The reactions of gluconeogenesis convert mitochondrial pyruvate to cytosolic glucose 6-phosphate which in turn can be hydrolyzed to glucose and exported from the cell. Gluconeogenesis is confined to cells of the liver and kidney and enables glucose synthesis from molecules such as lactate and alanine and other amino acids when exogenous glucose is not available. The process of gluconeogenesis as diagrammed below occurs in two parts: a network of possible reactions mediates the conversion of mitochondrial pyruvate to cytosolic phosphoenolpyruvate, which in turn is converted to glucose 6-phosphate in a single sequence of cytosolic reactions.
In the chicken, three variants of the process can be distinguished, two restricted to the kidney and one that takes place in both kidney and liver. 1) In the kidney, carbon atoms from mitochondrial oxaloacetate, via a series of transport and transamination reactions, are used to generate cytosolic oxaloacetate, which is converted to phosphoenolpyruvate by a cytosolic, hormonally regulated form of phosphoenolpyruvate carboxykinase, PCK1. This variant allows regulated glucose synthesis from lactate. 2) In the kidney, mitochondrial oxaloacetate is reduced to malate, which is exported to the cytosol and re-oxidized to oxaloacetate. This variant provides reducing equivalents to the cytosol, needed for glucose synthesis from amino acids such as alanine and glutamine. 3) In both liver and kidney, constitutively expressed mitochondrial phosphoenolpyruvate carboxykinase, PCK2, catalyzes the conversion of mitochondrial oxaloacetate to phosphoenolpyruvate which in turn is transported to the cytosol. This third variant also allows glucose synthesis from lactate (Soling et al. 1973; Wallace and Newsholme 1967; Watford et al. 1981; Weldon et al. 1990).
In all cases, the metabolism of a molecule of pyruvate requires the generation and consumption of one reducing equivalent as cytosolic NADH + H+. For pyruvate derived from lactate (variants 1 and 3), NADH + H+ is generated with the oxidation of lactate to pyruvate in the cytosol (a reaction of pyruvate metabolism not shown in the diagram). For pyruvate derived from amino acids (variant 2), mitochondrial NADH + H+ generated by glutamate dehydrogenase (a reaction of amino acid metabolism, not shown) is used to reduce oxaloacetate to malate, which is transported to the cytosol and re-oxidized, generating cytosolic NADH + H+. The metabolism of each molecule of pyruvate also requires the consumption of three high-energy phosphates, two from ATP and one from GTP.
In the second part of gluconeogenesis, cytosolic phosphoenolpyruvate, however derived, is converted to fructose 1,6-bisphosphate by reactions that are the reverse of steps of glycolysis. Hydrolysis of fructose 1,6-bisphosphate to fructose 6-phosphate is catalyzed by fructose 1,6-bisphosphatase, and fructose 6-phosphate is reversibly isomerized to glucose 6-phosphate.
