3/1/151Biochemistry401Lecture 25 The Pentose Phosphate Pathway(PPP)Overview of the pathwayNADH, NADPHThe PPP in depthReactions Function Oxidative NADPH production Non-oxidative Inter-conversion of sugars Products of the PPP • NADPH for reductions • Ribose for nucleotide synthesis • Glycolytic intermediates Pentose Phosphate Pathway Cytosolic pathway, many purposes Two Phases: Oxidative, Non-oxidative NADP+ H NAD+ Cellular NADH/NAD+ ratios favor NAD+ NAD+ oxidizes substrates in catabolic pathways NADH powers ATP synthesis Cellular NADPH/NADP+ ratios favor NADPH NADPH reduces substrates in reductive synthetic reactions in anabolic pathways. Reactivates glutathione.3/1/152• NAD+ oxidizes substrates to produce reducing equivalents: NADH - Citric acid cycle - Fatty Acid Oxidation • NADH reduces electron carriers to power ATP synthesis - Electron Transport Chain Mitochondrial Matrix Cytoplasm • NADPH reduces substrates in reductive synthetic reactions: - Fatty Acid Synthesis - Cholesterol Synthesis - Nucleotide Synthesis - Neurotransmitter Synthesis NADPH serves many purposes ➢ Reducing equivalents for biosynthesis ➢ Combats infection. – In neutrophils, used in synthesis of reactive oxygen species (superoxides, peroxide) from O2 to kill microbes – Synthesis of nitric oxide (NO) • Short range hormone • Vasodilator • Used in synthesis of reactive nitrogen species to kill bacteria ➢ Reduces and thereby reactivates glutathione, which is a potent antioxidant Glutathione in its reduced state is an important tripeptide that protects against oxidative stress: Oxidized Glutathione is re-reduced ultimately by NADPH, thus sufficient levels of NADPH are important to keep our cells safe from reactive oxygen species.3/1/153Red blood cells are especially sensitive to oxidative stress because they rely on pentose phosphate pathway for most of their reducing power. Glutathione is important as a sulfhydryl buffer to keep important cysteine residues in Hb and other red cell molecules reduced. Glutathione is also important for reducing peroxides and other ROS species that can damage membranes and other cellular structures Heinz bodies - clumps of denatured hemoglobin adhere to membranes Oxidative Pentose Phosphate Pathway3/1/1541. Oxidation 2. Hydration 3. Oxidation Reaction 1 is committed step, G6PD is regulated 1 2 G6PD Oxidative Pentose Phosphate Pathway 3 Regulated by NADPH/NADP+ High NADPH/NADP+ inhibits G6PD Low NADPH/NADP+ activates G6PD G6PD Hydration Unstable3/1/155Second oxidation Decarboxylation To make a 5C Sugar Pentose Ketose Epimerization Isomerization Ketose Aldose Ketose You are here C3 Epimer Ketose Aldose Ketose Two reactions at this point Nucleic acid synthesis3/1/156Three main reactions generate glycolytic intermediates from excess ribose 5-P either from the oxidative reactions of the PPP or from dietary nucleic acid: Reaction 1 : Transketolase C5+C5 <-> C3+C7 Reaction 2 : Transaldolase C3+C7 <-> C6+C4 Reaction 3 : Transketolase C4+C5 <-> C6+C3 C6 & C3 that we generate in the end are : Fructose 6-P & Glyceraldehyde 3-P, respectively. 3 Ribose 5-phosphate 2 F6P + 1 GAP Reaction 1 : Transketolase C5+C5 <-> C3+C7 Substrate only if C3 OH has configuration of Xylose (reason for the epimerization of Ribulose 5-P3/1/157Reaction 2 : Transaldolase C3+C7 <-> C6+C4 3 Ribose 5-phosphate 2 F6P + 1 GAP Here we have two products that can enter glycolytic pathway Reaction 3 : Transketolase C4+C5 <-> C6+C3 Ketose Aldose3/1/158Much more Ribose 5-P much less NADPH Rapidly dividing cells need ribose 5-P for nucleotide synthesis Products from glycolysis used to make ribose 5-P3/1/159Drive Non-OPPP backwards NADPH and Ribose 5-P In roughly equal amounts Oxidative PPP used Much more NADPH than Ribose 5-P Use both OPPP and Non-OPPP, then send products through Gluconeogenesis pathway to make more G6P for PPP OPPP Non-OPPP Gluconeogenesis3/1/1510Both NADPH and ATP needed Glycolysis Both OPPP and Non-OPPP are used. Products are sent to Glycolytic pathway TCA Cycle ATP Regulation of PPP: NADP+/NADPH ratio The reaction catalyzed by Glucose 6-P Dehydrogenase is essentially irreversible, and is rate-limiting. This is the point of regulation in the PPP pathway. Glucose 6-P Dehydrogenase is regulated by levels of NADP+/NADPH in the cell. Low levels of NADP+ are slow rxn 1: 1) NADP+ is necessary for redox, 2) NADPH is a competitive inhibitor. The reactions of the non-oxidative phase are regulated at the level of substrate availability.3/1/1511Deficiency of glucose 6-P Dehydrogenase Most common enzyme deficiency. X-linked Usually subclinical Increased susceptibility to oxidative injury. Hemolytic anemia, especially in response to oxidative challenge In RBCs, PPP = only source of NADPH. Cannot keep important molecules in a reduced state. Oxidative challenge: Oxidative Drugs -Some: antibiotics, anitmalarials, fever reducers Favism - Hemolytic anemia precipitated by fava bean ingestion Infection - Peroxides are made in neutrophils mobilized in response to infection -:-: -:-: -:-: -:-: -:-: -:-: -:-: -:-: -:-: -:-: -:-: -:-: -:-: -:-: -:-: Prolonged Neonatal Jaundice may result from G6PDH deficiency usually seen in first four days after birth Favism Fava beans (broad beans) contain active oxidants (vicine, convicine) Main staple in Mediterranean diet Can induce symptoms in persons with a certain type of mutation in G6PDH called Favism. Genetic Mutations Resulting in G6PDH Deficiency High prevalence in areas where malaria is endemic. May afford some protection from death from malarial infection This marks the end of Lecture 25.Have a great
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