Glucose Oxidation & Gluconeogenesis PDF

Glucose oxidation Shereen El Tarhouny Professor of Medical Biochemistry Krebs Cycle The tricarboxylic acid cycle (TCA cycle, also called the Krebs cycle or the citric acid cycle) plays several roles in metabolism. The cycle occurs totally in the mitochondria and is, therefore, in c...

Glucose oxidation Shereen El Tarhouny Professor of Medical Biochemistry Krebs Cycle The tricarboxylic acid cycle (TCA cycle, also called the Krebs cycle or the citric acid cycle) plays several roles in metabolism. The cycle occurs totally in the mitochondria and is, therefore, in close proximity to the reactions of electron transport, which oxidize the reduced coenzymes produced by the cycle. The TCA cycle is thus an aerobic pathway, because O2 is required as the final electron acceptor.  It is the final common pathway for the oxidation of carbohydrates, amino acids, and fatty acids to CO2 and H20. This oxidation provides energy for the production of the majority of ATP in humans.  1 2 3 8 7 4 6 5  Two carbon atoms enter the cycle as acetyl CoA and leave as CO2. The cycle does not involve net consumption or production of oxaloacetate or of any other intermediate. Four pairs of electrons are transferred during one turn of the cycle: three pairs of electrons reducing NAD to NADH and one pair reducing FAD to FADH2. Oxidation of one NADH by the electron transport chain leads to formation of approximately three ATP, whereas oxidation of FADH2 yields approximately two ATP.  Energy-producing Number of ATP reaction produced 3 NADH → 3 NAD+ 9 FADH2 → FAD 2 GDP + Pi → GTP 1 12 ATP/acetyl CoA oxidized Regulation of Citric Acid Cycle Increases its reaction rate when low levels of ATP or NAD+ activate isocitrate dehydrogenase. Slows when high levels of ATP or NADH inhibit citrate synthetase (first step in cycle).  Synthesis of glucose from non- carbohydrate sources. It occurs principally in the liver (90%) and Gluconeogenesis to some extent in the kidney (10% in fasting, and up to 50% during prolonged starvation). It occurs in the cytosol and mitochondria. Gluconeogenesis is the chief source of blood glucose during fasting for more than 18 hours. Normal glucose curve Plasma glucose levels mg/dL 250 - Renal threshold (180 mg/dL) - 200 150 - - 100 - 50 - F 1h 1.5h 2h 4h 16h 24h I I I I I I I I Time of blood samples Glycogenolysis Gluconeogenesis  Glucose is necessary as a source of energy for the nervous system and the erythrocytes, renal cortex, cornea, and eye lens. It is the main source of energy for skeletal muscles during severe exercise and for the fetus during pregnancy. During lactation, glucose is needed by the mammary glands for lactose synthesis. A- Lactate Liver Glucose Gluconeogenesis Lactate Blood Lactate Glucose Lactate RBCs Exercising Glycolysis muscle Glucose Lactate is released into the blood by exercising skeletal muscle and by red blood cells. “Cori cycle”. The principal pathway of gluconeogenesis is essentially a reversal of glycolysis. However, the reactions catalyzed by 1- glucokinase (hexokinase) 2- Phosphofructokinase-1 3- pyruvate kinase are irreversible and require 4 enzymes unique to gluconeogenesis for their reversal.  Glucose Glucose 6-phosphate Fructose 6-phosphate Fructose 1,6-bisphosphate Glyceraldehyde 3- Dihydroxyacetone Glycerol phosphate phosphate 1,3-diphosphoglycerate 3-phosphoglycerate 2-phosphoglycerate Phosphoenolpyruvate Oxaloacetate Some amino acids Lactate Pyruvate Some amino acids  Regulation of gluconeogenesis Substrate availability Decreased levels of insulin (anabolic hormone) favor mobilization of amino acids from muscle protein and provide carbon skeleton for gluconeogenesis. Glycerol is available due to accelerated lipolysis in adipose tissue  Liver Gluconeogenesis Glucose Glutarate Blood Pyruvate Glucose Alanine -ketoglutaric acid Glucose Glycolysis Muscle Pyruvate Glutamate Alanine -ketoglutaric acid Alanine cycle Gluconeogenesis and glycolysis are reciprocally regulated that is one pathway is relatively inactive while the other is highly active. Blood glucose level plays an important role in determining whether glucose is to be degraded (Glycolysis) or synthesized (Gluconeogenesis). PENTOSE PHOSPHATE PATHWAY Occurs in the cytosol. Consists of 2 reaction pathways: 1. irreversible oxidative reactions. 2. reversible sugar-phosphate interconversion. 2 NADP+ 2 NADPH + H+ Glucose CO2 G6P Oxidative Ribulose 5-P Fructose 6-P Non-oxidative Ribose 5-P Glyceraldeyde 3-P Nucleotide biosynthesis  No ATP is directly consumed or produced in the cycle. C1 of glucose 6-phosphate is released as CO2. The pathway provides: 1. A major portion of the body's NADPH which functions as a biochemical reductant 2. ribose 5-phosphate for the biosynthesis of nucleotides.  Irreversible oxidative reactions These reactions are important in tissues that require primarily NADPH. e.g. 1. liver , mammary glands, and adipose tissue for the synthesis of fatty acids. 2. Adrenal cortex for the synthesis of steroids. 3. RBCs to keep glutathione reduced. 4. skeletal muscles are nearly lacking in this pathway.  II- Reversible non-oxidative reactions. In all cell types synthesizing nucleotides and nucleic acids.  oxidative reactions of pentose phosphate pathway Ribose Glyceraldehyde Fructose 6- Glucose C5 C6 5-phosphate 3-phosphate phosphate 6-phosphate C3 C6 transketolase transaldolase Xylulose Sedoheptulose Erythrose Fructose C5 5-phosphate 7-phosphate 4-phosphate 6-phosphate C7 C4 C6 transketolase Xylulose Glyceraldehyde C5 5-phosphate 3-phosphate C3  GLUCOSE 6-PHOSPHATE DEHYDROGENASE DEFICIENCY Glucose 6-phosphate dehydrogenase (G6PD) deficiency is an inherited disease characterized by hemolytic anemia caused by the inability to detoxify oxidizing agents. G6PD deficiency is X-linked. The life span of many individuals with G6PD deficiency is somewhat shortened as a result of complications arising from chronic hemolysis. Effect of diminished G6PD activity in red blood cells: Diminished G6PD activity impairs the ability of the cell to form the NADPH that is essential for the maintenance of the reduced glutathione pool. This results in a decrease in the cellular detoxification of free radicals and peroxides formed within the cell.  Precipitating factors in G6PD deficiency Oxidants drugs. Fava beans (Favism). Infections.  Glucose 6-phosphate dehydrogenase deficiency impairs the ability of an Oxidant stress erythrocyte to form NADPH, resulting certain drugs in hemolysis. Infections Fava beans Glucose Glucose Glucose 6-phosphate Erythrocyte (G6P) HMP NADP+ 2 GSH H2O2 2 ADP Glucose 6-phosphate Glutathione Glutathione Glycolytic dehydrogensae reductase peroxidase pathway 6-Phospho- NADPH GS-SG 2 H2O gluconate + H+ 2 ATP 2 Lactate Pathways of glucose 6-phosphate metabolism in the erythrocyte  Uronic Acid pathway This is an alternative oxidative pathway in which glucose gives glucuronic acid as an intermediate. It principally occurs in the liver.  OH O-PO32 ˉ I I H–C H–C I I H – C – OH H – C – OH I Phosphoglucomutase I HO – C – H O HO – C – H O I I H – C – OH H – C – OH UTP I I H–C H–C I I CH2O-PO32ˉ CH2OH UDPG Pyrophosphorylase Glucose 6-P Gluccose 1-P PPi O - UDP 2 (NADH+ + H+) O - UDP I 2 NAD+ I H–C H–C I I H – C – OH H – C – OH I I HO – C – H O UDP-glucose O HO – C – H I dehydrogenase H2O I H – C – OH H – C – OH I I H–C H–C I I COOH CH2OH UDP-Glucuronic acid UDP-Glucose The uronic acid pathway Importance Formation of Glucuronic Acid: This pathway provides UDP- glucuronic acid, which is the active donor of glucuronic acid for conjugation with steroid hormones, drugs, and bilirubin, as well as for the biosynthesis of the glycosaminoglycans. The reaction is catalyzed by UDP-glucuronyltransferase. Conjugation of a compound with glucuronic acid produces products that are strongly acidic, highly polar, more soluble in water, and easily excreted by the kidneys. In humans, development of this conjugation mechanism takes several days to 2 weeks after birth.