Glucagon Signaling and Metabolic Regulation During Fasting

Glucagon Signaling Pathway During Fasting

During fasting, the predominant hormone is glucagon; there is an increase in the Glucagon/Insulin (G/I) ratio, which promotes the binding of glucagon to its receptor, causing a conformational change in the associated G protein. The G protein consists of three subunits: alpha (α), beta (β), and gamma (γ). The alpha subunit is initially bound to GDP. The conformational change in the G protein after ligand binding causes the alpha-GDP subunit to decrease its affinity for GDP and increase its affinity for GTP. This displacement of GDP by GTP generates a new conformational change, causing the alpha subunit to lose affinity for the beta and gamma subunits. The alpha-GTP subunit then dissociates to diffuse through the membrane and activate adenylate cyclase, an enzyme that catalyzes the formation of cAMP from ATP.

Activation of Protein Kinase A (PKA)

cAMP acts as a positive allosteric modulator of Protein Kinase A (PKA). PKA is a tetrameric protein consisting of four subunits: two regulatory and two catalytic. cAMP binds to the regulatory subunits, freeing the catalytic subunits to phosphorylate pyruvate kinase (in the liver) and the bifunctional (dual) enzyme.

Regulation of the Bifunctional Enzyme

The bifunctional enzyme possesses dual catalytic activities:

• Kinase activity: Represented by Phosphofructokinase-2 (PFK-2).
• Phosphatase activity: Represented by Fructose 2,6-bisphosphatase.

Impact on Glycolysis and Gluconeogenesis

During fasting, the phosphorylation of this dual enzyme promotes its phosphatase activity. Consequently, it synthesizes fructose 6-phosphate from fructose 2,6-bisphosphate. This leads to a decrease in the concentration of fructose 2,6-bisphosphate, which is a positive allosteric modulator of Phosphofructokinase-1 (PFK-1), the regulatory enzyme of glycolysis. A decrease in this modulator inhibits glycolysis. Furthermore, because fructose 2,6-bisphosphate is a negative allosteric modulator of fructose 1,6-bisphosphatase (a key regulatory enzyme of the gluconeogenic pathway), its reduction promotes gluconeogenesis.