Carbohydrate Metabolism III & IV
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Carbohydrate Metabolism III & IV - Gluconeogenesis - - Anaplerosis and Cataplerosis - - Pentose Phosphate Pathway – - Regulation of Glycolysis & Gluconeogenesis - FScN4621W Food Science and Nutrition University of Minnesota 1 Unit III Gluconeogenesis Anaplerosis and Cataplerosis 2 Glycolysis Glycolysis under anaerobic conditions C6H12O6 (6 carbon) Cytosol 2X Pyruvate (3 carbon) NADH NAD+ 2X lactate (3 carbon) 3 1 Glycolysis &TCA Oxidation of glucose or breakdown of glucose C6H12O6 (6 carbon) Cytosol glycolysis 2X Pyruvate (3 carbon) PDH CO2 2X Acetyl CoA (2 carbon) aerobic conditions TCA O2 6 CO2 + 6H2O + 2ATP + 6NADH & 2FADH Oxidative phosphorylation Mito ATPs 4 Respiratory Chain and Oxidative Phosphorylation Respiratory chain consists of four complexes Complex I: NADH dehydrogenase Complex II: Succinate dehydrogenase Complex III: Cytochrome bc1 complex Complex IV: Cytochrome c oxidase Complex I, III, and IV are proton pumps ATP synthase is an ATP-dependent proton pump A series of oxidation-reduction reactions make up the electron transport. Oxidation: a chemical substance takes on oxygen or loses electrons. Reduction: a chemical substance gives off oxygen or takes on electrons Electrons pass through four complexes and generate energy This energy is transferred to ADP for synthesis of ATP by establishing proton gradient concentration across the inner membrane 5 Respiratory Chain FADH2 NADH → Complex I → Q → Complex III → cytochrome c → Complex IV → O2 ↑ ↑ Complex II 6 2 Glycerol 3-Phosphate Shuttle 7 Malate- Aspartate Shuttle Respiratory chain 8 Gluconeogenesis Biosynthesis of glucose from non- carbohydrate precursors Maintenance of blood glucose within normal ranges It occurs primarily in _____ and _____ (tissues) It occurs primarily in ___________ and ________(intracellular compartments) NOT exactly a reversal of glycolysis 9 3 Common Precursors for Gluconeogenesis Pyruvate Lactate Glycerol Propionate Glucogenic amino acids AA converted to Pyruvate: Ala, Ser, Gly, Thr, Cys, Tryp AA converted to Oxaloacetate: Asp AA converted to -Ketoglutarate: Glu, Gln, Pro, His, Arg 10 Three Irreversible Reactions in Glycolysis Hexokinase Glucokinase Glucose glucose 6-phosphate 6-Phosphofructo-1-kinase Fructose 6-phosphate Fructose 1,6-bisphosphate Pyruvate kinase Phospoenolpyruvate pyruvate 11 Three Bypass Reactions in Gluconeogenesis Pyruvate kinase Phospoenolpyruvate pyruvate 2 1 Phosphoenolpyruvate Pyruvate carboxykinase carboxylase 6-Phosphofructo-1-kinase Fructose 6-phosphate Fructose 1,6-bisphosphate Fructose 1,6-biphosphatase Hexokinase Glucose glucose 6-phosphate Glucose 6-phosphatase 12 4 Gluconeogenesis * Occurs in mitochondria and cytosol Mitochondria: • Generation of PEP • 2 key reactions • 2 key enzymes PEP Cytosol: • Conversion of PEP to glucose • 2 key reactions * • 2 key enzymes Lactate/pyruvate: 4 reactions PEP Glutamate: 3 reactions * Glycerol: 2 reactions 13 * Conversion of PEP Pyruvate to PEP 2 2 key reactions 2 key enzymes * 2 Mitochondrion PEP 1 * * 14 Gluconeogenesis Glycogen * Cytosol: • Conversion of PEP to glucose • 2 key reactions • 2 key enzymes PEP 15 5 Gluconeogenesis Conversion of F-1,6-BisP to F-6-P glycolysis GNG * * Conversion of G-6-P to Glucose GNG * * glycolysis Catalyzed by Glucose-6-phosphatase Its presence determines whether a tissue can contribute to circulating glucose In which tissues is this enzyme expressed? 16 Cori Cycle Specific gluconeogenic pathway involving de novo synthesis of glucose from Lactate Production of lactate mainly by muscle and red blood cells Lactate lactate glucose Transport to the liver Typical gluconeogenic pathway – four key reactions/enzymes are invovled Glucose into the circulation Red blood cell Muscle 17 Alanine Cycle Specific gluconeogenic pathway involving de novo synthesis of glucose from Alanine Alanine is released to the circulation by skeletal muscle In muscle it is derived by transamination of pyruvate In liver, deamination of alanine generates pyruvate glucose circulation muscle Alanine Alanine Alanine deamination transamination Glucose Pyruvate Glucose Pyruvate Gluconeogenesis involves four key reactions/enzymes 18 6 Glutamate as a precursor Gluconeogenic pathway involving TCA cycle and bypassing the reaction by pyruvate carboxylase Glutamate is converted to Oxaloacetate via TCA cycle GNG using Glu as a precursor bypasses pyruvate kinase catalyzed reaction * PEP 19 Glucose Fatty acid TCA Cycle Amino acid Harper’s Chapter 16, Figure 16- 3 Glutamate 20 Glycerol as a precursor Gluconeogenic pathway only in cytosol Two last key reactions/enzymes * * 21 7 Energy Utilization in GNG 6 molecules of ATP are utilized to produce 1 molecule of glucose from 2 molecules of pyruvate In liver, this energy required for de novo synthesis of glucose comes from fatty acid oxidation or partial amino acid oxidation 22 Gluconeogenesis during the starvation Glucose goes to the partial oxidation in the liver and muscle for sparing carbon backbone Brain (pyruvate and lactate) for Red blood cells gluconeogensis in the liver Glucose Cori cycle Lactate Lactate GNG OXA X X Alanine Liver Alanine cycle Muscle 23 Clinical Correlation During fasting Defects in PEPCK What happens to blood glucose control? What happens to blood lactate levels? What happens to glycogen store? PEPCK: phosphoenolpyruvate carboxykinase 24 8 Clinical Correlation Defects in Glucose-6-phosphatase What happens to blood glucose control? What happens to blood lactate levels? What happens to glycogen store? 25 Glucose-6-phosphatase 26 Gluconeogenesis in the Kidney It occurs during prolonged starvation Contributes 10% of gluconeogenically derived glucose After 14-16h fasting, 20-25% of gluconeogenically derived glucose provides ~50% of the net glucose synthesis during prolonged starvation Precursors: mostly glucogenic amino acids Glutamine mainly released from muscle Gluconeogenic pathway: It involves anaplerotic and cataplerotic reactions 27 9 Anaplerosis and Cataplerosis Series of enzymatic reactions or pathways that replenish the pools of metabolic intermediates in the TCA cycle. Keep the pool size of TCA intermediates not to be largely changed during high energy consumption (e.g. exercise) or during lower energy consumption (e.g. fasting). Anaplerosis – Entry of 4- and 5-carbon intermediates into the TCA cycle Cataplerosis – Exit of 4- and 5-carbon intermediates from the TCA cycle Anaplerosis and cataplerosis need to be balanced to avoid the accumulation of intermediates and to remain the normal function of TCA cycle 28 Anaplerosis and cataplerosis in the TCA cycle Owen, O. E. et al. J. Biol. Chem. 2002;277:30409-30412 29 Role of TCA Cycle in Gluconeogenesis Owen, O. E. et al. J. Biol. Chem. 2002;277:30409-30412 30 10 Role of Anaplerosis and Cataplerosis in Glutamine Metabolism in the small intestine Glutamine is metabolized for energy in the small intestine. PEPCK Cytosol Mito Owen, O. E. et al. J. Biol. Chem. 2002;277:30409-30412 31 Role of Anaplerosis and Cataplerosis in Glyceroneogenesis in Adipose Tissue Amino acids Glucose Provides glyceride- glycolysis glycerol from non-sugar precursor Glycerol-P pyruvate Anaplerosis Glycerol-P Cataplerosis 32 Owen, O. E. et al. J. Biol. Chem. 2002;277:30409-30412 Unit IV Pentose Phosphate Pathway Regulation of Glycolysis & Gluconeogenesis 33 11 Pentose Phosphate Pathway G-6-P Dehydrogenase Glucose-6-Phosphate 6-Phosphogluconolactone + NADP+ NADPH + H H2O H+ 6-Phosphogluconolactonase 6-Phosphogluconate Phosphogluconate Dehydrogenase NADP+ Ribulose-5-Phosphate + CO NADPH + H+ 2 34 Pentose Phosphate Pathway • A secondary pathway of glucose oxidation • A source of reducing equivalents: NADPH • Production of 5-carbon sugar phosphates - Ribose 5-phosphate - Xylulose 5-phosphate • Takes place in the cytosol 35 Pentose Phosphate Pathway • NADPH is required for synthesis of fatty acids, cholesterol, and sterols - mammary gland, adipose tissue, liver, adrenal cortex and testis • NADPH is required for maintenance of the integrity of the red blood cells • Ribose 5-phosphate is essential for biosynthesis of nucleotide and nuclear acids (ATP, coenzyme A, NAD, NADP, FAD, RNA and DNA) 36 12 Regulation of Glycolysis & Gluconeogenesis Glucose transport Substrate cycle Substrate levels can control the rate of metabolic pathways Coordinates glycolysis and gluconeogenesis Hormonal regulation Enzyme activity Gene expression 37 Glucose Transporters Transporter Major tissue site Affinity for Km glucose GLUT1 Brain, Placenta High Low GLUT2 Liver, Pancreatic β-cells Low High GLUT3 Brain, Placenta High Low GLUT4 Skeletal and cardiac High Low muscle, Adipose tissue GLUT5 Small intestine *Transport fructose 38 Michaelis Constant Km • The Michaelis constant Km is defined as the substrate concentration at 1/2 the maximum velocity. • The amount of the enzyme is kept constant and the substrate concentration is then gradually increased, the reaction velocity will increase until it reaches a maximum. 39 13 Glucose Transporters GLUT1 and GLUT3: Low Km or high affinity for glucose Active when blood glucose levels are low Responsible for basal glucose uptake in most tissues GLUT2: High Km or low affinity for glucose Active when blood glucose levels are high A key transporter responding to elevated blood glucose GLUT4: Low Km or high affinity for glucose Transport basal glucose uptake, BUT to a small extent – WHY? Mainly responsible for insulin-stimulated glucose uptake GLUT2 and GLUT4 transport glucose into the