Gluconeogenesis

Gluconeogenesis pathway with key molecules and enzymes. Many steps are the opposite of those found in the glycolysis.

Gluconeogenesis (abreviated GNG) is a metabolic pathway that results in the generation of glucose from non-carbohydrate carbon substrates such as 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. Gluconeogenesis is also a target of therapy for type II diabetes, such as metformin, which inhibit glucose formation and stimulate glucose uptake by cells.

Entering the pathway

Several non-carbohydrate carbon substrates can enter the gluconeogenesis pathway. One common substrate is lactic acid, formed during anaerobic respiration in skeletal muscle. 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 can also function as substrates for gluconeogenesis. Amino acids must have their amino group removed by transamination or deamination before entering the cycle directly (as pyruvate or oxaloacetate), or indirectly via the citric acid cycle.

Fatty acids cannot be converted into glucose in animals, the exception being odd-chain fatty acids which yield propionyl CoA, a precursor for succinyl CoA. In plants, specifically 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. Glycerol, which is a part of all triacylglycerols, can also 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. The pathway can begin in the mitochondria or cytoplasm, depending on the substrate being used. Many of the reactions are reversible steps found in glycolysis.

• Gluconeogenesis begins in the mitochondria with the formation of oxaloacetate through carboxylation of pyruvate at the expense of one molecule of ATP. This reaction is catalyzed by pyruvate carboxylase, which is stimulated by high levels of acetyl-CoA (when fatty acid oxidation is high in the liver) and inhibited by high levels of ADP.
• Oxaloacetate must then be reduced into malate using NADH in order to be transported out of the mitochondria.
• In the cytoplasm, malate is oxidized to oxaloacetate using NAD⁺, where the remaining steps of gluconeogenesis occur.
• Oxaloacetate is then decarboxylated and phosphorylated to produce phosphoenolpyruvate by phosphoenolpyruvate carboxykinase. One molecule of GTP is hydrolyzed to GDP in the course of this reaction.
• 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 can then be used for glucose generation or in other metabolic pathways. Free glucose is not generated automatically because glucose, unlike glucose-6-phosphate, tends to freely diffuse out of the cell.
• The final reaction of gluconeogenesis, the formation of glucose, is carried out in the lumen of the endoplasmic reticulum. Glucose-6-phosphate is hydrolyzed by glucose-6-phosphatase to produce glucose. Glucose is then shuttled into the cytosol by glucose transporters located in the membrane of the endoplasmic reticulum.

Regulation

While most steps in gluconeogenesis are the reverse of those found in glycolysis, three regulated and strongly exergonic reactions are replaced with more kinetically favorable reactions. Hexokinase/glucokinase, phosphofructokinase, and pyruvate kinase enzymes of glycolysis are replaced with glucose-6-phosphatase, fructose-1,6-bisphosphatase, and PEP carboxykinase. 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. 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 gluconeogenesis. 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). Due to the reciprocal control of the cycle, acetyl-CoA and citrate also have inhibitory roles in the activity of pyruvate kinase.

External links

• Overview at indstate.edu
• Interactive diagram at uakron.edu
• The chemical logic behind gluconeogenesis