Gluconeogenesis is the formation of glucose from non-carbohydrate precursors such as pyruvate, lactate, glycerol, and alpha keto acids (the carbon skeletons of amino acids can be used to make glucose). It takes place in both the cytosol and mitochondria. The major site is the liver, followed by the kidneys.
Substrates for gluconeogenesis:
Glycerol: Derived from adipose tissue triacylglycerols (triglycerides, TGs) by the action of the enzyme hormone sensitive lipase. In the liver, glycerol is phosphorylated to glycerol phosphate by glycerol kinase. Glycerol phosphate is then converted to DHAP (dihydroxyacetone phosphate) by the enzyme glycerol phosphate dehydrogenase. DHAP is an intermediate of glycolysis, so it can be converted to glucose via gluconeogenesis.
Lactate: In the Cori cycle, lactate released by RBC glycolysis and exercising skeletal muscle is taken up by the liver. The liver converts lactate to glucose by gluconeogenesis. This glucose enters the blood and can be taken up again by skeletal muscle and RBCs.
Amino acids: Alpha keto acids are derived from the metabolism of glucogenic amino acids. These carbon skeletons can enter the TCA cycle (for example, as alpha ketoglutarate) and be converted to OAA (oxaloacetate), which can then form PEP (phosphoenol pyruvate). All amino acids except leucine and lysine can be converted to glucose.
Important steps in gluconeogenesis:
Most steps in gluconeogenesis are the reverse of glycolysis. However, gluconeogenesis uses four alternate “roundabout” reactions to bypass the three irreversible reactions of glycolysis.
First roadblock is to overcome the irreversible reaction of PEP to pyruvate by pyruvate kinase. This is done in 2 steps -
Pyruvate is converted to OAA by pyruvate carboxylase. This reaction requires biotin and occurs in the mitochondria of liver and kidney cells. The enzyme is allosterically activated by high levels of acetyl CoA (e.g., during fasting, acetyl CoA comes from the beta oxidation of FA from lipolysis) and is inhibited by low levels of acetyl CoA. Acetyl CoA also inhibits PDH, so pyruvate is directed away from PDH and becomes available for gluconeogenesis. OAA cannot cross the inner mitochondrial membrane, so it is converted to malate, which can reach the cytosol. In the cytosol, malate is converted back to OAA.
In the second step, OAA is converted to PEP by PEP carboxykinase.
The second roadblock is the PFK 1 reaction, which converts fructose 6 phosphate to fructose 1,6 biphosphate. This is bypassed by fructose 1,6 biphosphatase, which converts fructose 1,6 biphosphate back to fructose 6 phosphate. This enzyme is inhibited by elevated AMP (an energy-poor state) and by fructose 2,6 biphosphate. It is stimulated by high ATP and low AMP. Remember: enzymes of glycolysis and the TCA cycle are activated by dephosphorylation, while enzymes of gluconeogenesis are activated by phosphorylation (think kinases). The body avoids running glycolysis and gluconeogenesis at the same time.
Third roadblock is to reverse the reaction glucose to glucose 6 phosphate by hexokinase/ glucokinase. This bypass is done by glucose 6 phosphatase, which converts glucose 6 phosphate to glucose. Only the liver and kidney have this enzyme. That’s why muscle glycogen cannot be used to raise blood sugar levels in hypoglycemia. The same enzyme is also required for glycogen degradation. So, its deficiency causes Type 1a Glycogen Storage Disorder with severe fasting hypoglycemia.
Energy is required for gluconeogenesis: 2 NADH, 2 ATP and 2 GTP.
Role of glucagon in gluconeogenesis: It is secreted by alpha cells of the pancreas. Glucagon is the principal regulator of gluconeogenesis by following steps:
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