Lecture 12 Modified from internet resources, journals and boks Gluconeogenesis Lecture 12 Modified from internet resources, journals and boks

Gluconeogenesis biosynthesis of new glucose, (i.e. not glucose from glycogen) production of glucose from other metabolites  necessary for use as a fuel source by the brain, testes, erythrocytes and kidney medulla (glucose is the sole energy source for these organs) During starvation  brain can derive energy from ketone bodies (are converted to acetyl-CoA) Synthesis of glucose from three and four carbon precursors  essentially a reversal of glycolysis

Gluconeogenesis from 2 moles of pyruvate to 2 moles of 1,3-bisphosphoglycerate consumes 6 moles of ATP makes the process of gluconeogenesis very costly from an energy standpoint several steps are required in going from 2 moles of 1,3-bisphosphoglycerate to 1 mole of fructose-1,6-bisphosphate In hepatocytes  the glucose-6-phosphatase reactions allows the liver to supply the blood with free glucose

continued In the liver or kidney cortex and in some cases skeletal muscle  the glucose-6-phosphate (G6P) produced by gluconeogenesis can be incorporated into glycogen Skeletal muscle lacks glucose-6-phosphatase  cannot deliver free glucose to the blood and undergoes gluconeogenesis exclusively as a mechanism to generate glucose for storage as glycogen

Pyruvate to Phosphoenolpyruvate (PEP), Bypass 1 Conversion of pyruvate to PEP requires the action of two mitochondrial enzymes pyruvate  is carboxylated to form oxaloacetate (OAA) CO2 in this reaction is in the form of bicarbonate (HCO3-) conversion of pyruvate to PEP  PEP carboxykinase (PEPCK) For gluconeogenesis to proceed  the OAA produced by PC needs to be transported to the cytosol

continued If OAA is converted to PEP by mitochondrial PEPCK  transported to the cytosol where it is a direct substrate for gluconeogenesis and nothing further is required Transamination of OAA to aspartate  allows the aspartate to be transported to the cytosol where the reverse transamination occurs yielding cytosolic OAA

Fructose-1,6-bisphosphate to Fructose-6-phosphate, Bypass 2 Fructose-1,6-bisphosphate (F1,6BP) conversion to fructose-6-phosphate (F6P) = the reverse of the rate limiting step of glycolysis the reaction = a simple hydrolysis is catalyzed by fructose-1,6-bisphosphatase (F1,6BPase) the regulation of glycolysis occurring at the PFK-1 reaction, the F1,6BPase reaction is a major point of control of gluconeogenesis

Glucose-6-phosphate (G6P) to Glucose (or Glycogen), Bypass 3 G6P is converted to glucose through the action of glucose-6-phosphatase (G6Pase) the brain and skeletal muscle, as well as most non-hepatic tissues  lack G6Pase activity  any gluconeogenesis that occurs in these tissues is not utilized for blood glucose supply In the kidney, muscle and especially the liver  G6P be shunted toward glycogen if blood glucose levels are adequate

Substrates for Gluconeogenesis Lactate: predominate source of carbon atoms for glucose synthesis by gluconeogenesis During anaerobic glycolysis in skeletal muscle  pyruvate is reduced to lactate by lactate dehydrogenase (LDH) This reaction serves to a critical functions during anaerobic glycolysis: lactate produced by the LDH reaction is released to the blood stream and transported to the liver where it is converted to glucose  glucose is then returned to the blood for use by muscle as an energy source and to replenish glycogen stores = the Cori cycle

The Cori cycle invloves the utilization of lactate, produced by glycolysis in non-hepatic tissues, (such as muscle and erythrocytes) as a carbon source for hepatic gluconeogenesis the liver can convert the anaerobic byproduct of glycolysis, lactate, back into more glucose for reuse by non-hepatic tissues

continued Pyruvate: generated in muscle and other peripheral tissues, can be transaminated to alanine which is returned to the liver for gluconeogenesis the transamination reaction requires an α-amino acid as donor of the amino group, generating an α-keto acid in the process the pathway is termed the glucose-alanine cycle

continued The glucose-alanine cycle  an indirect mechanism for muscle to eliminate nitrogen while replenishing its energy supply major function of the glucose-alanine cycle is to allow non-hepatic tissues to deliver the amino portion of catabolized amino acids to the liver for excretion as urea Within liver  alanine is converted back to pyruvate and used as a gluconeogenic substrate or oxidized in the TCA cycle the amino nitrogen is converted to urea in the urea cycle and excreted by the kidneys

continued glucose-alanine cycle  used primarily as a mechanism for skeletal muscle to eliminate nitrogen while replenishing its energy supply Glucose oxidation  produces pyruvate which can undergo transamination to alanine during periods of fasting  skeletal muscle protein is degraded for the energy value of the amino acid carbons and alanine is a major amino acid in protein alanine  enters the blood stream and is transported to the liver  converted back to pyruvate which is then a source of carbon atoms for gluconeogenesis  newly formed glucose can then enter the blood for delivery back to the muscle

continued Amino Acids: All 20 of the amino acids, excepting leucine and lysine, can be degraded to TCA cycle intermediates the carbon skeletons of the amino acids  converted to oxaloacetate and subsequently into pyruvate  utilized by the gluconeogenic pathway glycogen stores are depleted, in muscle during exertion and liver during fasting  catabolism of muscle proteins to amino acids contributes the major source of carbon for maintenance of blood glucose levels

continued Glycerol: Oxidation of fatty acids  yields enormous amounts of energy, however, the carbons of the fatty acids cannot be utilized for net synthesis of glucose carbon unit of acetyl-CoA derived from β-oxidation of fatty acids  can be incorporated into the TCA cycle glycerol backbone of lipids  can be used for gluconeogenesis

Regulation of Gluconeogenesis regulation of gluconeogenesis will be in direct contrast to the regulation of glycolysis generally, negative effectors of glycolysis are positive effectors of gluconeogenesis regulation of the activity of PFK-1 and F1,6BPase  the most significant site for controlling the flux toward glucose oxidation or glucose synthesis

Regulation of glycolysis and gluconeogenesis by fructose 2,6-bisphosphate (F2,6BP) major sites for regulation of glycolysis and gluconeogenesis  the phosphofructokinase-1 (PFK-1) and fructose-1,6-bisphosphatase (F-1,6-BPase) catalyzed reactions level of F2,6BP will decline in hepatocytes in response to glucagon stimulation as well as stimulation by catecholamines

continued Each of these signals  elicited through activation of cAMP-dependent protein kinase (PKA) One substrate for PKA is PFK-2 (enzyme responsible for the synthesis and hydrolysis of F2,6BP) PFK-2 is phosphorylated by PKA  it acts as a phosphatase leading to the dephosphorylation of F2,6BP with an increase in F1,6Bpase activity and a decrease in PFK-1 activity F1,6Bpase activity is regulated by the ATP/ADP ratio; when this is high, gluconeogenesis can proceed maximally

continued Gluconeogenesis is also controlled at the level of the pyruvate to PEP bypass hepatic signals elicited by glucagon or epinephrine lead to phosphorylation and inactivation of pyruvate kinase (PK)  allows for an increase in the flux through gluconeogenesis