Pentose Phosphate Pathway PDF

By: Dr. Safinaz Hamdy El Khoulany Lecturer of Medical Biochemistry and Molecular Biology  Pentose phosphate pathway (PPP) As the aim of the pathway is to form Pentose sugar Hexose monophosphate (HMP) shunt As we begin with G6P and can convert it into Ribose 6-phosphogluco...

By: Dr. Safinaz Hamdy El Khoulany Lecturer of Medical Biochemistry and Molecular Biology  Pentose phosphate pathway (PPP) As the aim of the pathway is to form Pentose sugar Hexose monophosphate (HMP) shunt As we begin with G6P and can convert it into Ribose 6-phosphogluconate pathway As the end product of the first phase is 6- phosphogluconate  Definition and Aim It is an alternative pathway for glucose oxidation The aim of this pathway is not to generate ATP or energy. But, the aim is to : 1- provides a major portion of the body’s NADPH, which functions as a biochemical reductant (hydrogen carrier). (It has no role in energy production. In ETC, we use NADH+H+ and FADH2) 2- produces ribose 5-phosphate, required for the biosynthesis of nucleotides  HMP Ribose-5- phosphate NADPH+H+ Synthesis of Nucleotides in: 1- Synthesis of : DNA -Fatty acids RNA -Aminoacids Energy carrieres: ATP and GTP -Steroids Hydrogen carriers: NAD+ and FAD+ 2- For the action of many Coenzymes : CoASH enzymes: -Glutathione reductase -NADPH oxidases in phagocytic cells  Site and Function Site in the cell: Cytosol Organs or tissues: This pathway is particularly important in: liver, lactating mammary glands, and adipose: NADPH-dependent biosynthesis of fatty acids Testes, ovaries, placenta and adrenal cortex: NADPH-dependent biosynthesis of steroid hormones Erythrocytes: Require NADPH to keep glutathione reduced in order to detoxify hydrogen peroxide Mucsle: Need Ribose 5 phosphate for Synthesis of : DNA, RNA, Nucleotides, Energy carrieres: ATP and GTP and Hydrogen carriers: NAD+ and FAD+ Phagocytic cells: NADPH oxidases Steps HMP IRREVERSIBLE REVERSIBLE NON-OXIDATIVE OXIDATIVE REACTIONS REACTIONS  IRREVERSIBLE OXIDATIVE REACTIONS -Three reactions -Lead to the formation of ribulose 5-phosphate, CO2, and two molecules of NADPH for each molecule of glucose 6- phosphate oxidized -This portion of the pathway is particularly important in: Ribose-5-phosphate 1- the liver, lactating mammary glands, and adipose Tissue 2-erythrocytes 3-testes, ovaries, placenta and adrenal cortex 4- Phagocytic cells  REVERSIBLE NON-OXIDATIVE REACTIONS -Occur in all cell types synthesizing nucleotides and nucleic acids. -These reactions catalyze the interconversion of sugars containing three to seven carbons. These reversible reactions permit ribulose 5-phosphate (produced by the oxidative portion of the pathway) Ribose-5-phosphate to be converted either to : -ribose 5-phosphate (needed for nucleotide synthesis or - intermediates of glycolysis—fructose 6-phosphate and glyceraldehyde 3- phosphate.  Cells that carry out reductive biosynthetic reactions have a greater need for NADPH than for ribose 5-phosphate The ribulose 5-phosphate produced as an endproduct of the oxidative reactions is converted by NON-OXIDATIVE REACTIONS to glyceraldehyde 3-phosphate Ribose-5-phosphate and fructose 6-phosphate, which are intermediates of glycolysis.  In contrast, under conditions in which the demand for ribose 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 glyceraldehyde 3-phosphate and fructose 6-phosphate in the absence of the oxidative Ribose-5-phosphate steps A) IRREVERSIBLE OXIDATIVE REACTIONS  A. Dehydrogenation of glucose 6-phosphate Glucose 6-phosphate dehydrogenase (G6PD) A catalyzes an irreversible oxidation of glucose 6-phosphate (6C) to 6- phosphogluconolactone (6C) producing NADPH+H+ The pentose phosphate pathway is regulated primarily at the G6PD reaction.  NADPH is a potent competitive inhibitor of the enzyme,  B. Formation of 6-phosphogluconate 6-Phosphogluconolactone is hydrolyzed by 6-phosphogluconolactone hydrolase B (6-phosphogluconolactone producing 6-phosphogluconate hydrolase)  C. Formation of ribulose 5-phosphate  Oxidative decarboxylation of 6-phosphogluconate is catalyzed by 6-phosphogluconate dehydrogenase.  This irreversible reaction produces: (6-phosphogluconolactone hydrolase) - a pentose sugar–phosphate (ribulose 5-phosphate) -CO2 - a second molecule of C NADPH REVERSIBLE NON-OXIDATIVE REACTIONS  Transketolase Transaldolase -Transfers two-carbon units transfers three-carbon units -Needs thiamine pyrophosphate (TPP) [vit. B1] as a coenzyme  TPP is needed in the following enzyme complexes: - Pyruvate dehydrogenase complex - α-ketoglutarate dehydrogenase of the TCA cycle -Transketolases of HMP shunt -Branched-chain amino acid metabolism Niacin Flavin -NAD -FAD -NADP -FMN Thus, B vitamins especially B1 have an important role in metabolism especially, carbohydrate metabolism  Ribulose and Ribose are Isomers as they have identical molecular formulas that is, same number of atoms of each molecule but one is aldehyde and the other is Ketone 5C 1 5C 5C Ribulose and Xylulose are Epimers as they vary in one position (C3) for the placement of the -OH group.  5C 7C 5C 3C 1 Ribulose -5- P is converted into Ribose-5-P by Ribose 5-phosphate isomerase Ribulose -5- P is converted into Xylulose 5-P by phosphopentose epimerase  Transketolase Transaldolase -Transfers two-carbon units transfers three-carbon units -Needs Thiamine pyrophosphate (TPP) [vit. B1] as a coenzyme  5C 7C 5C 3C Interconversion of sugars containing three to seven carbons  5C 7C 5C 3C 2 Transketolase transfer 2 carbon unit from Xylulose 5 P (Ketone) to Ribose 5 P (Aldehyde) Decrease 2 C Increase 2 C Thus, formation of : Glyceraldehyde 3 P (Aldehyde) and Sedoheptulose 7 P (Ketone)  5C 7C 5C 3C 3 Transaldolase transfer 3carbon unit from Sedoheptulose 7 P(Ketone) to Glyceraldehyde 3 P (Aldehyde) Decrease 3C Increase 3C Thus, formation of : Erythrose 4P (Aldehyde) and Fructose 6 P (Ketone)  5C 7C 5C 3C 4 Transketolase transfer 2 carbon unit from Xylulose 5 P (Ketone) to Erythrose 4P (Aldehyde) Decrease 2C Increase 2C Thus, formation of Glyceraldehyde 3 P (Aldehyde) and Fructose 6 P (Ketone) Reactants of Products of oxidative Removal oxidative Reactants of Products of 3 molecules of 3CO2 3 molecules non- non- of G6P (3C) of oxidative oxidative 3x6=18C Ribulose 5 P 15C 15C 3x5=15C (5+5+5) (6+6+3) (18-3) USES OF NADPH A. Reductive biosynthesis: It can be used in reductive biosynthesis i.e. reactions requiring an electron donor It is important in synthesis of : -Fatty acids -Aminoacids -Steroids B. Reduction of hydrogen peroxide Reactive oxygen species (ROS) They are formed from the partial reduction of molecular oxygen These compounds are formed continuously : -as by-products of aerobic metabolism -through reactions with drugs and environmental toxins - when the level of antioxidants is diminished All creating the condition of oxidative stress. Dangers: 1- They can cause serious chemical damage to DNA, proteins, and unsaturated lipids, and can lead to cell death. 2- They have been implicated in a number of pathologic processes, including reperfusion injury, cancer, inflammatory disease,and aging.  NADPH provides the reducing equivalents required by the protective mechanisms that minimize the toxic potential of ROS 1- Reduced glutathione, a tripeptide-thiol (γ-glutamylcysteinylglycine) present in most cells, can chemically detoxify hydrogen peroxide 2- The cell regenerates reduced glutathione in a reaction catalyzed by glutathione reductase, using NADPH as a source of reducing equivalents. Thus, NADPH indirectly provides electrons for the reduction of hydrogen peroxide D. Phagocytosis by white blood cells (Respiratory burst) 1- Attachment of the microorganism to certain receptors on phagocytic cells 2- Internalization of it into the phagocytic cells and activation of NADPH oxidase 3- Reduction of the oxygen from the surrounding tissues by NADPH oxidase to superoxide (O2– ), a free radical. By itself, it can kill bacteria. 4- Superoxide is converted to hydrogen peroxide (a ROS), either spontaneously or catalyzed by superoxide dismutase (SOD). D. Phagocytosis by white blood cells (Respiratory burst) 5- Fate of peroxide: - In the presence of MPO, peroxide plus chloride ions are converted to hypochlorous acid (HOCl), which kills the bacteria. -The peroxide can also be partially reduced to the hydroxyl radical (OH ), a ROS -It can be fully reduced to water by catalase or glutathione peroxidase. GLUCOSE 6-P DEHYDROGENASE DEFICIENCY  It is an inherited (X-linked) disease characterized by hemolytic anemia caused by the inability to detoxify oxidizing agents.  C/P: In addition to hemolytic anemia, a clinical manifestation of G6PD deficiency is neonatal jaundice appearing 1–4 days after birth.  Diminished G6PD activity Impairment of the ability of the cell to form the NADPH Impairment of regeneration of reduced glutathione pool A decrease in the cellular detoxification of free radicals and peroxides formed within the cell Increased membrane fragility of cells especially Red Blood Cells Rupture and hemolysis of RBCs  Precipitating factors in G6PD deficiency Patients with G6PD deficiency develop hemolytic anemia if exposed to Precipitating factors which are: 1. Oxidant drugs: Antibiotics: (for example, sulfamethoxazole and chloramphenicol) Antimalarials (for example, primaquine but not quinine) Antipyretics (for example, Asprin but not acetaminophen)  Precipitating factors in G6PD deficiency Patients with G6PD deficiency develop hemolytic anemia if exposed to Precipitating factors which are: 2. Favism: Some forms of G6PD deficiency, for example the Mediterranean variant, are particularly susceptible to the hemolytic effect of the fava bean, a common diet in the Mediterranean region.  Precipitating factors in G6PD deficiency Patients with G6PD deficiency develop hemolytic anemia if exposed to Precipitating factors which are: 3. Infection: Infection is the most common precipitating factor of hemolysis in G6PD deficiency. 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.  Notes Although G6PD deficiency occurs in all cells of the affected individual, it is most severe in erythrocytes as: 1- The pentose phosphate pathway provides the only means of generating NADPH. Other tissues have alternative sources for NADPH production that can keep glutathione reduced. 2-The erythrocyte has no nucleus or ribosomes and cannot renew its supply of the enzyme. 3- It carries oxygen in Hemoglobin and thus, more susceptible for generation of ROS In muscle: Ribose 5 p is needed and the 2 dehydrogenases of phase I are deficient or not present In this case, Ribose 5 P is provided by reversal of phase 2 of HMP from intermediates of glycolysis Ribose-5-phosphate