Pentose_Phosphate_Pathway_M2P_202425_updated.pptx

PENTOSE PHOSPHATE PATHWAY M2P Fall 2024 Wendy S. Innis, Ph.D. [email protected] Session Objectives Compareand contrast the oxidative and non-oxidative phases of the pentose phosphate pathway. Describe the pentose phosphate pathway, including its...

PENTOSE PHOSPHATE PATHWAY M2P Fall 2024 Wendy S. Innis, Ph.D. [email protected] Session Objectives Compareand contrast the oxidative and non-oxidative phases of the pentose phosphate pathway. Describe the pentose phosphate pathway, including its reactants, products, cellular localization, and fate of its various products. Describe the role of the pentose phosphate pathway in generating NADPH and ribose for the synthesis of nucleotides in dividing cells. Describe the role of the pentose phosphate pathway, reduced glutathione, and NADPH in protecting cells from oxidative stress. Differentiate the roles of glucose 6-phosphate dehydrogenase, transketolase, and transaldolase in the pentose phosphate pathway. Explain the biochemical basis of the drug-induced hemolytic anemia observed in glucose 6-phosphate dehydrogenase deficiency Predict the biochemical consequences of enzyme (G6PD) and vitamin (B1) deficiencies on the pentose phosphate pathway. Describe how sugars and sugar derivatives are utilized to synthesize glycoproteins and glycolipids Explain how specific enzyme deficiencies can result in sphingolipidoses Overview of PPP Occurs in cytosol Consists of 2 phases Oxidativepathway generates intermediates for nucleotide synthesis Non-oxidativegenerates intermediates for glycolysis Allowsglycolysis intermediates or five-carbon sugars to be generated depending on cellular needs Where does glucose come from to enter PPP? Free glucose Glucose-1- phosphate from glycogen phosphorolys is Gluconeogen esis  Oxidative Phase Irreversible rxns Each oxidation step produces NADPH + H + NADPH donates electrons in cellular pathways Cellular NADPH/NADP+ is >1 NADPH is inhibitor of G6PDH Non-oxidative phase Reversible reactions Allows glycolysis intermediates or five-carbon sugars to be generated depending on cellular needs Consistsof several rearrangement and transfer reactions Generates glyceraldehyde-3- P and fructose-6-P Important enzymes in non-oxidative phase: transketolase and transaldolase Both E catalyze transfer of carbon moiety from a ketone done to an aldose donor Transketolase – transfer of 2-carbon molecule Transaldolase – transfer of 3-carbon molecule TK requires TPP as a coenzyme R-5-P is the end of PPP in many cells PPP activity is high in rapidly- dividing cells that need to synthesize large amts of DNA Not only R5P needed, but also NADPH for reduction of R5P to deoxy-R5P [by ribonucleotide reductase] Summary of pentose pathway roles CELLULAR NEED DIRECTION OF PATHWAY Oxidative reactions produce NADPH; nonoxidative reactions NADPH only convert ribulose 5-P to glucose 6-P to produce more NADPH. Oxidative reactions produce NADPH and ribulose 5-P; the NADPH + ribose 5-P isomerase converts ribulose 5-P to ribose 5-P. Only the nonoxidative reactions. High NADPH inhibits glucose 6-P dehydrogenase, so transketolase and Ribose 5-P only transaldolase are used to convert fructose 6-P and glyceraldehyde 3-P to ribose 5-P. Both the oxidative and nonoxidative reactions are used. The oxidative reactions generate NADPH and ribulose 5-P. The NADPH and pyruvate nonoxidative reactions convert the ribulose 5-P to fructose 6-P and glyceraldehyde 3-P, and glycolysis converts these intermediates to pyruvate.  Pentose phosphate pathway in RBCs 2) Remainder (~10%) of glucose 3) Required to keep is used to glutathione in reduced generate the sole state (protects cell from source of NADPH free radical damage) 1) Nearly all (~90%) glucose entering red blood cells is used to generate energy Are both phases needed? Relate enz deficiencies to defects in CHO metabolism RBCs susceptible to defects in glu metab pathways An enz def may result from a genetic variation that limits production of the protein, directly impacts its catalytic power, or affects its regulation Def of glycolytic enz usually have severe consequences, particularly in tissues that rely heavily of glycolysis for ATP production RBCs lack mito to produce ATP by ox phos, so use the ATP from glycolysis to power the Na,K-ATPase that maintains cells’ ion conc gradients; low glycolytic activity reduces the supply of ATP, and ion imbalance results—leading to osmotic swelling and bursting of RBCs Anemia develops (loss of RBCs) Glucose 6-phosphate dehydrogenase deficiency G6PDH deficiency causes hemolytic anemia X-linked recessive trait; African, Asian, and Mediterrean descent Severity correlates with relative residual enzyme activity ↑ oxidative stress Oxidizing drugs RBCs are destroyed faster than made exacerbate oxidative stress with demand for more NADPH ↓ RBCs ↓ GSH Shortens life span of RBCs ↓ NADPH Hemolytic anemia in G6PDH def-- Bite cells - erythrocytes with an irregular membrane which result from splenic macrophage-mediated removal of denatured hemoglobin molecules. G6PDH-deficient red cells in combination with high levels of oxidants causes a cross-linking of sulfhydryl groups on globin chains which causes a denaturing and formation of Heinz body precipitates of hemoglobin (denatured Hb) Deficiency of G6PDH Most common human enzyme deficiency Defect decreases cellular production of NADPH—which normally participates in certain redox processes and helps protect cells from oxidative damage RBCs, where the O2 conc is high, are cells most at risk G6PDH def affects people mostly in Africa, tropical South America, and southeast Asia—areas with historically high rates of malaria The enz defect causes anemia, but the release of heme from damaged RBCs triggers anti-inflammatory responses that increase an individual’s chance of surviving malaria Same effect seen in people carrying HbS variant (sickling) G6PDH continued… While variants of G6PDH deficiency appear to provide partial protection against malaria it can also cause hemolysis after exposure to certain triggers, such as the ingestion of certain foods (fava beans), infection (Hepatitis viruses A and B, cytomegalovirus, pneumonia, and typhoid fever) and exposure to oxidant drugs Drug-induced G6PD deficiency-related hemolysis has been reported following therapy with a range of antimalarial drugs, including primaquine, methylene blue, and the sulphone drug dapsone Favism – Associated with severely deficient variants of G6PDH Toxic compounds in fava beans increase PPP activity in RBCs Divicine and isouramil produce ROS which rapidly oxidize NADPH and glutathione Oxidative damage to cellular structures, particularly cell wall of RBCs Cross-linking of membrane proteins leads to distortion of RBCs Rapid clearance of damaged cells from the circulation Presence of bite cells Abnormal shape/size Acute hemolytic anemia presents around 24 hrs after ingestion Interconversion of sugars Several pathways for interconversion of sugars or the formation of sugar derivatives using activated sugars attached to nucleotides All different sugars found in glycosaminoglycans, glycoproteins and glycolipids can be synthesized from glucose Lactose synthesis in the mammary gland Activated sugars Sugars activated by attachment to nucleotides: convert to other sugars oxidize to sugar acids modify proteins, lipids, or other sugars through glycosidic bonds See Table 27.4 in Marks’  Sugar nucleotides used by transferases to glycosylate proteins and lipids Glycoproteins contain short carbohydrate chains covalently linked to either serine/threonine or asparagine residues in Glycolipids secreted from cells I-cell (inclusion cell) the protein. or segregated in lysosomes disease is a rare Activated sugars are the precursors for the condition in which addition of sugars lysosomal enzymes lack the mannose phosphate marker Protein that targets them to lysosomes. Glycolipids Cerebrosides are synthesized from ceramide and UDP-glucose or UDP- galactose Sphingolipids have ceramide backbone Gangliosides contain Types of sphingolipids include oligosaccharides cerebrosides and produced from UDP- sugars and CMP- gangliosides NANA Involved in intercellular communication Components of cell membranes, especially prevalent in nervous tissue Degraded in lysosomes Clinical relevance: Sphingolipidoses Glycolipids are degraded by lysosomal enzymes Genetic deficiencies of lysosomal enzymes required for glycolipid metabolism causes accumulation of non- degraded molecules in the cell Fall under larger group of lysosomal storage disorders Defective enzymes in gangliosidoses (LSDs) Table 27.5 DISEASE ENZYME DEFICIENCY ACCUMULATED LIPIDa Fucosidosis α-Fucosidase Cer-Glc-Gal-GalNAc-Gal:Fuc H-isoantigen Generalized gangliosidosis GM1-β-galactosidase Cer-Glc-Gal(NeuAc)-GalNAc:Gal GM 1 ganglioside Tay-Sachs disease Hexosaminidase A Cer-Glc-Gal(NeuAc):GalNAc GM2 ganglioside Tay-Sachs variant or Sandhoff Cer-Glc-Gal-Gal:GalNAc globoside plus Hexosaminidase A and B disease GM2 ganglioside Fabry disease α-Galactosidase Cer-Glc-Gal:Gal globotriaosylceramide Ceramide lactosidase (β- Ceramide lactoside lipidosis Cer-Glc:Gal ceramide lactoside galactosidase) Metachromatic leukodystrophy Arylsulfatase A Cer-Gal:OSO33− sulfogalactosylceramide Krabbe disease β-Galactosidase Cer:Gal galactosylceramide Gaucher disease β-Glucosidase Cer:Glc glucosylceramide Niemann-Pick disease Sphingomyelinase Cer:P-choline sphingomyelin Farber disease Ceramidase Acyl:sphingosine ceramide