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Gluconeogenesis abbreviated GNG is a metabolic pathway that results in glucoegnolisis generation of glucose from non-carbohydrate carbon substrates such as lactate, glycerol, and glucogenic amino acids. It is one of the two main mechanisms humans and many other animals use to keep blood glucose levels from dropping too low hypoglycemia. The other means of maintaining blood glucose levels is through the degradation of glycogen glycogenolysis.
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Gluconeogenesis is a ubiquitous process, present in plants, animals, fungi, bacteria, and other microorganisms. In animals, gluconeogenesis takes place mainly in the liver and, to a lesser extent, in the cortex of kidneys.
This process occurs during periods of fasting, starvation, low-carbohydrate diets, or intense exercise and is highly endergonic. For example, the pathway leading from phosphoenolpyruvate to glucosephosphate requires 6 molecules of ATP.
Gluconeogenesis is often associated with ketosis. Gluconeogenesis is also a target of therapy for type II diabetes, such as metformin, which inhibits glucose formation and stimulates glucose uptake by cells. Lactate is transported back to the liver where it is converted into pyruvate by the Cori cycle using the enzyme lactate dehydrogenase. Pyruvate, the first designated substrate of the gluconeogenic pathway, can then be used to generate glucose.
All citric acid cycle intermediates, through conversion to oxaloacetate, amino acids other than lysine or leucine, and glycerol tluconeognesis also function as substrates for gluconeogenesis. Transamination or deamination of amino acids facilitates entering of their carbon skeleton into the cycle directly glucogenoliis pyruvate or oxaloacetateor indirectly via the citric guconeognesis cycle.
Whether fatty acids can be converted into glucose in animals has been a longstanding question in biochemistry. It is known that odd-chain fatty acids can be oxidized to yield propionyl CoA, glkconeognesis precursor for succinyl CoA, which can be converted to pyruvate and enter into gluconeogenesis.
In plants, to be specific, in seedlings, the glyoxylate cycle can be used to convert fatty acids acetate into the primary carbon source of the organism. The glyoxylate cycle produces four-carbon dicarboxylic acids that can enter gluconeogenesis.
Inresearchers identified the glyoxylate cycle in nematodes. In addition, the glyoxylate enzymes malate synthase and isocitrate lyase have been found in animal tissues. Genes coding for malate synthase gene have been identified in other [metazoans] including arthropods, echinoderms, and even some vertebrates.
Mammals found to possess these genes include monotremes platypus and marsupials opossum but not placental mammals. Genes for isocitrate lyase are found only in nematodes, in which, it is apparent, they originated in horizontal gene transfer from bacteria. The existence of glyoxylate cycles in humans has not been established, and it is widely held that fatty acids cannot be converted to glucose in humans directly.
However, carbon has been shown to end glucogenolisie in glucose when it is supplied in fatty acids. Glucogenolisie these findings, it is considered unlikely that the 2-carbon acetyl-CoA derived from the glucogejolisis of fatty acids would produce a net yield of glucose via the citric acid cycle.
However, it is glucogejolisis that, with additional sources of carbon via other pathways, glucose could be synthesized from acetyl-CoA. Glycerol, which is a part of the triacylglycerol molecule, can be used in gluconeogenesis.
In humans, gluconeogenesis is restricted to the liver and to a lesser extent the kidney. In all species, the formation of oxaloacetate from pyruvate and TCA cycle intermediates is restricted to the mitochondrion, and the enzymes that convert PEP to glucose are found in the glconeognesis. The location of gluconwognesis enzyme that links these two parts of gluconeogenesis by converting oxaloacetate to PEP, PEP carboxykinase, is variable by species: Transport of PEP across the mitochondrial membrane is accomplished by dedicated transport proteins; however no such proteins exist for oxaloacetate.
Therefore species that lack intra-mitochondrial PEP, oxaloacetate must be gluxogenolisis into malate or asparate, exported from the mitochondrion, and converted back into oxaloacetate in order to allow gluconeogenesis to continue.
Gluconeogenesis is a pathway consisting of eleven glucogneolisis reactions. The pathway can begin in the mitochondria or cytoplasm, depending on the substrate being used.
Many glucoenolisis the reactions are the reversible steps found gluconfognesis glycolysis. Gluconeogenesis begins in the mitochondria with the formation of oxaloacetate through carboxylation of pyruvate.
This reaction also requires one molecule of ATP, and is catalyzed by pyruvate carboxylase. Oxaloacetate is reduced to malate using NADH, a step required for transport out of the mitochondria. Oxaloacetate is decarboxylated and phosphorylated to produce phosphoenolpyruvate by phosphoenolpyruvate carboxykinase.
The next steps in the reaction are the same as reversed glycolysis.
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However, fructose-1,6-bisphosphatase converts fructose-1,6-bisphosphate to fructose 6-phosphate, requiring one water molecule and releasing one phosphate. This is also the rate-limiting step of gluconeogenesis. Glucosephosphate is formed from fructose 6-phosphate by phosphoglucoisomerase.
Glucosephosphate can be used in other metabolic pathways or dephosphorylated to free glucose.
Whereas free glucose can easily diffuse in and out of the cell, the phosphorylated form glucosephosphate is locked in the gluocneognesis, a mechanism by which intracellular glucose levels are controlled by cells.
The final reaction of gluconeogenesis, the formation of glucose, occurs in the lumen of the endoplasmic reticulum, where glucosephosphate is hydrolyzed by glucosephosphatase to produce glucose.
Glucose is shuttled into the cytosol by glucose transporters located in the membrane of the endoplasmic reticulum. While most steps in gluconeogenesis are the reverse of glcogenolisis found in glycolysis, three regulated and strongly exergonic reactions are replaced with more kinetically favorable reactions. This system of reciprocal control allow glycolysis and gluconeogenesis to inhibit each other and prevent the formation of a futile cycle.
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 an isozyme located in both the mitochondrion and the cytosol. 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 glucogrnolisis or expression of key enzymes. However, both acetyl CoA and citrate activate gluconeogenesis enzymes pyruvate carboxylase and fructose-1,6-bisphosphatase, respectively.
Due to the reciprocal control of the cycle, acetyl-CoA and citrate also have inhibitory roles in the activity of pyruvate kinase. During gluconeogenesis, pyruvate carboxylase is the first enzyme in the pathway that synthesizes phosphoenolpyruvate PEP from pyruvate. The enzyme pyruvate carboxylase acts within the mitochodrial matrix to convert pyruvate to oxaloacetate OAAutilizing the energy from the hydrolysis of one molecule of ATP.
In the next step, OAA is then decarboxylated and simultaneously phosphorylated, which is catalyzed by one of two isoforms of phosphoenolpyruvate carboxykinase PEPCK either in the cytosol or in the mitochondria to produce PEP. Under ordinary gluconeogenic condition, OAA gglucogenolisis converted into PEP by mitochondrial PEPCK; the resultant PEP is then transported out of the mitochondria via the citric acid cycle carrier system, and converted into glucose by cytosolic gluconeogenic enzymes.
However, during starvation when cytostolic NADH concentration is low and yluconeognesis NADH levels are high, oxaloacetate can be used as a shuttle of reducing equivalents. Very high levels of PC activity, together with high activities of other gluconeogenic enzymes including PEPCK, fructose-1,6-bisphosphatase and glucosephosphatase in liver and kidney cortex, suggest that a primary role of PC is to participate in gluconeogenesis in these organs.
During fasting or starvation when endogenous glucose is required for certain tissues brain, white blood cells and kidney medullaexpression of PC and other gluconeogenic enzymes is elevated. In rats and mice, alteration of nutrition status has been shown to affect hepatic PC activity.
Fasting promotes hepatic glucose production sustained by yluconeognesis increased pyruvate flux, and increases in PC activity and protein concentration; Diabetes similarly increases gluconeogenesis through enhanced uptake of substrate and increased flux through liver PC in mice and rats Similarly to other gluconeogenic enzymes, PC is positively regulated by glucagon glucineognesis glucocorticoids while negatively regulated by insulin. Further supporting the key role of PC in gluconeogenesis, in dairy cattle, which have hexose absorption ability at adequate nutrition levels, PC and the associated gluconeogenic enzyme PEPCK are markedly elevated during the transition to lactation in proposed support of lactose synthesis for milk production.
Aside from the role of PC in gluconeogenesis, PC serves an anaplerotic role an enzyme catalyzed reaction that can replenish the supply of intermediates in the citric acid cycle for the tricarboxylic acid cycle essential to provide oxaloacetatewhen intermediates are removed for different biosynthetic purposes. Phosphoenolpyruvate carboxykinase PEPCK is an enzyme in the lyase family used in the metabolic pathway of gluconeogenesis.
It converts oxaloacetate into phosphoenolpyruvate and carbon dioxide.
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It is found in two forms, cytosolic and mitochondrial. It has been shown that PEPCK catalyzes the rate-controlling step of gluconeogenesis, the process whereby glucose is synthesized. The enzyme has therefore been thought to be essential in glucose homeostasis, as evidenced by laboratory mice that contracted diabetes mellitus type 2 as a result of the overexpression of PEPCK. A recent study suggests that the role that PEPCK plays in gluconeogenesis may be mediated by the citric acid cycle, the activity of which was found to be directly related to PEPCK abundance.
PEPCK levels alone were not found to be highly correlated with gluconeogenesis in the mouse liver, as previous studies have suggested. Therefore, the role of PEPCK in gluconeogenesis may be more complex and involve more factors than was previously believed. This gene belongs to the GPI family whose members encode multifunctional phosphoglucose isomerase proteins involved in energy pathways.
The protein encoded by this gene is a dimeric enzyme that catalyzes the reversible isomerization of glucosephosphate and fructosephosphate. The protein has different functions inside and outside the cell.
In the cytoplasm, the protein is involved in glycolysis and gluconeogenesis, while outside the cell it functions as a neurotrophic factor for spinal and sensory neurons. The same protein is also secreted by cancer cells, where it is called autocrine motility factor and stimulates metastasis.
Defects in this gene are the cause of nonspherocytic hemolytic anemia and a severe enzyme deficiency can be associated with hydrops fetalis, immediate neonatal death and neurological impairment. The Cori cyclenamed after its discoverers, Carl Cori and Gerty Corirefers to the metabolic pathway in which lactate produced by anaerobic glycolysis in the muscles moves to the liver and is converted to glucose, which then returns to the muscles and is converted back to lactate.
Muscular activity requires energy, which is provided by the breakdown of glycogen in the skeletal muscles. The breakdown of glycogen, a process known as glycogenolysis, releases glucose in the form of glucosephosphate GP. GP is readily fed into glycolysis, a process that provides ATP to the muscle cells as an energy source.
During muscular activity, the store of ATP needs to be constantly replenished. When the supply of oxygen is sufficient, this energy comes from feeding pyruvate, one product of glycolysis, into the Krebs cycle. When oxygen supply is insufficient, typically during intense muscular activity, energy must be released through anaerobic gluconeogneais.
Anaerobic respiration converts pyruvate to lactate by lactate dehydrogenase. Refer to the main articles on glycolysis and fermentation for the details. Instead of accumulating inside the muscle cells, lactate produced by anaerobic fermentation is taken up by the liver. This initiates the other half of the Cori cycle. In the liver, gluconeogenesis occurs.
From an intuitive perspective, gluconeogenesis reverses both glycolysis and fermentation glufogenolisis converting lactate first into pyruvate, and finally back to glucose. The glucose is then supplied to the muscles through the bloodstream; it is ready to be fed into further glycolysis reactions.
If muscle activity has stopped, the glucose is used to replenish the supplies of glycogen through glycogenesis. Overall, the glycolysis part of the cycle produces 2 ATP molecules at a cost of 6 ATP molecules consumed in the gluconeogenesis part.
Each iteration of the cycle must be maintained by a net consumption of 4 ATP molecules. As a result, the cycle cannot be sustained indefinitely. The intensive consumption of ATP molecules indicates that the Cori cycle shifts the metabolic burden from the muscles to the liver.