Gluconeogenesis

              Gluconeogenesis is the generation of glucose from non-sugar carbon substrates like pyruvate, lactate, glycerol, and glucogenic amino acids.

The vast majority of gluconeogenesis takes place in the liver and, to a smaller extent, in the cortex of kidneys. This process occurs during periods of fasting, starvation, or intense exercise and is highly endergonic. Gluconeogenesis is often associated with ketosis.

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Entering the pathway

Many 3- and 4-carbon substrates can enter the gluconeogenesis pathway. lactate from anaerobic respiration in skeletal muscle is easily converted to pyruvate in the liver cells; this happens as part of the Cori cycle. However, the first designated substrate in the gluconeogenic pathway is pyruvate.

Oxaloacetate (an intermediate in the citric acid cycle) can also be used for gluconeogenesis. The gluconeogenic pathway can also generate glucose from amino acids, with the exception of lysine and leucine. Following removal of the amino group (by transamination or deamination) from the amino acid, the remaining carbon skeleton can enter gluconeogenesis directly (as pyruvate or oxaloacetate), or indirectly, e.g., via the citric acid cycle, converting α-ketoglutarate to oxaloacetate.

Fatty acids cannot be converted into glucose in animals, the exception being odd-chain fatty acids, which can yield propionyl CoA, a precursor for succinyl CoA. In plants, i.e., mainly in the seeds, the glyoxylate cycle allows conversion of fats into glucose, which is then used in the synthesis of complex carbohydrates, such as cellulose and glucans required for formation of new cell walls during germination. However, normally fatty acids are broken down into the two-carbon acetyl CoA, which is then used to fuel the citric acid cycle, and thus becomes unavailable to gluconeogensis. In contrast, glycerol, which is a part of all triacylglycerols, can be used in gluconeogenesis. In organisms in which glycerol is derived from glucose (e.g., humans and other mammals), glycerol is sometimes not considered a true gluconeogenic substrate, as it cannot be used to generate new glucose.

Pathway

• Gluconeogenesis is a pathway consisting of eleven enzyme-catalyzed reactions.
• Gluconeogenesis begins with the formation of oxaloacetate through carboxylation of pyruvate at the expense of one molecule of ATP, but is inhibited in the presence of high levels of ADP. This reaction is catalyzed by pyruvate carboxylase, which is stimulated by high levels of acetyl-CoA, i.e., when fatty acid oxidation is high in the liver.
• Oxaloacetate is then decarboxylated and simultaneously phosphorylated by phosphoenolpyruvate carboxykinase to produce phosphoenolpyruvate. One molecule of GTP is hydrolyzed to GDP in the course of this reaction. Oxaloacetate has to be reduced into malate using NADH in order to be transported out of the mitochondria. In the cytoplasm, malate is again oxidized to oxaloacetate using NAD⁺ where said reactions can occur.
• The next steps in the reaction are the same as reversed glycolysis. However fructose-1,6-bisphosphatase converts fructose-1,6-bisphosphate to fructose-6-phosphate. The purpose of this reaction is to overcome the large negative ΔG.
• Glucose-6-phosphate is formed from fructose-6-phosphate by phosphoglucoisomerase. Glucose-6-phosphate is used in other pathways. Free glucose is not generated automatically because glucose, unlike glucose-6-phosphate, tends to freely diffuse out of the cell. The reaction of actual glucose formation is carried out in the lumen of the endoplasmic reticulum. Here, glucose-6-phosphate is hydrolyzed by glucose-6-phosphatase, the last enzyme in gluconeogenesis, to produce glucose. Glucose is then shuttled into the cytosol by glucose transporters located in the membrane of the endoplasmic reticulum.

Regulation

Gluconeogenesis cannot be considered to be simply a reverse process of glycolysis, as the three irreversible steps in glycolysis are bypassed in gluconeogenesis. This is done to ensure that glycolysis and gluconeogenesis are not operating at the same time in the cell, making it a futile cycle. Therefore, glycolysis and gluconeogenesis follow reciprocal regulation, that is, cellular conditions, which inhibit glycolysis, may in turn activate gluconeogenesis.

Glucose-6-phosphate regulates the enzyme glucose-6-phosphatase in the lumen of ER by inducing its activity. In contrast, its accumulation will feed-back inhibit hexokinase in glycolysis. Once again, it follows the principle of reciprocal regulation.

The majority of the enzymes responsible for gluconeogenesis are found in the cytoplasm; the exceptions are mitochondrial pyruvate carboxylase, and, in animals, phosphoenolpyruvate carboxykinase. The latter exists as isozymes located in both the mitochondrion and the cytosol ^[1]. As there is no known mechanism to transport phosphoenolpyruvate from the mitochondrion into the cytosol, the cytosolic enzyme is believed to be the isozyme important for gluconeogeneis. The rate of gluconeogenesis is ultimately controlled by the action of a key enzyme, fructose-1,6-bisphosphatase, which is also regulated through signal tranduction by cAMP and its phosphorylation.

Most factors that regulate the activity of the gluconeogenesis pathway do so by inhibiting the activity or expression of key enzymes. However, both acetyl CoA and citrate activate gluconeogenesis enzymes (pyruvate carboxylase and fructose-1,6-bisphosphatase, respectively). Notably, acetyl-CoA and citrate also play inhibitory roles in pyruvate kinase activity in glycolysis.

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