5.1: Gluconeogenesis and glycogenolysis (2023)

amino acids

The primary GNG substrates are derived from glucogenic amino acids released by cortisol-mediated protein catabolism. In the fasting state, cortisol is elevated and supports the fasting pathways by activating protein catabolism (in skeletal muscle) and increasing the transcription of enzymes necessary for gluconeogenesis (specifically phosphoenolcarboxykinase (PEPCK)). As skeletal muscle releases amino acids, primarily glutamine and alanine, they are absorbed by the liver. For use in glucose synthesis, they are transaminated to generate a useful intermediate of the TCA cycle, primarily \(\alpha\)-ketoglutaramate and pyruvate (see Figures 5.3 and 5.10). In the case of alanine, it can be transaminated to generate pyruvate. Glutamine is first deaminated by glutaminase, and the remaining glutamate is transaminated to form \(\alpha\)-ketoglutaramate (see Figure 5.11). Both pyruvate and \(\alpha\)-ketoglutaramate increase substrates in the TCA cycle, which ultimately increases the amount of malate available for transport out of mitochondria. Through this process of protein catabolism and transamination, glucogenic amino acids contribute to the synthesis of oxaloacetate (OAA), which is necessary for gluconeogenesis.

lactate

Lactate is mainly produced by the Cori cycle, or the anaerobic oxidation of glucose. (Note: The Cori cycle, or lactic acid cycle, refers to the metabolic pathway in which lactate, produced by anaerobic glycolysis in muscle or red blood cells, travels to the liver and is converted to glucose. Glucose returns to peripheral tissues and is converted to lactate and metabolized there). In the liver, lactate can be oxidized back to pyruvate via the reverse reaction catalyzed by lactate dehydrogenase (Figure 5.3).

Glycerin

When epinephrine or glucagon stimulates lipolysis, activation of hormone-sensitive lipase in adipose tissue allows hydrolysis of triacylglycerol into free three-chain fatty acids and glycerol. Glycerol released into the circulation is absorbed by the liver. Once in the liver, it can be converted to dihydroxyacetone phosphate (DHAP), a glycolytic intermediate. In this way, additional carbon can be obtained for the synthesis of glucose (fig. 5.4).

\(\beta\)-Oxidation

The \(\beta\) oxidation process supports gluconeogenesis in two ways:

1. IT WASN'T LONG₂resulting from \(\beta\)-oxidation is oxidized to ATP in the electron transport chain. This ATP provides the energy necessary for the synthesis of glucose. It also supplies energy to the urea cycle for nitrogen removal.
2. \(\beta\)-oxidation also generates acetyl-CoA. This compound is required to allosterically activate pyruvate carboxylase (Figure 5.5).

The acetyl-CoA produced by \(\beta\)-oxidation is not itself a substrate for gluconeogenesis, but is required for allosteric activation of pyruvate carboxylase, which is the first step of GNG. Again, acetyl-CoA is not a substrate for this process; it is completely oxidized in the TCA cycle and does not provide additional carbons to be exported from the TCA cycle as malate. Therefore, the cell depends on carbon backbones of amino acids, glycerol, and lactate as substrates for glucose production.Section 5.2).

Pyruvate-carboxylase and phosphoenol-carboxykinase (PEPCK)

Gluconeogenesis is essentially the opposite of glycolysis, with four major regulatory steps that allow the three irreversible steps of glycolysis to be bypassed (Figure 5.2). This first step of GNG starts in the mitochondria using pyruvate carboxylase (Figure 5.5). This enzyme converts pyruvate to oxaloacetate in mitochondria and requires biotin as a cofactor. This enzyme is allosterically activated by acetyl-CoA. The OAA produced is reduced to malate, which is transported out of the mitochondria by malate-aspartate transport. In the cytosol, malate is oxidized back to OAA and decarboxylated by the enzyme phosphoenolcarboxykinase (PEPCK) to generate phosphoenolpyruvate (Figure 5.3). The combination of these two enzymes, pyruvate carboxylase and PEPCK, allows the cell to bypass the irreversible step catalyzed by pyruvate kinase.

Once phosphoenolpyruvate (PEP) is synthesized, it continues through the reverse process using glycolytic enzymes until it reaches its next irreversible transformation.

Fructose-1,6-bisphosphatase (FBP1)

As PEP continues through the reversal of glycolysis, fructose-1,6-bisphosphate is produced. To bypass the irreversible step of glycolysis catalyzed by phosphofructokinase 1 (PFK1), the enzyme fructose-1,6-bisphosphatase (FBP1) is present and dephosphorylates fructose-1,6-bisphosphate to produce fructose-6-phosphate. This enzyme, FBP1, is inhibited by AMP and fructose-2,6-bisphosphate (Figure 5.2).

As with glycolysis, there is additional regulation by the bifunctional enzyme phosphofructokinase 2 (PFK2)/fructose-2,6-bisphosphatase (Figure 4.1). When fed, this bifunctional enzyme acts as a kinase (PFK2) and produces fructose-2,6-bisphosphate, which allosterically activates PFK1. In the fasting state, the enzyme is phosphorylated by glucagon-activated protein kinase A, which activates the phosphatase activity of the enzyme. The enzyme dephosphorylates fructose-2,6-bisphosphate and thus decreases the allosteric activation of PFK1 by facilitating the reverse reaction of fructose-1,6-bisphosphatase (Figure 5.2).

Glucose-6-phosphatase

Finally, glucose-6-phosphatase is required to dephosphorylate glucose-6-phosphate so that it can be released from the liver. This is an important step in both glycogenolysis and gluconeogenesis, and a deficiency of this enzyme can lead to severe episodes of fasting hypoglycemia.

hepatic glycogenolysis

In the liver, glucagon initiates glycogenolysis through a GPCR-mediated signaling cascade. This leads to activation of adenylyl cyclase and an increase in cAMP. cAMP activates protein kinase A, which phosphorylates and activates glycogen phosphorylase. Glycogen phosphorylase initiates the breakdown of glycogen. Even under these conditions, glycogen synthase is phosphorylated and inactivated by the same mechanism, ensuring that glycogen synthesis does not occur simultaneously (Figure 5.6).

Epinephrine can also increase hepatic glycogenolysis by binding to a \(\alpha\) receptor agonist. This stops the cleavage of phosphatidylinositol-4,5-bisphosphate (PIP₂) in inositol-1,4,5-triphosphate (IP₃) and diacylglycerol (DAG) by phospholipase C. IP₃stimulates the release of \(\ce{Ca}^{2+}\) from the endoplasmic reticulum and causes:

1. phosphorylation and activation of glycogen phosphorylase and
2. Phosphorylation and inactivation of glycogen synthase.

In all cases, glucose-6-phosphate released from glycogen stores is dephosphorylated by glucose-6-phosphatase and released from the liver.

Skeletal muscle glycogenolysis

Skeletal muscle glycogen is not affected by glucagon, but responds to AMP, \(\ce{Ca}^{2+}\), and epinephrine (Figure 5.7).

• The main regulator of this process is AMP. Elevated AMP will allosterically activate glycogen phosphorylase independent of phosphorylation.
• Then glycogen phosphorylase can be activated by \(\ce{Ca}^{2+}\). Similar to the previous cascade, calcium activates the \(\ce{Ca}^{2+}\) calmodulin complex, which in turn activates phosphorylase kinase, resulting in the phosphorylation and activation of glycogen phosphorylase.
• Finally, epinephrine can also stimulate skeletal muscle glycogenolysis through an increase in cAMP (the cascade of events is the same as in glucagon-stimulated hepatic glycogenolysis).