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Pathway

Glycolysis

Homo sapiens · Cytosol · 10 reactions

Glycolysis breaks one glucose molecule into two pyruvate molecules. The cell spends 2 ATP early on, splits the sugar in half, then harvests 4 ATP and 2 NADH from the two halves. The result is a net gain of 2 ATP per glucose, without needing oxygen.

REACTOME IDENTIFIERR-HSA-70171View source record ↗

Net yield per glucose

COMPUTED FROM REACTION STOICHIOMETRY
2Pyruvate2 made
2ATP4 made − 2 spent
2NADH2 made
OVERALL REACTION

Glucose + 2 ADP + 2 NAD+ + 2 Pi → 2 ATP + 2 NADH + 2 H+ + 2 H2O + 2 Pyruvate

Overview

EXPLORE BY LEVEL

Glycolysis is how a cell starts getting energy from sugar. It splits one molecule into two smaller molecules called pyruvate and keeps a little energy along the way as ATP and . It does not need oxygen.

WHY THE FLOW CHANGES

Regulation

Compare fed and fasting

Cells tune glycolysis to energy demand and tissue state.

Can speed upAMP and ADP

Low-energy signals activate phosphofructokinase-1 (PFK-1) in many tissues.

Can slow downATP and citrate

High-energy and abundant-carbon signals restrain PFK-1.

Can speed upFructose 2,6-bisphosphate

In liver, this strong PFK-1 activator rises with insulin signaling and falls with glucagon signaling.

How to read this map

This map shows the ten main reactions of glycolysis. Other routes that use its molecules are linked from the steps and records.

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. Energy investment

    Two ATP are spent to phosphorylate the sugar and commit it to the pathway.

    1. Step 1Irreversible · ATP → ADP
    2. Step 2Reversible
    3. Step 3Irreversible · ATP → ADP

  2. Cleavage

    The six-carbon sugar splits into two three-carbon branches that converge on glyceraldehyde 3-phosphate.

    1. Step 4Reversible
    DIRECT BRANCH

    Passes through unchanged

    Aldolase releases glyceraldehyde 3-phosphate, which goes straight to the payoff phase.

    ISOMERIZED BRANCH
    1. Step 5Reversible

    Aldolase releases DHAP, which triose-phosphate isomerase converts into glyceraldehyde 3-phosphate.

    The branches converge: continue to the next stage.

  3. Energy payoff

    ×2 per glucose

    Each of the two G3P molecules runs through steps 6–10, so every reaction happens twice per glucose.

    1. Step 6Reversible · NAD+ + Pi → NADH + H+
    2. Step 7Reversible · ADP → ATP
    3. Step 8Reversible
    4. Step 9Reversible · — → H2O
    5. Step 10Irreversible · ADP → ATP

Reaction steps

1Phosphorylation of glucoseGlucose → Glucose 6-phosphateIrreversible

One ATP is spent to add a phosphate to glucose. The charged phosphate traps glucose inside the cell.

REACTION · PER TURN + yields +
Enzyme
EC 2.7.1.1 ↗
Cofactors
Reversibility
Irreversible under cellular conditions. This is a control point.
Runs per glucose
Once
Stage
Energy investment
MECHANISM & CONTEXT

Hexokinase transfers the γ-phosphate of Mg-ATP to carbon 6 of glucose. Under cellular conditions the reaction is effectively irreversible. Hexokinases 1–3 are inhibited by their product, glucose 6-phosphate.

Evidence for this step: Glycolysis (R-HSA-70171) · Molecular Biology of the Cell: glycolysis (NBK26882)

2Isomerization to fructose 6-phosphateGlucose 6-phosphate ⇌ Fructose 6-phosphateReversible

The ring is rearranged, turning glucose 6-phosphate into fructose 6-phosphate.

REACTION · PER TURNreversibly yields
Enzyme
EC 5.3.1.9 ↗
Cofactors
No metal cofactor listed.
Reversibility
Reversible, operating near equilibrium in cells.
Runs per glucose
Once
Stage
Energy investment
MECHANISM & CONTEXT

An aldose-to-ketose isomerization that runs near equilibrium. It creates a free hydroxyl on carbon 1 for the next phosphorylation.

Evidence for this step: Glycolysis (R-HSA-70171) · Molecular Biology of the Cell: glycolysis (NBK26882)

3Phosphorylation of fructose 6-phosphateFructose 6-phosphate → Fructose 1,6-bisphosphateIrreversible

A second ATP adds a phosphate to carbon 1. This commits the sugar to glycolysis and is the pathway's main control point.

REACTION · PER TURN + yields +
Enzyme
EC 2.7.1.11 ↗
Cofactors
Reversibility
Irreversible under cellular conditions. This is a control point.
Runs per glucose
Once
Stage
Energy investment
MECHANISM & CONTEXT

PFK-1 catalyzes the first committed, irreversible step. High ATP and citrate slow it. AMP and fructose 2,6-bisphosphate speed it up, which ties glycolytic flux to the cell's energy state and to hormonal signals.

Evidence for this step: Glycolysis (R-HSA-70171) · Molecular Biology of the Cell: glycolysis (NBK26882)

4Cleavage into two triose phosphatesFructose 1,6-bisphosphate ⇌ Dihydroxyacetone phosphate + Glyceraldehyde 3-phosphateReversible

The six-carbon sugar splits into two three-carbon sugars: DHAP and glyceraldehyde 3-phosphate.

REACTION · PER TURNreversibly yields +
Enzyme
EC 4.1.2.13 ↗
Cofactors
No metal cofactor listed.
Reversibility
Reversible, operating near equilibrium in cells.
Runs per glucose
Once
Stage
Cleavage
MECHANISM & CONTEXT

A reverse aldol cleavage between carbons 3 and 4. Its standard free-energy change is unfavorable, but in cells it proceeds because the products are removed quickly.

Evidence for this step: Glycolysis (R-HSA-70171) · Molecular Biology of the Cell: glycolysis (NBK26882)

5Interconversion of triose phosphatesDihydroxyacetone phosphate ⇌ Glyceraldehyde 3-phosphateReversible

DHAP is converted into a second glyceraldehyde 3-phosphate, so both halves of glucose continue down the same path.

REACTION · PER TURNreversibly yields
Enzyme
EC 5.3.1.1 ↗
Cofactors
No metal cofactor listed.
Reversibility
Reversible, operating near equilibrium in cells.
Runs per glucose
Once
Stage
Cleavage
MECHANISM & CONTEXT

The equilibrium favors DHAP. Continuous use of G3P in step 6 pulls the reaction toward G3P, so each glucose delivers two G3P to the payoff phase.

Evidence for this step: Glycolysis (R-HSA-70171) · Molecular Biology of the Cell: glycolysis (NBK26882)

6Oxidation of glyceraldehyde 3-phosphateGlyceraldehyde 3-phosphate ⇌ 1,3-BisphosphoglycerateReversible×2

G3P is oxidized. NAD+ picks up electrons to become NADH, and a phosphate is added to form a high-energy acyl-phosphate.

REACTION · PER TURN + + reversibly yields + +
Enzyme
EC 1.2.1.12 ↗
Cofactors
NAD+ is consumed as a co-substrate (see reaction).
Reversibility
Reversible, operating near equilibrium in cells.
Runs per glucose
2 times, once for each three-carbon half
Stage
Energy payoff
MECHANISM & CONTEXT

A catalytic cysteine attacks the aldehyde, and hydride transfer to NAD+ forms a thioester. Phosphorolysis by inorganic phosphate then releases 1,3-bisphosphoglycerate. NAD+ acts here as a co-substrate and must be regenerated for glycolysis to continue.

Evidence for this step: Glycolysis (R-HSA-70171) · Molecular Biology of the Cell: glycolysis (NBK26882)

7First ATP-generating step1,3-Bisphosphoglycerate ⇌ 3-PhosphoglycerateReversible×2

1,3-Bisphosphoglycerate gives a phosphate to ADP and makes ATP. Across both three-carbon halves, this repays the two ATP invested.

REACTION · PER TURN + reversibly yields +
Enzyme
EC 2.7.2.3 ↗
Cofactors
Reversibility
Reversible, operating near equilibrium in cells.
Runs per glucose
2 times, once for each three-carbon half
Stage
Energy payoff
MECHANISM & CONTEXT

Substrate-level phosphorylation. Although PGK is named for the reverse reaction and is reversible, coupling to step 6 drives the pair of reactions forward.

Evidence for this step: Glycolysis (R-HSA-70171) · Molecular Biology of the Cell: glycolysis (NBK26882)

8Phosphate shift3-Phosphoglycerate ⇌ 2-PhosphoglycerateReversible×2

The phosphate group moves from carbon 3 to carbon 2.

REACTION · PER TURNreversibly yields
Enzyme
EC 5.4.2.11 ↗
Cofactors
No metal cofactor listed.
Reversibility
Reversible, operating near equilibrium in cells.
Runs per glucose
2 times, once for each three-carbon half
Stage
Energy payoff
MECHANISM & CONTEXT

Human phosphoglycerate mutases are 2,3-bisphosphoglycerate-dependent. They transfer phosphate through a phosphohistidine intermediate rather than moving a single phosphate intramolecularly.

Evidence for this step: Glycolysis (R-HSA-70171) · Molecular Biology of the Cell: glycolysis (NBK26882)

9Dehydration to phosphoenolpyruvate2-Phosphoglycerate ⇌ PhosphoenolpyruvateReversible×2

Removing a water molecule forms phosphoenolpyruvate, whose phosphate has very high transfer potential.

REACTION · PER TURNreversibly yields +
Enzyme
EC 4.2.1.11 ↗
Cofactors
Reversibility
Reversible, operating near equilibrium in cells.
Runs per glucose
2 times, once for each three-carbon half
Stage
Energy payoff
MECHANISM & CONTEXT

Mg2+-dependent dehydration. It barely changes the total free energy but redistributes it, so hydrolysis of the enol phosphate in PEP releases much more energy than hydrolysis of 2-phosphoglycerate.

Evidence for this step: Glycolysis (R-HSA-70171) · Molecular Biology of the Cell: glycolysis (NBK26882)

10Second ATP-generating stepPhosphoenolpyruvate → PyruvateIrreversible×2

PEP gives its phosphate to ADP, forming ATP and pyruvate. This second ATP-making step is where glycolysis turns a net profit.

REACTION · PER TURN + yields +
Enzyme
EC 2.7.1.40 ↗
Cofactors
Reversibility
Irreversible under cellular conditions. This is a control point.
Runs per glucose
2 times, once for each three-carbon half
Stage
Energy payoff
MECHANISM & CONTEXT

The enol product tautomerizes to pyruvate, making the reaction strongly exergonic and irreversible in cells. Feed-forward activation by fructose 1,6-bisphosphate couples this exit step to flux through PFK-1.

Evidence for this step: Glycolysis (R-HSA-70171) · Molecular Biology of the Cell: glycolysis (NBK26882)

Research sources, claims, and curation

Glycolysis (R-HSA-70171)

Reactome · Homo sapiens · Curated human pathway record: reaction sequence, participants, and enzymes.

Molecular Biology of the Cell: glycolysis (NBK26882)

Alberts et al. · NCBI Bookshelf · Textbook account of the ten steps, the investment and payoff phases, and the net yield.

Claims

Glycolysis converts one glucose to two pyruvate through ten enzyme-catalyzed steps.

Glycolysis produces a net 2 ATP and 2 NADH per glucose.

Curation notes

  • The ten-step sequence, enzymes, and reversibility follow Reactome's human Glycolysis pathway and Molecular Biology of the Cell.
  • Enzyme gene lists name the principal human genes for each step. They are not an exhaustive isozyme catalog.
  • Protons are written only where they accompany NADH. Ionization states are simplified to textbook names.
  • Regulation is described in the text but not yet modeled as structured regulatory relationships.
  • ChEBI, NCBI Gene, and EC identifiers were checked against their source databases on 2026-09-25.