Gluconeogenesis Notes

Gluconeogenesis

Objectives

• Definition of Gluconeogenesis.

• Different Substrates of Gluconeogenesis and their point of entry.

• Steps of gluconeogenesis.

• Regulation of gluconeogenesis.

Utilization of Glucose

• Glucose is the primary energy source for the brain, skeletal muscle, and red blood cells.

• Deficiency in glucose can impair the brain and nervous system.

Sources of Blood Glucose

• Fed State: Dietary carbohydrate is converted to glucose in the gut, which is then used by the brain, red blood cells (RBCs), and other tissues. Excess glucose is stored as glycogen in the liver.

• Fasting State: Glycogen is broken down into glucose (glycogenolysis) to maintain blood glucose levels. Gluconeogenesis also occurs, using substrates like glycerol, amino acids, and lactate.

• Starved State: Gluconeogenesis becomes the primary source of blood glucose, utilizing glycerol, amino acids, and lactate. The liver is the main organ responsible for glucose production.

Definition of Gluconeogenesis

• The synthesis of glucose from carbon atoms of non-carbohydrate compounds.

• Gluconeogenesis meets the body's needs for glucose when sufficient carbohydrate is not available from the diet or glycogen reserves.

• Failure of gluconeogenesis has fatal consequences.

Gluconeogenesis: Glucose Synthesis

• Involves the synthesis of glucose from pyruvate.

• Lactate from the Cori cycle and amino acids can be converted to pyruvate, which then enters gluconeogenesis.

• Occurs primarily in the liver.

Importance of Gluconeogenesis

• Glycogen stores are depleted within the first 12-18 hours of fasting.

• During starvation, gluconeogenesis maintains blood glucose levels.

• On prolonged starvation, gluconeogenesis is accelerated, and protein catabolism provides glucogenic amino acids as substrates.

Site of Gluconeogenesis

• The major gluconeogenic tissues are:

  • Liver (mainly).

  • Kidney (to a lesser extent, in the renal cortex).

• Cellular location:

  • Partly mitochondrial.

  • Partly cytoplasmic.

Gluconeogenesis vs. Glycolysis

• Gluconeogenesis is almost the reverse of the glycolysis pathway.

• Most reactions are the reverse of glycolysis (7 steps) and use the same enzymes.

• Exceptions: Three irreversible reactions in glycolysis are bypassed by different enzymes in gluconeogenesis.

• Glycolysis starts with glucose, while gluconeogenesis ends with glucose.

• Gluconeogenesis synthesizes glucose from different substrates that enter the pathway at different points of glycolysis.

Substrates of Gluconeogenesis

• Gluconeogenesis derives glucose from non-carbohydrate precursors such as:

  • Lactate.

  • Some amino acids (glucogenic amino acids).

  • Glycerol.

• All substrates are converted to pyruvate or other intermediates.

Substrate Point of Entry

• Various substrates enter gluconeogenesis at different points, eventually leading to glucose synthesis.

Lactate

• Arises from anaerobic glycolysis in RBCs or active muscle.

• Enters gluconeogenesis as pyruvate.

• Pyruvate is converted to lactate in muscle, which can accumulate. Lactate diffuses and is taken up by the liver, where it is reconverted to pyruvate to regenerate glucose.

Cori Cycle

• A process in which glucose is converted to lactate in the muscle, and in the liver, this lactate is re-converted to glucose.

• In actively contracting muscle, pyruvate is reduced to lactic acid, which may accumulate.

• The body uses the Cori cycle to prevent lactate accumulation.

Cori Cycle Details

• Lactic acid from muscle diffuses into the blood and reaches the liver, where it is oxidized to pyruvate.

• Pyruvate enters gluconeogenesis, regenerating glucose.

• Regenerated glucose can enter the blood and then return to the muscle.

• This process also occurs in red blood cells.

• The Cori cycle involves glycolysis in peripheral tissues (like muscle) and gluconeogenesis in the liver.

• The liver expends 6 ATP to produce glucose via gluconeogenesis, while the muscle generates 2 ATP via glycolysis.

Amino Acids

• Called glucogenic amino acids.

• The carbon skeleton of these amino acids (all except leucine and lysine) results in the formation of or enter as:

  • Pyruvate.

  • Intermediates of the citric acid cycle.

• Ultimately, they result in the synthesis of glucose.

Alanine

• Transport of glucose and nitrogen between muscle and liver.

• In muscle, pyruvate is transaminated to alanine, which is then transported to the liver.

• In the liver, alanine is converted back to pyruvate, which is used for gluconeogenesis. The resulting ammonia is converted to urea.

Glycerol

• Source: Adipose tissue via the hydrolysis of fats (triacylglycerols).

• Activated by the enzyme glycerokinase (in liver and kidney, absent in adipose tissue) forming glycerol 3-phosphate.

• Enters gluconeogenesis as DHAP (dihydroxyacetone phosphate) via glycerol 3-phosphate dehydrogenase.

Glycerol Conversion

• Glycerol is converted to glycerol 3-phosphate by glycerol kinase using ATP.

• Glycerol 3-phosphate is converted to dihydroxyacetone phosphate (DHAP) by glycerol phosphate dehydrogenase, producing NADH.

Glycerol and Gluconeogenesis

• Glycerol can be converted to glucose in the liver, which is then used by extrahepatic tissues.

• In adipocytes, triglycerides are broken down into fatty acids and glycerol. Glycerol is transported to the liver for gluconeogenesis.

Propionate

• Source: Oxidation of odd-chain fatty acids and the breakdown of some amino acids (methionine, isoleucine, valine).

• Enters gluconeogenesis as succinyl CoA.

• Propionyl CoA carboxylase acts on propionyl CoA in the presence of ATP and biotin, converting it to methylmalonyl CoA.

• Methylmalonyl CoA is then converted to succinyl CoA in the presence of vitamin B12.

• Succinyl CoA enters gluconeogenesis.

Propionate Conversion

• Propionyl CoA is carboxylated to D-methylmalonyl CoA by propionyl CoA carboxylase, which requires biotin and ATP.

• D-methylmalonyl CoA is converted to L-methylmalonyl CoA by methylmalonyl-CoA racemase.

• L-methylmalonyl CoA is isomerized to succinyl-CoA by methylmalonyl-CoA isomerase, which requires vitamin B12.

Gluconeogenesis Steps

• Overview of the steps involved in gluconeogenesis.

Reactions of Gluconeogenesis

• Gluconeogenesis is almost the reverse of the glycolysis pathway.

• Most reactions are the reverse of glycolysis (7 steps) and use the same enzymes.

• Exceptions: Three irreversible reactions in glycolysis are bypassed by different enzymes in gluconeogenesis.

• Reaction 1: Hexokinase/glucokinase.

• Reaction 3: Phosphofructokinase.

• Reaction 10: Pyruvate kinase.

Bypass of Irreversible Reactions

• The three irreversible reactions are bypassed by new, different enzymes specific to gluconeogenesis.

• These are:

  • Glucose-6-phosphatase.

  • Fructose-1,6-bisphosphatase.

  • Pyruvate carboxylase.

  • Phosphoenolpyruvate carboxykinase.

Pyruvate to Phosphoenolpyruvate

• Phosphoenolpyruvate is formed from pyruvate via oxaloacetate.

• Two enzymes are involved: pyruvate carboxylase and phosphoenolpyruvate carboxykinase.

• Pyruvate carboxylase is a biotin-dependent mitochondrial enzyme, while other enzymes of gluconeogenesis are cytoplasmic.

Pyruvate Carboxylase

• This enzyme converts pyruvate to oxaloacetate in the presence of ATP and $CO_2$.

• Regulates gluconeogenesis and requires acetyl CoA for its activity.

Oxaloacetate Transport

• Oxaloacetate is synthesized in the mitochondrial matrix but cannot diffuse out of the mitochondria.

• It is converted to malate and transported to the cytosol.

Malate Dehydrogenase

• The reversible conversion of oxaloacetate to malate is catalyzed by an NADH-linked malate dehydrogenase.

• Malate is transported across the mitochondrial membrane and reoxidized to oxaloacetate in the cytosol.

• In the cytosol, phosphoenolpyruvate carboxykinase converts oxaloacetate to phosphoenolpyruvate.

Phosphoenolpyruvate Carboxykinase

• GTP is used in this reaction, and $CO_2$ is liberated.

• For the conversion of pyruvate to phosphoenolpyruvate, 2 ATP equivalents are utilized.

Phosphoenolpyruvate to Fructose-1,6-bisphosphate

• Phosphoenolpyruvate is converted to fructose-1,6-bisphosphate using the same enzymes as in glycolysis (reversible steps).

Fructose 1,6-bisphosphate to Fructose 6-phosphate

• Loss of a phosphate from fructose-1,6-bisphosphate forms fructose-6-phosphate and $P_i$.

• Catalyzed by the enzyme fructose-1,6-bisphosphatase.

• A reversible reaction then converts fructose-6-phosphate to glucose-6-phosphate.

Glucose 6-phosphate to Glucose

• Glucose 6-phosphatase converts glucose 6-phosphate to glucose.

• It is present in the liver and kidney.

• Absent in muscle, brain, and adipose tissue.

• The liver can replenish blood sugar through gluconeogenesis because glucose 6-phosphatase is present mainly in the liver.

Regulation of Gluconeogenesis

• Gluconeogenesis and glycolysis are reciprocally regulated.

• One pathway is relatively inactive when the other is active.

• High glucose levels and insulin promote glycolysis.

• Low glucose levels and glucagon promote gluconeogenesis.

Regulatory Enzymes

• Pyruvate Carboxylase.

• Fructose-1,6-bisphosphatase.

Pyruvate Carboxylase Regulation

• It is an allosteric enzyme.

• Acetyl CoA is an activator of pyruvate carboxylase so that the generation of oxaloacetate is favored when the acetyl CoA level is high.

Fructose-1,6-bisphosphatase Regulation

• Citrate is an activator.

• Fructose-2,6-bisphosphate and AMP are inhibitors.

• All these three effectors have an exactly opposite effect on phosphofructokinase (PFK).

• ATP also affects this enzyme.