Physiology, Gluconeogenesis

The brain, eye, and kidney are some of the organs that have glucose as the sole metabolic fuel source. Prolonged fasting or vigorous exercise depletes glycogen stores, making the body switch to de-novo glucose synthesis to maintain blood levels of this monosaccharide. Gluconeogenesis is the process that allows the body to form glucose from non-hexose precursors, particularly glycerol, lactate, pyruvate, propionate, and glucogenic amino acids. Gluconeogenesis essentially reverses glycolysis (see . Gluconeogenesis). Four enzymes facilitate glucose synthesis by this pathway by reversing 3 highly exergonic glycolytic steps, namely, pyruvate carboxylase, phosphoenol pyruvate carboxykinase (PEPCK), fructose-1,6-bisphosphatase, and glucose-6-phosphatase. However, these enzymes are not present in all cell types. Therefore, gluconeogenesis can only occur in specific tissues. In humans, gluconeogenesis takes place primarily in the liver and, to a lesser extent, the renal cortex. This article discusses gluconeogenesis and its clinical correlates.