Gluconeogenesis: Overview & Regulation PDF

Gluconeogenesis Objectives 1. Learn the enzymes and gluconeogenesis pathway. steps of the 2. Learn the similarities and difference the between gluconeogenesis and glycolysis pathways. 3. Understand how the gluconeogenesis pathway is regulated vs glycolysis. Gluconeogenesis meets the needs of the bod...

Gluconeogenesis Objectives 1. Learn the enzymes and gluconeogenesis pathway. steps of the 2. Learn the similarities and difference the between gluconeogenesis and glycolysis pathways. 3. Understand how the gluconeogenesis pathway is regulated vs glycolysis. Gluconeogenesis meets the needs of the body for glucose when carbohydrate is not available in sufficient amounts from the diet or from glycogen reserves. A supply of glucose is necessary especially for the nervous system and erythrocytes. Failure of gluconeogenesis is usually fatal Gluconeogenesis clears lactate produced by muscle and erythrocytes and glycerol produced by adipose tissue Gluconeogenesis Gluconeogenesis – (“formation of new sugar”)- Synthesis of glucose from noncarbohydrate precursors – Glucose primary fuel for brain & red blood cells, as well as the erythrocytes, testes, renal medulla, and embryonic tissues – Body uses 160g glucose/day, brain uses 120g/day Gluconeogenic pathway, pyruvate → glucose – – – – Major precursors → lactate, glycerol, certain amino acids Lactate → pyruvate Amino acids → pyruvate or oxaloacetate Glycerol → DHAP Takes place mainly in the liver, and to a lesser extent in renal cortex Gluconeogenesis: Precursor for Carbohydrates Major precursors alanine. → lactate, glycerol, certain amino acids, particularly Leucine and lysine can gluconeogenesis not supply carbon for net synthesis of glucose by The Cori (glucose–lactate) and Alanin cycle allows recycling of lactate and alanine back to glucose Triacylglycerols are composed of three fatty acids each in ester linkage with a single glycerol Glycerol’s Entry in Gluconeogenesis or Glycolysis Glycerol is released during the hydrolysis of triacylglycerols in adipose tissue, and is delivered by the blood to the liver. Glycerol is phosphorylated by glycerol kinase to glycerol phosphate, which is oxidized by glycerol phosphate dehydrogenase to dihydroxy acetone phosphate—an intermediate of glycolysis Animals cannot produce glucose from fatty acids Product of fatty degradation is acetyl-CoA acid Cannot have a net converstion of acetyl-CoA to oxaloacetate Glucose Can Be Synthesized from Odd Chain Fatty Acids Most fatty acids found in humans have straight chains with an even number of carbon atoms. Since acetyl CoA and other intermediates of even numbered fatty acid oxidation cannot be converted to oxaloacetate or any other intermediate of gluconeogenesis, it is impossible to synthesize glucose from fatty acids – Except: fatty acids with an odd number of carbon atoms Propionate is a good precursor for gluconeogenesis, generating oxaloacetate 9 Acetyl coAs Glucose Is Synthesized from Other Sugars – Fructose 2 molecules of dihydroxyacetone phosphate obtainable from one molecule of fructose can be converted to glucose – Galactose UDPglucose serves as a recycling intermediate in the overall process of converting galactose into glucose. – Mannose It is phosphorylated by hexokinase and then converted into fructose 6-phosphate; which then can be used in either glycolysis or gluconeogenesis General Features Tissues Liver (~90 %) Kidney (~10%) Subcellular Location of Gluconeogenic Enzymes (1) Glucose 6-phosphatase (endoplasmic reticulum) (2) Fructose 1,6 biphosphatase (cytosol) (2) PEPCK (cytosol and/or mitochondria) (3) Pyruvate carboxylase (mitochondria) Horton et al., 2000 Chap 13.6 Pyruvate to Phosphoenolpyruvate Requires two energyconsuming steps First step, pyruvate carboxylase converts pyruvate to oxaloacetate – Carboxylation using a biotin cofactor This is an anaplerotic (refilling) reaction – this reaction replenishes the oxaloacetate in the TCA cycle – And provides substrate for the synthesis of glucose Pyruvate carboxylase is a mitochondrial enzyme Mitochondrial membrane has no transporter for oxaloacetate. Oxaloacetate is transported from a mitochondrion in the form of malate After malate has been transported across the mitochondrial membrane, it is reoxidized to oxaloacetate Synthesis of Oxaloacetate Carboxylation of pyruvate Biotin is a CO2 carrier Pyruvate carboxylase is the first regulatory enzyme in the gluconeogenic pathway Acetyl-CoA activator is allosteric Second step, phosphoenolpyruvate carboxykinase (PEPCK) converts oxaloacetate to PEP – GTP provides a source of energy as well as the phosphate group of PEP. – PEPCK is located either in the cytosol or in mitochondria – In humans, the enzyme is distributed about equally in each compartment. PEP carboxykinase Oxaloacetate is converted to phosphoenolpyruvate by PEP carboxykinase Alternative Paths from Pyruvate to Phosphoenolpyruvate The relative importance of the two pathways depends on the availability of lactate or pyruvate and the cytosolic requirements for NADH for gluconeogenesis. Provides an important balance between NADH produced and consumed in the cytosol during gluconeogenesis The path on the right predominates when lactate is the precursor, because cytosolic NADH is generated in the lactate dehydrogenase reaction and does not have to be shuttled out of the mitochondrion Additional Bypasses Fructose bisphosphatase-1 (FBPase-1) Catalyze reverse reaction of opposing step in glycolysis Irreversible Fructose 1,6-bisphosphate → Fructose 6-Phosphate – By Mg+2-dependent – Coordinately/oppositely regulated with PFK Glucose 6-phosphate → Glucose – By Mg+2-dependent -Glucose 6-phosphatase – This enzyme is found on the lumenal side of the endoplasmic reticulum of hepatocytes and renal cells Gluconeogenesis is expensive 2 Pyruvate + 4 ATP + 2 GTP + 2 NADH + 2 H+ + 4 H2O → Glucose + 4 ADP + 2 GDP + 6 Pi + 2 NAD+ Costs 4 ATP, 2 GTP, and 2 NADH – But physiologically necessary – ATP is provided in large part by fatty acid oxidation Brain, nervous system, and red blood cells generate ATP ONLY from glucose When glycogen stores are depleted we need to get glucose – During starvation or vigorous exercise Regulation of Gluconeogenesis Although gluconeogenesis fasting, it is also stimulated : occurs during – during prolonged exercise – by a high-protein diet – and under conditions of stress. Availability of substrate Changes in the activity or amount of certain key enzymes ACTIVITY OR AMOUNT OF KEY ENZYMES Three sequences in the pathway gluconeogenesis are regulated: This avoids futile cycles of The increased concentration of acetyl-CoA inhibits the pyruvate dehydrogenase complex, slowing the formation of acetyl-CoA from pyruvate, and stimulates gluconeogenesis by activating pyruvate carboxylase, allowing excess pyruvate to be converted to glucose. Pyruvate carboxylase is inhibited by ADP Pyruvate dehydrogenase is inactive under conditions of – fasting – insulin levels are low – glucagon levels are elevated Pyruvate carboxylase is active: – Elevated levels of Acetyl CoA Phosphoenolpyruvate carboxykinase is induced by: – Glucagon – Cortisol Regulation of Phosphofructokinase 1 and Fructose 1,6-Bisphosphatase Goes to----- glycolysis if AMP is high and ATP is low Goes to ------ gluconeogenesis if AMP is low High levels of ATP and citrate: indicate that the energy charge is high and that biosynthetic intermediates are abundant High levels of AMP: Energy is needed Hormonal Control of Gluconeogenesis Is Critical for Homeostasis Glucagon and insulin regulate gluconeogenesis by influencing the state of phosphorylation of hepatic enzymes subject to covalent modification Glucagon – induces gluconeogenesis – increases plasma fatty acids by promoting lipolysis in adipose tissue Insulin has the opposite effect Fructose 2,6-Bisphosphate Plays a Unique Role in the Regulation of Glycolysis & Gluconeogenesis in Liver Fructose 2,6-bisphosphate – positive allosteric effector of PFK-1 – inhibitor of fructose-1,6-bisphosphatase The Balance Between Glycolysis and Gluconeogenesis in the Liver is Sensitive to BloodGlucose Concentration PFK-2 The concentration of F-2,6-BP rise and fall with blood glucose levels. How? Glucagon stimulates gluconeogenesis at the conversion of fructose 1,6 bisphosphate to fructose-6phosphate by decreasing the concentration of fructose 2,6 bisphosphate in liver When glucose is abundant (insulin ) – the concentration of fructose 2,6-bisphosphate increases – Stimulates glycolysis by activating PFK-1 – Inhibits fructose-1,6-bisphosphatase When glucose is low (glucagon ) – inactivates PFK-2 – activates fructose 2,6-bisphosphatase Glucagon: α cells of pancreatic islets Decreases the level of fructose 2,6-bisphosphate Mediates the phosphorylation of hepatik PK (inactive) Induces the transcription of PEP-carboxykinase Dephosphorylation of glucose 6-phosphate It is present in liver and kidney but absent from muscle and adipose tissue, which, therefore, cannot export glucose into the bloodstream Induced during (increase in transcription) fasting gene