Glucose Production by the Liver and Kidneys

Gluconeogenesis

The biosynthesis of glucose from pyruvate, lactate, or other precursors is known as gluconeogenesis. It is not a direct reversal of glycolysis, since several steps of glycolysis are irreversible. Gluconeogenesis occurs mainly in the liver and less so in the kidney. These tissues contain all the necessary enzymes for gluconeogenesis and, furthermore, for the enzymatic activity of glycerol kinase, which allows glycerol to enter the gluconeogenic pathway at the level of glyceraldehyde 3-phosphate (Figure 2).

It is vital that the organism synthesizes glucose for those tissues that are unable to synthesize glucose. In humans, liver glycogen stores can sustain the body for 18 h without the ingestion of dietary carbohydrates. After this period, the liver must produce glucose for transport to other organs. The liver is the main gluconeogenic contributor (90%), while the kidney contributes gluconeogenically produced glu-

Glucose t

Glucose 6-phosphate

Fructose 6-phosphate t

Fructose 1,6-bisphosphate r h aCetone*^ + G^1^'

, , . 3-phosphate phosphate

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