Text of Glycolysis and Gluconeogenesis Alice Skoumalová. Metabolism of glucose - overview
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Glycolysis and Gluconeogenesis Alice Skoumalov
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Metabolism of glucose - overview
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1. Glycolysis
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Glucose: the universal fuel for human cells Sources: diet (the major sugar in our diet) internal glycogen stores blood (glucose homeostasis) Glucose oxidation: after a meal: almost all tissues during fasting: brain, erythrocytes
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Glycolysis: oxidation and cleavage of glucose ATP generation (with and without oxygen) all cells in the cytosol (the reducing equivalents are transferred to the electron-transport chain by the shuttle) ATP is generated: 1. via substrate-level phosphorylation 2. from NADH 3. from oxidation of pyruvate Regulation of glycolysis: 1. Hexokinase 2. Phosphofructokinase 3. Pyruvate Kinase Generation of precursors for biosynthesis: fatty acids amino acids ribosis-5-P
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Anaerobic glycolysis a limited supply of O 2 no mitochondria increased demands for ATP Lactic acidemia in hypoxia
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Phosphorylation of glucose: irreversible Glucose 6-P: cannot be transported back across the plasma membrane a precursor for many pathways that uses glucose Hexokinases Glucokinase (liver, -cell of the pancreas) high K m
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Michaelis-Menten kinetics
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1. Conversion of glucose 6-P to the triose phosphates 2. Oxidation and substrate-level phosphorylation
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1. Conversion of glucose 6-P to the triose phosphates irreversible regulation essential for the subsequent cleavage
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Substrate-level phosphorylation 2. Oxidation and substrate-level phosphorylation
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Summary of the glycolytic pathway: Glucosis + 2 NAD + + 2 P i + 2 ADP 2 pyruvate + 2 NADH + 4 H + + 2 ATP + 2 H 2 O G 0 = - 22 kcal (it cannot be reversed without the expenditure of energy!)
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Clinical correlations: Hypoxemia (lack of oxygen in tissues) Acute hemorrhage (hypotension, lost of erythrocytes) - anaerobic glycolysis - lactate formation, metabolic acidosis Chronic obstructive pulmonary disease (an insuficient ventilation) - anaerobic glycolysis, lactate formation, metabolic acidosis - accumulation of CO 2, respiratory acidosis Myocardial infarction (lack of oxygen in myocardium) - anaerobic glycolysis, lactate formation - lack of ATP
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Aerobic glycolysis: involving shuttles that transfer reducing equivalents across the mitochondrial membrane
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Glycerol 3-phosphate shuttle:
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Malate-aspartate shuttle:
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Anaerobic glycolysis: Energy yield 2 mol of ATP dissociation and formation of H +
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Daily lactate production115 (g/d) Erythrocytes29 Skin20 Brain17 Sceletal muscle16 Renal medulla15 Intestinal mucosa8 Other tissues10 Major tissues of lactate production: (in a resting state)
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Cori cycle: Lactate can be further metabolized by: heart, sceletal muscle Lactate dehydrogenase: a tetramer (subunits M and H)
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Lactate dehydrogenase Pyruvate + NADH + H + lactate + NAD + LD 5 isoenzymes: Heart (lactate) Muscle (pyruvate)
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Biosynthetic functions of glycolysis:
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Clinical correlations: Long-intensity exercise (for example a sprint) - the need for ATP exceeds the capacity of the mitochondria for oxidative phosphorylation, anaerobic glycolysis lactate formation, muscle fatigue and pain - a training the amounts of mitochondria and myoglobin increase
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Regulation
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Fructose 2,6-bis-phosphate: is not an intermediate of glycolysis! Phosphofructokinase-2:inhibited through phosphorylation - cAMP-dependent protein kinase (inhibition of glycolysis during fasting-glucagon) tissue-specific isoenzymes (low K m, a high afinity) glucokinase (high K m ) the rate-limiting, allosteric enzyme tissue-specific isoenzymes
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the liver isoenzyme - inhibition by cAMP-dependent protein kinase (inhibition of glycolysis during fasting) Lactic acidemia: increased NADH/NAD + ratioinhibition of pyruvate dehydrogenase
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2. Gluconeogenesis
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Gluconeogenesis: synthesis of glucose from noncarbohydrate precursors to maintain blood glucose levels during fasting liver, kidney fasting, prolonged exercise, a high- protein diet, stress Specific pathways: 1.Pyruvate Phosphoenolpyruvate 2.Fructose-1,6-P Fructose-6-P 3.Glucose-6-P Glucose
Conversion of pyruvate to phosphoenolpyruvate 1. Pyruvate Oxaloacetate Pyruvate carboxylase 2. Oxaloacetate PEP Phosphoenolpyruvate- carboxykinase
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Conversion of phosphoenolpyruvate to glucose 3. Fructose-1,6-P Fructose-6-P Fructose 1,6-bisphosphatase (cytosol) 4. Glucose-6-P Glucose Glucose 6-phosphatase (ER)
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Clinical correlations: Alcoholism - excessive ethanol consumption increase NADH/NAD + ratio that drive the lactate dehydrogenase reaction toward lactate - lack of precursors for gluconeogenesis its inhibition - insuficient diet - reduced glucose in the blood, consumption of glycogen in the liver hypoglycemia
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3. Regulations
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TypeMechanismExample Substrate concentrationSaturation kinetics (according Michaelis- Menten) Glucokinase (activation after meal- high K m ) AllostericConformational changes induced by the binding of the allosteric efector Enzymes of glycolysis and glukoneogenesis (allosteric effectors ATP, AMP, citrate) Covalent modificationConformational changes induced by phosphorylation by proteinkinases Phosphorylation og glycogensynthase and glycogenphosphorylase (glucagon) Protein-protein interaction Conformational changes as a result of different protein binding Muscle glycogenphosphorylase (activated by Ca 2+ -calmodulin) Proteolytic cleavageActivation by proteolytic cleavage of precursor molecule Proteins of coagulation cascade (zymogens) Enzyme synthesisInduction or repression of the enzyme synthesis Enzymes of gluconeogenesis (induced during fasting) Enzyme regulation:
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Second messenger - cAMP: Glucagon Adenylate- cyclase
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Regulation of gluconeogenesis: concomitant inactivation of the glycolytic enzymes and activation of the enzymes of gluconeogenesis 1. Pyruvate PEP Phosphoenolpyruvate carboxykinase - induced by glucagon, epinephrine, and cortisol 2. Fructose 1,6-P Fructose 6-P Fructose 1,6-bisphosphatase - inhibited by fructose 2,6-P 3. Glucose 6-P Glucose Glucose 6-phosphatase - induced during fasting
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Summary Glycolysis Generation of ATP (with or without oxygen) The role of glycolysis in different tissues Lactate production Regulation (3 key enzymes) Gluconeogenesis Activation during fasting, prolonged exercise, after a high- protein diet Precursors: lactate, glycerol, amino acids 3 key reactions:Pyruvate PEP Fructose-1,6-P Fructose-6-P Glucose-6-P Glucose Regulation
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Pictures used in the presentation: Marks Basic Medical Biochemistry, A Clinical Approach, third edition, 2009 (M. Lieberman, A.D. Marks)
