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metabolism_lecture_4

metabolism_lecture_4 - Pentose Phosphate Pathway Maingoals...

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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
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Oxidative damage protection by NADPH:
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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 )
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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
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Interconversion of pentoses: ketose aldose Nucleotide biosynthesis Isomerase:  general acid-base catalyzed reaction Epimerization Isomerization Flipping one stereo  center
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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)
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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
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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:
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Non-oxidative reactions allow oxidative reactions  to continue Pathway from 6 pentoses to 5 hexoses (continuous oxidation of glucose 6-phosphate)
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High [NADPH] inhibits  glucose-6-phosphate dehydrogenase ->  NADPH regulates partitioning into  glycolysis vs. pentose phosphate pathway Regulation of the pentose phosphate pathway:
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