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Pathway

Pentose phosphate pathway

Homo sapiens · Cytosol · 8 reactions

The pentose phosphate pathway branches from glucose 6-phosphate. Its oxidative branch makes NADPH and ribulose 5-phosphate; its reversible non-oxidative reactions interconvert pentoses with glycolytic intermediates to match biosynthetic demand.

REACTOME IDENTIFIERR-HSA-71336View source record ↗

Pathway scope

NETWORK MAP

The oxidative and non-oxidative branches can carry different net fluxes, so this network map does not assign one overall yield.

Overview

EXPLORE BY LEVEL

Pentose phosphate is a set of connected chemical steps in human cells. The pentose phosphate branches from 6-phosphate. Its oxidative branch makes and ribulose 5-phosphate; its reversible non-oxidative reactions interconvert pentoses with glycolytic intermediates to match biosynthetic demand.

WHY THE FLOW CHANGES

Regulation

Compare fed and fasting

The oxidative branch responds to the cell’s need for NADPH.

Can speed upNADP+ availability

NADP+ availability supports glucose-6-phosphate dehydrogenase activity and NADPH production.

Can slow downNADPH

NADPH product feedback restrains the first oxidative step.

Context mattersBiosynthetic demand

Cells can route more glucose 6-phosphate through this pathway when reducing power or ribose is needed.

How to read this map

This is an instructional map of selected reactions, not a complete inventory of every reaction in the body. Some steps are grouped; transport, alternate routes, tissue differences, or full molecule balances may be summarized. The Research notes explain this map’s specific limits.

Explore the reaction map and enzyme steps

Pathway map

GENERATED FROM STRUCTURED REACTIONS

Select a molecule or enzyme to inspect it. Select a step number to open its full reaction detail.

  1. Oxidative branch

    Three one-way reactions convert glucose 6-phosphate to ribulose 5-phosphate while producing two NADPH and releasing carbon dioxide.

    1. Step 1Irreversible · NADP+ → NADPH
    2. Step 2Irreversible · H2O → —
    3. Step 3Irreversible · NADP+ → CO2 + NADPH

  2. Pentose interconversion

    Isomerase and epimerase reactions make ribose 5-phosphate and xylulose 5-phosphate.

      RIBOSE BRANCH
      1. Step 4Reversible

      Ribulose 5-phosphate isomerase supplies ribose 5-phosphate for nucleotide synthesis.

      XYLULOSE BRANCH
      1. Step 5Reversible

      Ribulose 5-phosphate epimerase supplies xylulose 5-phosphate for carbon rearrangement.

    1. Sugar rearrangement

      Transketolase and transaldolase reactions reconnect pentose carbon to fructose 6-phosphate and glyceraldehyde 3-phosphate.

      1. Step 6Reversible
      2. Step 7Reversible
      3. Step 8Reversible

    Reaction steps

    1Oxidation of glucose 6-phosphateGlucose 6-phosphate → 6-Phosphoglucono-1,5-lactoneIrreversible

    G6PD oxidizes glucose 6-phosphate and reduces NADP+ to NADPH.

    REACTION · PER TURN + yields +
    Enzyme
    EC 1.1.1.49 ↗
    Cofactors
    No metal cofactor listed.
    Reversibility
    Irreversible under cellular conditions. This is a control point.
    Runs per glucose 6-phosphate
    Once
    Stage
    Oxidative branch
    MECHANISM & CONTEXT

    This first oxidative-branch reaction is a major control point. NADPH feedback and substrate availability help match the pathway to cellular redox and biosynthetic needs.

    Evidence for this step: Pentose phosphate pathway (R-HSA-71336)

    2Hydrolysis of the lactone6-Phosphoglucono-1,5-lactone → 6-PhosphogluconateIrreversible

    6-Phosphogluconolactonase opens the lactone ring to form 6-phosphogluconate.

    REACTION · PER TURN + yields
    Enzyme
    EC 3.1.1.31 ↗
    Cofactors
    No metal cofactor listed.
    Reversibility
    Irreversible under cellular conditions. This is a control point.
    Runs per glucose 6-phosphate
    Once
    Stage
    Oxidative branch
    MECHANISM & CONTEXT

    This hydrolysis prepares the intermediate for the second NADPH-generating oxidation.

    Evidence for this step: Pentose phosphate pathway (R-HSA-71336)

    3Oxidative decarboxylation6-Phosphogluconate → Ribulose 5-phosphateIrreversible

    6-Phosphogluconate dehydrogenase releases CO2 and makes a second NADPH.

    REACTION · PER TURN + yields + +
    Enzyme
    EC 1.1.1.44 ↗
    Cofactors
    No metal cofactor listed.
    Reversibility
    Irreversible under cellular conditions. This is a control point.
    Runs per glucose 6-phosphate
    Once
    Stage
    Oxidative branch
    MECHANISM & CONTEXT

    One six-carbon glucose 6-phosphate yields a five-carbon ribulose 5-phosphate, carbon dioxide, and NADPH in this oxidative segment.

    Evidence for this step: Pentose phosphate pathway (R-HSA-71336)

    4Ribose 5-phosphate formationRibulose 5-phosphate ⇌ Ribose 5-phosphateReversible

    Ribose-5-phosphate isomerase makes ribose 5-phosphate for nucleotide synthesis.

    REACTION · PER TURNreversibly yields
    Enzyme
    EC 5.3.1.6 ↗
    Cofactors
    No metal cofactor listed.
    Reversibility
    Reversible, operating near equilibrium in cells.
    Runs per glucose 6-phosphate
    Once
    Stage
    Pentose interconversion
    MECHANISM & CONTEXT

    This reversible reaction is favored as demand for ribose 5-phosphate rises.

    Evidence for this step: Pentose phosphate pathway (R-HSA-71336)

    5Xylulose 5-phosphate formationRibulose 5-phosphate ⇌ Xylulose 5-phosphateReversible

    Ribulose-phosphate epimerase makes xylulose 5-phosphate.

    REACTION · PER TURNreversibly yields
    Enzyme
    EC 5.1.3.1 ↗
    Cofactors
    No metal cofactor listed.
    Reversibility
    Reversible, operating near equilibrium in cells.
    Runs per glucose 6-phosphate
    Once
    Stage
    Pentose interconversion
    MECHANISM & CONTEXT

    Xylulose 5-phosphate participates in the carbon-transfer reactions that return pentose carbon to glycolysis.

    Evidence for this step: Pentose phosphate pathway (R-HSA-71336)

    6First two-carbon transferXylulose 5-phosphate + Ribose 5-phosphate ⇌ Glyceraldehyde 3-phosphate + Sedoheptulose 7-phosphateReversible

    Transketolase transfers a two-carbon unit to make glyceraldehyde 3-phosphate and sedoheptulose 7-phosphate.

    REACTION · PER TURN + reversibly yields +
    Enzyme
    EC 2.2.1.1 ↗
    Cofactors
    No metal cofactor listed.
    Reversibility
    Reversible, operating near equilibrium in cells.
    Runs per glucose 6-phosphate
    Once
    Stage
    Sugar rearrangement
    MECHANISM & CONTEXT

    The reaction is thiamine-pyrophosphate dependent and reversible; direction depends on the available sugar-phosphate pool.

    Evidence for this step: Pentose phosphate pathway (R-HSA-71336)

    7Three-carbon transferSedoheptulose 7-phosphate + Glyceraldehyde 3-phosphate ⇌ Erythrose 4-phosphate + Fructose 6-phosphateReversible

    Transaldolase transfers a three-carbon unit to form erythrose 4-phosphate and fructose 6-phosphate.

    REACTION · PER TURN + reversibly yields +
    Enzyme
    EC 2.2.1.2 ↗
    Cofactors
    No metal cofactor listed.
    Reversibility
    Reversible, operating near equilibrium in cells.
    Runs per glucose 6-phosphate
    Once
    Stage
    Sugar rearrangement
    MECHANISM & CONTEXT

    The products connect pentose metabolism back to glycolysis or into other biosynthetic routes.

    Evidence for this step: Pentose phosphate pathway (R-HSA-71336)

    8Second two-carbon transferXylulose 5-phosphate + Erythrose 4-phosphate ⇌ Glyceraldehyde 3-phosphate + Fructose 6-phosphateReversible

    A second transketolase reaction yields another fructose 6-phosphate and glyceraldehyde 3-phosphate.

    REACTION · PER TURN + reversibly yields +
    Enzyme
    EC 2.2.1.1 ↗
    Cofactors
    No metal cofactor listed.
    Reversibility
    Reversible, operating near equilibrium in cells.
    Runs per glucose 6-phosphate
    Once
    Stage
    Sugar rearrangement
    MECHANISM & CONTEXT

    Together, the non-oxidative reactions can recycle pentose carbon, supply ribose without NADPH production, or return carbon to glycolysis.

    Evidence for this step: Pentose phosphate pathway (R-HSA-71336)

    Research sources, claims, and curation

    Pentose phosphate pathway (R-HSA-71336)

    Reactome · Homo sapiens · Reviewed Reactome human pathway used for reaction sequence and context.

    Claims

    The pentose phosphate pathway branches from glucose 6-phosphate. Its oxidative branch makes NADPH and ribulose 5-phosphate; its reversible non-oxidative reactions interconvert pentoses with glycolytic intermediates to match biosynthetic demand.

    Curation notes

    • The non-oxidative reactions form a reversible network; their net direction changes with NADPH and ribose demand.
    • The eight canonical reactions and the two major pathway functions follow the reviewed Reactome human pathway.
    • Nucleotide synthesis consumes ribose 5-phosphate after its activation to PRPP; this downstream step is shown on the purine and pyrimidine pages.