletons (especially alanine), glycerol, and propionate-derived succinyl-CoA. It maintains blood glucose during fasting, prolonged exercise, and starvation after hepatic glycogen becomes depleted. The pathway is not simply glycolysis in reverse because the three strongly exergonic glycolytic reactions are bypassed by four distinct reactions.

Cellular location

Gluconeogenesis is concentrated in the liver and, during prolonged fasting or acidosis, the renal cortex; small-intestinal enterocytes can contribute under selected conditions. Its enzymes span the mitochondrial matrix (pyruvate carboxylase; often mitochondrial PEPCK), cytosol (most reactions), endoplasmic-reticulum lumen/membrane (glucose-6-phosphatase catalytic system), and the mitochondrial–cytosolic malate shuttle. Skeletal muscle lacks glucose-6-phosphatase and cannot release free glucose into blood.

Net energetics

From two pyruvate, gluconeogenesis consumes 4 ATP + 2 GTP + 2 NADH: 2 pyruvate + 4 ATP + 2 GTP + 2 NADH + 6 H2O → glucose + 4 ADP + 2 GDP + 6 Pi + 2 NAD+ + 2 H+. Thus it costs six high-energy phosphate bonds per glucose; it produces no net ATP, NADPH, FADH2, or GTP. If lactate is the precursor, lactate dehydrogenase generates the required cytosolic NADH as lactate is oxidized to pyruvate.

Clinical significance

The hepatic Cori cycle uses gluconeogenesis to convert lactate released by anaerobic muscle and erythrocytes back to glucose, transferring an energetic burden to the liver. Reciprocal control of PFK-1 and FBPase-1 prevents futile cycling between glycolysis and gluconeogenesis. Fructose-1,6-bisphosphatase deficiency causes fasting hypoglycemia, lactic acidosis, ketosis, and hyperventilation; defects in pyruvate carboxylase similarly impair anaplerosis and gluconeogenesis, producing lactic acidosis and neurologic disease.