Gluconeogenesis

Gluconeogenesis is the formation of glucose from precursors such as pyruvate, lactate, glycerol and alpha keto acids (carbon skeleton of amino acids is used for glucose). It takes place in the cytosol and mitochondria. Major site is the liver, followed by the kidneys.

Substrates for gluconeogenesis:

1. 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 enzyme glycerol phosphate dehydrogenase. DHAP is an intermediary of glycolysis, hence can be converted to glucose by gluconeogenesis.

2. Lactate: In the Cori Cycle, lactate released by RBC glycolysis and exercising skeletal muscle is taken up by the liver which converts it to glucose by gluconeogenesis and this glucose enters the blood and can be taken up again by skeletal muscle and RBCs.

3. Amino acids: Alpha keto acids are derived from the metabolism of glucogenic amino acids. They can then enter the TCA cycle as alpha ketoglutarate and are converted to OAA (oxaloacetate) which can 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 reversal of glycolysis steps. Four alternate roundabout reactions are used to overcome three of the irreversible reactions of glycolysis.

1. 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, which 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 that pyruvate becomes exclusively available for gluconeogenesis. OAA cannot cross the inner mitochondrial membrane and is converted to malate which reaches the cytosol. Now the malate is converted back to OAA in the cytoplasm.

   • In the second step, OAA is converted to PEP by PEP Carboxykinase.

2. The second roadblock is PFK 1 reaction which converts fructose 6 phosphate to fructose 1,6 biphosphate. This is reversed by fructose 1,6 biphosphatase which breaks fructose 1,6 biphosphate to fructose 6 phosphate. The enzyme is inhibited by elevated levels of AMP (energy poor state) and fructose 2,6 biphosphate and stimulated by high ATP and low AMP. Remember that enzymes of glycolysis and TCA cycle are activated by dephosphorylation while those of gluconeogenesis are activated by phosphorylation (think kinases)! The body doesn’t want Glycolysis and Gluconeogenesis to happen at the same time.

3. Third roadblock is to reverse the reaction glucose to glucose 6 phosphate by hexokinase/ glucokinase. The reversal is done by enzyme glucose 6 phosphatase which converts glucose 6 phosphate to glucose. Liver and kidney are the only organs which have this enzyme. That’s the reason muscle glycogen cannot be used to raise blood sugar levels in hypoglycemia. The same enzyme is also required for glycogen degradation. So note that 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:

1. It lowers the level of fructose 2,6 biphosphate (thus inhibiting glycolysis), causes activation of fructose 1,6 biphosphatase (stimulating gluconeogenesis) and inhibiting PFK 1.
2. Glucagon (via GPCR) raises cAMP, phosphorylating and inactivating pyruvate kinase of glycolysis.
3. It also increases the transcription of PEP Carboxykinase.