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Pentose Phosphate Pathway (PPP)

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PPP

  • also referred to as the hexose monophosphate shunt

  • another fate for G6P

  • occurs in cytoplasm

    • not in all tissues​

      • mostly adipose and other fat synthesizing tissues (liver, mammary glands, adrenal cortex, testes)​

      • also in RBCs

      • not much activity in muscle

  • oxidative vs non-oxidative

Oxidative

  • alternative to glycolysis

  • produces reduced coenzyme via 2 dehydration steps

    • 2 NADPH + 2H from NAPD​

  • also produces ribose-5-phosphate (5C sugar)

    • used to make ATP and other nucleotides (GTP, UTP)​

    • used to make NAD and FAD

    • used to make nucleic acids (DNA, RNA)

  • also makes CO2

  • There are two steps

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  • NADPH + H = reduced coenzyme

  • NADP = oxidized coenzyme

    • these are like NAD and NADH + H but with a phosphate group attached​

  • need for NADPH

    • reductive synthesis (adds an H)​

      • FA synthesis​

      • cholesterol synthesis

      • steroids

      • nucleotides

    • control of oxidative stress

  • FA synthesis

    • NADPH + H donates H for FA (and other) synthesis, which is going on simultaneously in the fed state from excess acetyl CoA thats not needed in the TCA cycle​

    • control of oxidative stress

      • is a coenzyme for glutathione reductase, which is part of the bodys natural antioxidant defenses, especially in RBC​

Non-oxidative

  • if don't need ribose

  • ribose-5-phosphate can undergo further transformation to produce

    • C3 sugars​

    • C4 sugars

    • C5 sugars

    • C6 sugars

    • C7 sugars

  • enzymes are transketolase and aldolase (convert ketones to aldehydes and vice versa)

  • transketolase requires TPP (thiamin pyrophosphate) as a coenzyme

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  • Summary of the flow of 15 Carbons (3 riboses) through PPP non-oxidative phase reactions by which 5C sugars are converted to 3C and 6C sugars

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PPP continued

  • the PPP is regulated primarily by the level of NADP

  • the first reaction in the pathway (dehydrogenation of G6P) is rate limiting

    • if not very active, whole pathway is not very active​

    • under control of insulin

    • can have an overshoot response​​

    • the non-oxidative branch is regulated by available substrates

  • the 2 phases of the PPP can operate depending on conditions in the cell

    • the products of the oxidative branch can be entirely converted into glycolysis intermediates​

    • this flexibility allows the cell to use this pathway in different ways to meet different cellular needs

©2023 by Syracuse University Dr.Margaret Voss

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