Gluconeogenesis: Glucose Synthesis

• Gluconeogenesis is the formation of glucose from non-carbohydrate sources.
• Sources include lactate, pyruvate, glycerol, propionic acid, and amino acids (glucogenic amino acids).

Location of Gluconeogenesis

• Occurs in the cytosol and mitochondria.
• Primarily happens in the liver (90%), with the kidney contributing (10%).

Functions of Gluconeogenesis

• Supplies the body with glucose.
• Glucose is the only energy source for nervous tissues, RBCs, and skeletal muscles during exercises.
• Glucose is a precursor for milk sugar (lactose) in the mammary gland.
• Glucose is vital during low carbohydrate diets, liver glycogen depletion after 12-18 hours of fasting and starvation.
• Clears waste products, such as lactate produced by muscles and RBCs, from the blood.

When Gluconeogenesis is Needed

• During normal physiological situations, such as between meals and during sleep.
• During exercise/work, to recycle lactate.
• After a protein-rich diet (glucogenic amino acids).
• During starvation (glucogenic amino acids).

Gluconeogenesis Reactions

• The synthesis of glucose from pyruvate utilizes many of the same enzymes as glycolysis.
• Three glycolytic reactions are essentially irreversible.
• These reactions are:
• Hexokinase (or Glucokinase)
• Phosphofructokinase I
• Pyruvate Kinase
• Three stages are bypassed by four enzymes specific to gluconeogenesis:
  • Pyruvate carboxylase
  • Phosphoenolpyruvate carboxykinase (PEPCK)
  • Fructose 1,6-Bis phosphatase
  • Glucose 6-phosphatase

First Bypass Reaction

• Conversion of pyruvate to phosphoenolpyruvate.
• Requires participation of mitochondrial and cytosolic enzymes.
• Pyruvate is transported from the cytosol into mitochondria via the mitochondrial pyruvate transporter, or generated within mitochondria via deamination of alanine.
• Pyruvate is converted to OAA by the biotin-requiring enzyme pyruvate carboxylase.
• Reaction: Pyruvate + HCO3- + ATP → oxaloacetate + ADP + Pi + H+
• Pyruvate carboxylase is a regulatory enzyme.
• Oxaloacetate is reduced to malate by mitochondrial malate dehydrogenase.
• Reaction: Oxaloacetate + NADH + H+ → L-malate + NAD+
• Malate exits the mitochondrion via the malate/α-ketoglutarate carrier.
• In the cytosol, malate is reoxidized to oxaloacetate via cytosolic malate dehydrogenase.
• Reaction: L-malate + NAD+ → oxaloacetate + NADH + H+
• Oxaloacetate is then converted to phosphoenolpyruvate (PEP) by phosphoenolpyruvate carboxykinase.
• Reaction: Oxaloacetate + GTP → phosphoenolpyruvate + CO2 + GDP
• The overall equation for the set of bypass reactions is: Pyruvate + ATP + GTP + HCO3- → phosphoenolpyruvate + ADP + GDP + Pi + H+ + CO2
• Synthesis of one molecule of PEP requires an investment of 1 ATP and 1 GTP.
• If either pyruvate or the ATP/ADP ratio is high, the reaction is pushed toward the right (biosynthesis direction).

Second Bypass Reaction

• Conversion of fructose 1,6-bisphosphate to fructose 6-phosphate.
• The third glycolytic reaction (phosphorylation of fructose 6-phosphate by PFK1) is irreversible.
• Fructose 6-phosphate must be generated from fructose 1,6 bisphosphate by a different enzyme: fructose 1,6-bisphosphatase.
• This reaction is also irreversible.
• Reaction: Fructose 1,6-bisphosphate + H2O → fructose 6-phosphate + Pi

Third Bypass Reaction

• Glucose 6-phosphate to glucose.
• The hexokinase reaction is irreversible.
• The final reaction of gluconeogenesis is catalyzed by glucose 6-phosphatase.
• Reaction: Glucose 6-phosphate + H₂O → glucose + Pi
• Glucose 6-phosphatase is present in the liver, but absent in muscle.
• Glucose produced by gluconeogenesis in the liver is taken by the bloodstream to the brain and muscle.

Summary of Gluconeogenesis Pathway

• Overall equation: 2 Pyruvate + 4 ATP + 2 GTP + 6 H2O + 2 NADH → Glucose + 4 ADP + 2 GDP + 6 Pi + 2 NAD+ + 6H+
• For the conversion of two molecules of pyruvate into one molecule of glucose, 4 molecules of ATP, 2 molecules of GTP and 2 molecules of NADH + H+ are utilized.
  • Two pyruvate → two oxaloacetate (-2ATP)
  • Two oxaloacetate → two phosphoenolpyruvate (-2GTP)
  • Two 3 phosphoglycerate → two 1,3 Bisphosphoglycerate (-2ATP)
  • Two 1,3 Bisphosphoglycerate → two glyceraldehyde 3 phosphate (-2 NADH + H+) groups

Precursors for Gluconeogenesis

• Amino acids.
• Lactate.
• Glycerol.
• Propionic acid.

Amino Acids

• Derived from the hydrolysis of tissue proteins, serving as a major glucose source during a fast.
• α-Ketoacids, such as oxaloacetate and α-ketoglutarate, are derived from the metabolism of glucogenic amino acids.
• Can enter the citric acid cycle and form oxaloacetate as a direct precursor of phosphoenolpyruvate.
• Acetyl CoA and compounds giving rise to acetyl CoA (e.g., acetoacetate and amino acids like lysine and leucine) cannot give rise to a net synthesis of glucose.
• This is due to the irreversible pyruvate dehydrogenase reaction, which converts pyruvate to acetyl CoA.
• These compounds give rise to ketone bodies and are termed ketogenic.

Lactate (Lactic Acid)

• In vigorous skeletal muscle activity, a large amount of lactic acid (lactate) produced by anaerobic glycolysis is passed to the liver through bloodstream.
• Lactate is converted into pyruvate and then glucose, which is then sent back to the muscles for energy (Cori cycle).

Glycerol

• Released during the hydrolysis of triacylglycerols in adipose tissue.
• Delivered by the blood to the liver.
• Glycerol is phosphorylated by glycerol kinase to glycerol phosphate.
• Which is oxidized by glycerol phosphate dehydrogenase to dihydroxyacetone phosphate, an intermediate of glycolysis.
• Glycerol kinase is only in the liver and kidneys.

Propionic Acid

• Is a normal human physiological metabolite, produced by the oxidation of odd chain fatty acids.
• Propionic acid serves as a substrate for hepatic gluconeogenesis via conversion to succinyl-CoA.
• Even-chain fatty acids can only be oxidized to acetyl-CoA, so they cannot be used for gluconeogenesis.

Gluconeogenesis Regulation

• Hormonal and allosteric effectors regulate gluconeogenesis.

Hormonal Regulation

• Glucocorticoids (e.g., cortisol): stimulate gluconeogenesis by inducing the synthesis of gluconeogenesis enzymes.
• Also stimulates protein catabolism by tissues increasing glucogenic amino acids available for gluconeogenesis.
• Glucagon: Stimulates gluconeogenesis by stimulating fructose 1,6-bisphosphatase.
• Insulin: Inhibits gluconeogenesis and acts as an inhibitor for synthesis of enzymes of gluconeogenesis.

Allosteric Effectors (Acetyl CoA and ATP)

• Stimulate gluconeogenesis by inhibiting glycolysis (through inhibiting phosphofructokinase-1).
• Also stimulate gluconeogenesis (by fructose 1,6-bisphosphatase).
• Acetyl CoA also stimulates pyruvate carboxylase (gluconeogenesis) and inhibits pyruvate dehydrogenase (oxidation).