Gluconeogenesis Dr. Tarek A Salem

Objectives of this lecture Definition of gluconeogenesis The steps of gluconeogenesis Site of occurrence and importance of gluconeogenesis. The sources of carbon atoms used in gluconeogenesis

Definition Glucneogenesis is making a new glucose from non-carbohydrate precursors In other words: Create new glucose from the products of its breakdown

Site of gluconeogenesis Major site of gluconeogenesis: Liver (90%) Secondary site: Kidney cortex and in small intestine under some conditions. (10 %) It takes place in the mitochondria and cytoplasm. The production of glucose is necessary for use as a fuel source by the brain, testes, erythrocytes, kidney medulla, lens and cornea of the eye and exercising muscle.

Overview Synthesis of glucose from pyruvate utilizes many of the same enzymes as Glycolysis. Three Glycolysis reactions have such a large negative ΔG that they are essentially irreversible Hexokinase (or Glucokinase) Phosphofructokinase Pyruvate Kinase. These steps must be bypassed in Gluconeogenesis.

First Bypass Reaction: Convervsion of Pyruvate to Phosphoenolpyruvate Enzymes involved: Pyruvate carboxylase PEP carboxykinase

In Mitochondria Pyruvate carboxylase (PC) presents in the mitochondria of liver and kidney but absent in muscle ATP, biotin, Mn++ and CO2 are required.

Transport of Oxaloacetate into cytosol as Malate

In cytosol

Summary of the first bypass

The overall equation for this set of bypass reactions Pyruvate + ATP + GTP + HCO3- phosphoenolpyruvate + ADP + GDP + Pi + H+ + CO2 Thus the synthesis of one molecule of PEP requires an investment of 1 ATP and 1 GTP. Note: when either pyruvate or the ATP/ADP ratio is high, the reaction is pushed toward the right (i.e., in the direction of biosynthesis).

Second Bypass Reaction: Conversion of Fructose 1,6- bisphosphate to Fructose 6-phosphate The second glycolytic reaction (phosphorylation of fructose 6-phosphate by PFK) is irreversible. Hence, for gluconeogenesis fructose 6-phosphate must be generated from fructose 1,6-bisphosphate by a different enzyme: Fructose 1,6-bisphosphatase. Fructose 1,6-bisphosphatse presents in liver and kidney. This reaction is also irreversible. Fructose 1,6-bisphosphate + H2O fructose 6-phosphate + Pi

Phosphofructokinase (In Glycolysis): fructose-6-P + ATP  fructose-1,6-bisP + ADP Fructose-1,6-bisphosphatase (In Gluconeogenesis): fructose-1,6-bisP + H2O  fructose-6-P + Pi

Third Bypass Reaction: Glucose 6-phosphate to Glucose Because the hexokinase reaction is irreversible, the final reaction of gluconeogenesis is catalyzed by Glucose 6-phosphatase. Glucose 6-phosphate + H2O glucose + Pi Glucose 6-phosphatase is present in the liver, kidney and small intestine but absent in brain and muscle. Thus, glucose produced by gluconeogenesis in the liver, is delivered by the bloodstream to brain and muscle.

Hexokinase or Glucokinase (In Glycolysis): glucose + ATP  glucose-6-phosphate + ADP Glucose-6-Phosphatase (In Gluconeogenesis): glucose-6-phosphate + H2O  glucose + Pi

Summary of Gluconeogenesis Pathway: Gluconeogenesis enzyme names in red. Glycolysis enzyme names in blue.

Carbon sources for gluconeogenesis Glycerol: is released during hydrolysis of triacylglycerols in adipose tissue and is delivered by the blood to the liver. Glycerol kinase Glycerol Glycerol 3-P G3P dehydrogenase Glycerol 3-P DHAP DHAP is converted into glyceraldehyde 3-P

Carbon sources for gluconeogenesis Lactate (Lactic acid): In vigorous skeletal muscle activity, large amount of lactic acid produced pass to liver through blood stream converted into pyruvic and lastly to glucose reach muscle again through blood stream to provide energy. (This called Cori cycle).

Lactate produced from pyruvate passes via the blood to the liver, where it may be converted to glucose. The glucose may travel back to the muscle to fuel Glycolysis.

Carbon sources for gluconeogenesis Propionic acid: product of odd number fatty acid degradation. It is converted into succinyl CoA which converted into oxaloactic acid that can form glucose.

Carbon sources for gluconeogenesis Glucogenic amino acids: amino acids by deamination can be converted into keto acids as pyruvic, α-ketoglutaric and oxaloacetic acid. Proteins are considered as one of the main sources of blood glucose especially after 18 hr due to depletion of liver glycogen.

Carbon sources for gluconeogenesis Glucose-alanine cycle: During starvation, alanine is formed from protein catabolism. Alanine is converted into pyruvic acid in liver which can give glucose.

Acetyl CoA can not produce glucose Acetyl CoA cannot give rise to a net synthesis of glucose. This is due to the irreversible nature of the pyruvate dehydrogenase reaction, which converts pyruvate to acetyl CoA. Pyruvate dehydrogenase Pyruvate acetyl CoA + CO2 NAD+ NADH+H+

Importance of gluconeogenesis 1- Maintenance of blood glucose during starvation, fasting and prolonged exercise. 2- Removal of lactic acid. 3- Removal of glycerol produced by lipolysis.

Regulation of gluconeogenesis In liver: ATP PK PC glycolysis is inhibited and gluconeogenesis is activited During starvation, the priority is to conserve blood glucose for the brain and muscle. Thus, under these conditions, PK in the liver is switched off. This occurs because the hormone glucagon is secreted into the blood­stream and activates a cAMP cascade that leads to the phosphorylation and inhibition of this enzyme.

Regulation of gluconeogenesis In liver: AMP PK PC glycolysis is activated and gluconeogenesis is stopped. In case of Acetyl CoA PK PC When acetyl CoA is abundant, gluconeogenesis is activated