Regulation of gluconeogenesis
- Gluconeogenesis is stimulated under conditions of fasting and starvation under the effect of glucagon and catecholamines.
- The key enzymes of regulation are pyruvate carboxylase, phosphoenol pyruvate carboxy kinase and fructose 1, 6 bisphosphatase which are stimulated if there is need for glucose production.
- Gluconeogenesis, like glycolysis, is under tight control of hormones to regulate blood glucose. Stress hormones such as glucagon or cortisol upregulate PEPCK and fructose 1, 6-bisphosphatase to stimulate gluconeogenesis.
Three mechanisms are responsible for regulating the activity of key enzymes of gluconeogenesis
(1) Changes in the rate of enzyme synthesis (Induction/Repression)
(2) Covalent modification by reversible phosphorylation
(3) Allosteric effects.
1) Induction & Repression of Key Enzymes
The amounts and the activities of essential enzymes (catalysing physiologically irreversible reactions) are regulated by hormones. Hormones affect gene expression primarily by changing the rate of transcription, as well as by regulating the degradation of mRNA.
Insulin: Concentration rises subsequent to hyperglycemia after meals. It stimulates the expression of glycolytic enzymes such as phosphofructokinase-1 and pyruvate kinase and also the bifunctional enzyme (PFK-2 – Fr-2,6 bisphosphatase) that makes and degrades F-2,6-BP. Thus insulin stimulates glycolysis and inhibits gluconeogenesis.
Glucagon: Concentration rises during starvation. It inhibits the expression of glycolytic enzymes and instead stimulates the production of two key gluconeogenic enzymes, phosphoenolpyruvate carboxy kinase and fructose 1, 6-bisphosphatase. As a result glycolysis is inhibited and gluconeogenesis is stimulated.
(Transcriptional control in eukaryotes is much slower than allosteric control; it takes hours or days in contrast with seconds to minutes).
2) Covalent Modification by Reversible Phosphorylation (rapid process)
- Glucagon and epinephrine: Hormones that decrease blood glucose level (Inhibit glycolysis and stimulate gluconeogenesis in the liver by increasing the concentration of cAMP).
- cAMP activates cAMP-dependent protein kinase, leading to the phosphorylation and inactivation of mainly two enzymes :
1) Pyruvate kinase
- Pyruvate kinase exists in two forms: phosphorylated form (inactive) and dephosphorylated form (active).
- Insulin activates this enzyme by causing dephosphorylation (By stimulating phosphatase enzyme).
- Glucagon inactivates this enzyme by bringing about cAMP mediated phosphorylation
When the pyruvate kinase is inactive, phosphoenol pyruvate is channeled towards glucose production (gluconeogenesis)
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2) PFK 2/Fructose 2, 6 bisphosphatase bifunctional enzyme
In phosphorylated state: Fructose 2, 6 bisphosphatase is active
In dephosphorylated state: PFK2 is active
When Fructose 2, 6 bisphosphatase is active, it lowers the concentration of fructose 2, 6 bisphosphate and favors gluconeogenesis.
3) Allosteric Modification (an instantaneous process)
a) Role of Acetyl co A
Acetyl-CoA is a positive allosteric modulator of pyruvate carboxylase (gluconeogenesis) and a negative modulator of pyruvate dehydrogenase.
Fate of pyruvate in the mitochondrion: Pyruvate can be converted either to acetyl-CoA (by the pyruvate dehydrogenase complex) to fuel the citric acid cycle or to oxaloacetate (by pyruvate carboxylase) to start the process of gluconeogenesis.
When fatty acids are readily available as fuels, their breakdown in liver mitochondria yields acetyl-CoA, a signal that further oxidation of glucose for fuel is not necessary.
- When the cell’s energetic needs are being met, oxidative phosphorylation slows, NADH rises relative to NAD+ and inhibits the citric acid cycle, and acetyl-CoA accumulates.
- The increased concentration of acetyl-CoA inhibits the pyruvate dehydrogenase complex, slowing the formation of acetyl-CoA from pyruvate, and stimulates gluconeogenesis by activating pyruvate carboxylase, allowing excess pyruvate to be converted to glucose.
The reciprocal relationship between these two enzymes alters the metabolic fate of pyruvate as the tissue changes from carbohydrate oxidation (glycolysis) to gluconeogenesis during the transition from the fed to fasting state.
b) Role of ATP, AMP and citrate
ATP: Allosteric activator of fructose 1, 6 bisphosphatase
AMP: Allosteric inhibitor of fructose 1, 6 bisphosphatase
Citrate: Allosteric activator of Fructose 1,6 bisphosphatase
ADP: Inhibits phosphoenolpyruvate carboxykinase and pyruvate carboxylase.
A high level of AMP indicates that the energy charge is low and signals the need for ATP generation. Conversely, high levels of ATP and citrate indicate that the energy charge is high and that biosynthetic intermediates are abundant. Under these conditions, glycolysis is nearly switched off and gluconeogenesis is promoted.
3) Role of fructose 2, 6 bisphosphate
- The most potent allosteric inhibitor of fructose 1, 6-bisphosphatase.
- It inhibits fructose 1,6-bisphosphatase by increasing the Kmfor fructose 1,6-bisphosphate.
- When there is an abundant supply of glucose, the concentration of fructose 2, 6-bisphosphate increases, stimulating glycolysis by activating phosphofructokinase-1 and inhibiting fructose 1,6-bisphosphatase.
- In the fasting state, glucagon stimulates the production of cAMP, activating cAMP-dependent protein kinase, which in turn inactivates phosphofructokinase-2 and activates fructose 2,6-bisphosphatase by phosphorylation. Hence, gluconeogenesis is stimulated by a decrease in the concentration of fructose 2, 6-bisphosphate, which inactivates phosphofructokinase-1 and relieves the inhibition of fructose 1, 6-bisphosphatase.
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