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Pentose Phosphate Pathway

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Pentose Phosphate Pathway CH 4. HMP Pentose Phosphate Pathway Pentose Phosphate Pathway Other names: Phosphogluconate Pathway Hexose Monophosphate Shunt The linear part of the pathway carries out oxidation and decarboxylation of the 6-C sugar glucose-6-P, producing the 5-C sugar ribulose-5-P. 1 2 CH 4. HMP Glucose metabolism via the oxidative pentose phosphate pathways and relationship with glycolysis. Principal cellular functions of the pathways are indicated, and roles of NADPH included. G6PDH, glucose 6-phosphate dehydrogenase; PGDH, 6- 3 phosphogluconate dehydrogenase; RPE, ribulose-5-phosphate 3-epimerase; RPI, ribose 5-phosphate isomerase; TKL, transketolase. CH 4. HMP Hexose Monophosphate Pathway CH 4. HMP 4 Hexose Monophosphate Pathway It consists of two irreversible oxidative reactions, followed by a series of reversible sugar-phosphate interconversions No ATP is directly consumed or produced in the cycle. Carbon 1 of glucose 6-phosphate is released as CO2, and two NADPH are produced for each glucose 6-phosphate entering the oxidative part of the pathway. 5 CH 4. HMP The rate and direction of the reactions at any given time are determined by the supply of and demand for intermediates in the cycle. The HMP occurs in the cytosol of the cell. The pathway provides a major portion of the cell's NADPH, which functions as a biochemical reductant. The HMP also produces ribose-phosphate, required for biosynthesis of nucleotides, 6 CH 4. HMP for each molecule of glucose 6-phosphate oxidized. 1. Dehydrogenation of glucose 6-phosphate 2. Hydrolysis of 6- phosphogluconolactone and formation of ribulose 5- phosphate. The oxidative portion of the HMP leads to the formation of -two molecules of NADPH -CO2, and -ribulose 5-phosphate, 7 CH 4. HMP Oxidative Reactions Reductive anabolic pathway H+ NADPH NADPH NADP+ NADP+ Xylulose5 G PO 6-Phospho Ribulose-5-PO 6 4 gluconate 4 phosphate 1 2 Oxidative Reaction Irreversible 8 CH 4. HMP Oxidative Reactions Glucose-6-phosphate Dehydrogenase catalyzes oxidation of the aldehyde at C1 of glucose-6-phosphate, to a carboxylic acid in ester linkage (lactone). NADP+ serves as electron acceptor. Lactone is hydrolyzed resulting in ring opening. The product is 6-phosphogluconate. 9 CH 4. HMP Oxidative Reactions CH 4. HMP Phosphogluconate Dehydrogenase catalyzes oxidative decarboxylation of 6-phosphogluconate, to yield the 5-C ketose ribulose-5-phosphate. The OH at C3 (C2 of product) is oxidized to a ketone. This promotes loss of the carboxyl at C1 as CO2. NADP+ again serves as oxidant (electron acceptor). Structure of NADPH Regulation: Glucose-6-phosphate Dehydrogenase is the committed step of the Pentose Phosphate Pathway. This enzyme is regulated by availability of the substrate NADP+. As NADPH is utilized in reductive synthetic pathways, the increasing concentration of NADP+ stimulates the Pentose Phosphate Pathway, to replenish NADPH. 10 11 CH 4. HMP Nucleic acid Nonoxidative reaction CH 4. HMP biosynthesis Xylulose-5- Erythrose-4- PO Ribose-5-PO Sedoheptulose 4 4 PO –7-PO4 4 Fructose-6- PO4 Glyceraldehydes Ribulose-5- -3-PO4 PO4 Xylulose-5- Glyceraldehyde Fructose-6- phosphate -3-PO4 PO4 Transketolase & Transaldolase catalyze Glycolic pathway transfer of 2-C or 3-C molecular fragments respectively, in each case from a ketose donor to an aldose acceptor. Transketolase (transfer 2-C unit) and Transaldolase (transfer 3-C unit) 12 The Oxidative Steps of the Pentose Phosphate Pathway (1) Glucose-6-Phosphate Dehydrogenase (2) Gluconolactonase (3) 6-Phosphogluconate Dehydrogenase The Nonoxidative Steps of the Pentose Phosphate Pathway (4) Phosphopentose Isomerase (5) Phosphopentose Epimerase (6) and (8) Transketolase (7) Transaldolase 13 CH 4. HMP Nonoxidative reactions CH 4. HMP Formation of ribose 5-phosphate from intermediates of glycolysis Under conditions where the demand for pentoses for incorporation into nucleotides and nucleic acids is greater than the need for NADPH, the nonoxidative reactions can provide the biosynthesis of ribose 5-phosphate from fructose 6-phosphate in the absence of the oxidative steps. 14 Uses of NADPH Reductive biosynthesis: The electrons in NADPH are destined for use in reductive biosynthesis rather than for transfer to oxygen as in the case with NADH. NADPH that can be used as source of electrons in biosynthesis of fatty acids and steroids. Reduction of hydrogen peroxide: • Hydrogen peroxide and other reactive oxygen intermediates are highly reactive and can cause serious damages, each with double bonds in fatty acid moieties of membrane lipids, making membranes leaky. •The cell has several protective mechanisms that serve to minimize the toxic potential of these compounds. enzymes that catalyze antioxidant reactions: Reduced glutathione can chemically detoxify hydrogen peroxide. Regeneration of glutathione reductase from the oxidizes form utilizes NADPH as source of electrons. 15 CH 4. HMP Structure of NADPH The cell has several protective mechanisms that serve to minimize the toxic potentials of these compounds. I) Enzymes that catalyze antioxidant reactions: Glutathione is a tripeptide that includes cysteine. Its functional group is the cysteine thiol. Glutathione has a role in degradation of hydroperoxides that arise spontaneously in the oxygen-rich environment within red blood cells. Reduced glutathione can chemically detoxify hydrogen peroxide . this reaction catalyzed by glutathione peroxidase forms oxidized glutathione. The cell regenerate reduced glutathione in a reaction catalyzed by electrons thus NADPH indirectly provides electrons for the reduction of hydrogen peroxide. 16 CH 4. HMP Reduced glutathione can chemically detoxify hydrogen peroxide . This reaction is catalyzed by glutathione peroxidase. The cell regenerate reduced glutathione in a reaction catalyzed by Glutathione Reductase NADPH indirectly provides electrons for the reduction of hydrogen peroxide. GSH + ROOH GSSG + ROH + H2O 2 GSSG + NADPH + H+ 2 GSH + NADP+ 17 CH 4. HMP II) Antioxidant chemicals: A number of intracellular reducing agents, such as ascorbate, vitamin E and - carotene are able to reduce and thus detoxify oxygen intermediates in cells III) Cytochrome p-450 system: NADPH is critical for the liver microsomal cytochrome P-450 monooxygenase system. This is the major pathway for hydroxylation of aromatic and aliphatic compounds, such as steroids, alcohols and many drugs. These oxidations also serve to detoxify drugs and foreign compounds by converting them into soluble forms more readily excreted through the kidney 18 CH 4. HMP IV) Phagocytosis by white blood cells: Neutrophils and monocytes have oxygen-dependent and oxygen-independent mechanisms for killing bacteria. The oxygen-dependent mechanism include the myeloperoxidase (MPO) system and another system that involves the generation of oxygen –derived free radicals. Oxygen-independent systems utilize pH changes in the phagolysosomes and lysosomal enzymes to destroy pathogens. After phagocytosis has occurred, NADPH oxidase, converts molecular oxygen into superoxide. (the respiratory burst). Next superoxide is converted into hydrogen peroxide by superoxide dismutase (SOD). In the presence of MPO, peroxide plus chloride ions are converted into hypochlorous acid that kills the bacteria. 19 CH 4. HMP Glucose 6 phosphate dehydrogenase deficiency: Glucose 6 phosphate dehydrogenase(G6PD) deficiency is an inherited disease (X-linked disorder) characterized by hemolytic anemia caused by the inability to detoxify oxidizing agents. G6PD deficiency is the most common disease producing enzyme abnormality in humans. The life span of many individuals with G6PD deficiency is shortened as a result of complications arising from chronic hemolysis. It is most common in the Mediterranean, the Middle East, South East Asia and West Africa. It is rare among Caucasians Role of G6PD in red blood cells: Diminished G6PD activity impairs the ability to form NADPH that is essential in the detoxification of free radicals and peroxides formed within the cell. All cells of the affected individual have enzyme deficiency. But it is most sever in erythrocytes where the HMP provides the only means of generating NADPH. Other tissues have other NADPH sources as NADP+ - dependent malate dehydrogenase). 20 CH 4. HMP CH 4. HMP Precipitating factors in G6PD deficiency: Some factors precipitate the hemolytic anemia in G6PD deficiency patients: 1. Oxidant drugs: like antibiotics e.g; sulfamethoxazole, Antimalarials e.g; premaquine 2. Favism: The hemolytic effect of ingesting fava beans is observed in patients with favism (G6PD deficiency). 3. Infection: The inflammatory response to infection results in the generation of free radicals in macrophages, which can diffuse into the red blood cells and cause oxidative damage. 4. Neonatal jaundice: Individuals with G6PD deficiency may experience neonatal jaundice, which may result from impaired hepatic catabolism or increased production of bilirubin. 21 The end 22 CH 4. HMP.
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