metabolism_lecture_4 - Pentose Phosphate Pathway Main...

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Unformatted text preview: Pentose Phosphate Pathway Main goals: Production of NADPH for anabolic reactions and ribose 5-phosphate for nucleotide synthesis (DNA, RNA, cofactors) Glucose 6-P + 2 NADP + + H 2 O Ribose 5-P + 2 NADPH + 2H + + CO 2 PPP also catalyzes transitions between trioses, tetroses, and pentoses NADP + P Pentose NADPH not only important for reductive anabolic reactions (e.g. fatty acid synthesis)-> provides cytosolic reducing potential for oxidative damage protection (glutathion) O 2- H-O Oxidative damage protection by NADPH: The 2 phases of the pentose phosphate pathway: If ribose-5-P is not used for biosynthesis: non-oxidative phase recycles 6x pentose 5x hexose-> continued production of NADPH (converting Glu-6-P CO 2 ) Oxidative phase: lactone Glucose 6-phosphate dehydrogenase: Oxidation at C1, formation of a lactone (intramolecular ester) H- transfer to NADP + Lactonase: Specific hydrolysis of the lactone 6-phosphogluconate dehydrogenase: Oxidative decarboxylation, Oxidation at C3, H- transfer to NADP + Products: 1 pentose, 2 NADPH Interconversion of pentoses: ketose aldose Nucleotide biosynthesis Isomerase: general acid-base catalyzed reaction Epimerization Isomerization Flipping one stereo center C 2 and C 3 transfer reactions by transketolases and transaldolases: Transketolase transfers C 2 Glycol aldehyde Dihydroxy acetone Transaldolase transfers C 3 Donor: always Ketose Acceptor: always Aldose has to be left (Xylulose) Mechanisms and cofactors of transaldolase and transketolase: Transketolase uses cofactor Thiamin pyrophosphate (TPP): stabilizes C 2 carbanion (cp. pyruvate decarboxylase) Transaldolase forms protonated Schiff base intermediate: stabilizes C 3 carbanion, allowing an aldose to react (mechanism similar to aldolase) Thiamin (Vit. B1) deficiency -> Wernicke Korsakoff Syndrome / Beriberi Xylusose 5-P Ribose 5-P Sedoheptulose 7-P Transaldolase Transketolase Fructose 6-P Erythrose 4-P Xylulose 5-P Transketolase C 7 C 4 C 6 glycolysis Fructose 6-P Glycer- aldehyde 3-P C 5 C 6 C 3 C 3 C 5 C 5 Glycer- aldehyde 3-P PPP connection to glycolysis: Non-oxidative reactions allow oxidative reactions to continue Pathway from 6 pentoses to 5 hexoses (continuous oxidation of glucose 6-phosphate) High [NADPH] inhibits glucose-6-phosphate dehydrogenase...
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This note was uploaded on 12/03/2009 for the course MCB 58168 taught by Professor Thorner during the Fall '09 term at University of California, Berkeley.

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metabolism_lecture_4 - Pentose Phosphate Pathway Main...

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