GLUCONEOGENESIS

1. The synthesis of glucose from non carbohydrates is known as glugoneogenesis GLUCONEOGENESIS

2. Gluco-glucose, neo-new , genesis- make Gluconeogenesis occurs mainly in cytosol , liver(90%). Gluconeogenesis occurs to a more limited extent in kidney(10%) & small intestine under some conditions, it involves cytosolic and mitochondrial enzymes.  The major precursors/substrates for this pathway are pyruvate ,lactate, glucogenic amino acids, propionate, glycerol. Synthesis of glucose from pyruvate utilizes many of the same enzymes as Glycolysis. Three Glycolysis reactions have such a large negative enthalpy that they are essentially irreversible. • Hexokinase(or Glucokinase) • Phosphofructokinase • PyruvateKinase. These steps must be bypassed in Gluconeogenesis. Two of the bypass reactions involve simple hydrolysis reactions. INTRODUCTION

3. Brain ,CNS, erythrocytes, kidneys, testes, medulla are dependant on glucose for continuous supply of glucose for energy. Glucose is the only source that supply energy to the skeletal muscle under anaerobic conditions. In fasting even more than a day gluconeogenesis must occur to meet the basal requirements of the body for glucose. Certain metabolites accumulates in the blood eg: lactate, glycerol, propionate etc which will be cleared by gluconeogenesis from the blood. significance

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

6. Glycolysis&Gluconeogenesisbotharespontaneous. If both pathways were simultaneously active in a cell, it would constitute a "futile cycle" that would waste energy. Glycolysis: glucose + 2 NAD+ + 2 ADP + 2 Pi 2 pyruvate + 2 NADH + 2 ATP Gluconeogenesis: 2 pyruvate + 2 NADH + 2 H + 6H20 + 4 ATP + 2 GTP  glucose + 2 NAD+ + 4 ADP + 2 GDP + 6 Pi +6H

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

8. Glucose-6-phosphatase enzyme is embedded in the endoplasmic reticulum (ER) membrane in liver cells. The catalytic site is found to be exposed to the ER lumen. Mostly present in liver & kidney but absent in muscle, brain & adipose tissues.

9. Phosphofructokinase (Glycolysis) catalyzes: fructose-6-P + ATP  fructose-1,6-bisP + ADP Fructose-1,6-bisphosphatase (Gluconeogenesis) catalyzes: fructose-1,6-bisP + H2O  fructose-6-P + Pi This enzyme is absent in smooth muscle and heart muscle . This requires mg2+ ions as cofactors,this is also a regulatory step in gluconeogenesis.

10. Bypass of PyruvateKinase (2 enzymes): PyruvateCarboxylase (Gluconeogenesis) catalyzes: pyruvate+HCO3-+ ATPoxaloacetate +ADP+Pi PEP Carboxykinase (Gluconeogenesis) catalyzes: oxaloacetate + GTP  PEP + GDP + CO2

11. Oxaloacetate is synthesized in mitochondria which has to be transported to cytosol for further pathway, due to membrane impermeability OAA cannot diffuse out ,so that it is converted in to malate which is catalysed by malatedehydrogenase (present in both cytosol &mitochondria)and the transported to the cytosol. In cytosol PEP carboxylase converts OAA to PEP.

12. PEP Carboxykinasecatalyzes GTP-dependent oxaloacetate PEP. It is thought to proceed in 2 steps: • Oxaloacetate is first decarboxylated to yield a pyruvateenolate anion intermediate. • Phosphate transfer from GTP then yields phosphoenolpyruvate (PEP).

13. The carbon skeletons of glucogenic amino acids (all except leucine & lysine) results in the formation of pyruvate or the intermediates which ultimately results in the synthesis of glucose GLUCONEOGENESIS FROM GLYCEROL Glycerol is liberated in adipose tissues up on hydrolysis of fats ,the enzyme glycerokinase activates glycerol to glycerol 3-phosphate,which is converted in to dihydroxyacetone phosphate which helps in the glucose production. GLUCONEOGENESIS FROM PROPIONATE Oxidation of some fatty acids and break down of amino acids yields a 3 carbon propionyl COA which converts in to succinyl COA that enters in to guconeogenesis via TCA cycle and helps in glucose synthesis. GLUCONEOGENESIS FROM AMINO ACIDS

14. The hormone GLUCAGON and the availability of substrates mainly regulates the pathway - Influence Of Glucagon It stimulates the pathway by 2 ways: It decrease the pyruvatekinase through cyclic AMP which results in the reduced conversion of phosphoenolpyruvate to pyruvate that leads to the diversion for the synthesis of glucose. Glucagon reduces the conc of fructose 2,6,bisphosphate ,this will inhibit phosphofructokinase and activates fructose 1,6,bis phosphate both favors in gluconeogenesis. Availability of substrates Glucogenic amino acids stimulates gluconeogenesis(DM patients). Acetyl COA promotes gluconeogenesis by activating pyruvatecarboxylase enzyme. Alcohol inhibits gluconeogenesis(increses the risk of hypoglycemia(reduced plasma glucose level)due to reduced gluconeogenesis). Regulation of gluconeogenesis

15. Gluconeogenesis from Lactate Sources and fate of lactate: Sources: Lactate produced by active skeletal muscle is a major precursor for gluconeogenesis. In anaerobic conditions pyruvate is reduced to lactate by lactate dehydrogenase. From glycolysis especially in RBCs due to absence of mitochondria and muscle during exercises due to oxygen lack. Lactate or pyruvate produced in the muscle cannot be utilized for the synthesis of glucose due to absence of key enzymes. Plasma membrane is freely permeable to lactate, it is carried from skeletal muscle through blood and handed over to liver, there it undergoes conversion and then again transported to skeletal muscle as glucose. CORI CYCLE

16. b) Fate: 1) Glucose formation: [through lactic acid (Cori cycle)]: • Lactate formed in muscles and RBCs may be diffuse to blood then to the liver. • In the liver, lactate is converted to glucose by gluconeogenesis. Glucose may diffuse back to the blood, then to red cells or muscles to be used for production of energy. This cycle is called: Lactic acid or Cori cycle. • Definition of Cori cycle: It is the conversion of glucose to lactate in peripheral tissues, followed by conversion of lactate into glucose in liver.

17. 2) Conversion into pyruvate: If oxygen gets available, lactate is converted into pyruvate, which proceeds into Krebs' cycle. 3) Lactate may be accumulated in muscles causing muscle fatigue. 4) Lactate may be excreted in urine and sweat. c) Blood level of lactate: • Normal blood lactate level is: 1-2 mmol/L.

18. 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.

19. The Cori cycle costs 6 ~Pin liver for every 2 ~P made available in muscle. The net cost is 4 ~P. Although costly in ~P bonds, the Cori Cycle allows the organism to accommodate to large fluctuations in energy needs of skeletal muscle between rest and exercise.

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