Gluconeogenesis
Gluconeogenesis 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.
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.