Glycolysis

1. 1

   Glucose + ATP -> glucose-6-phosphate + ADP

   Hexokinase I-III 2.7.1.1 / 2.7.1.2 ST-0001 Irreversible ATP Mg2+
   In Glucose

   Out Glucose-6-phosphate
   › Notes

   Hexokinase I–III in most tissues, or glucokinase (hexokinase IV) in liver and pancreatic beta cells, transfers the terminal phosphate of ATP to glucose; Mg2+ is required. This is an irreversible trapping step; it is not the committed step because glucose-6-phosphate can enter glycogenesis or the pentose phosphate pathway. Hexokinase is product-inhibited by glucose-6-phosphate, whereas glucokinase has a high Km and high Vmax and is regulated by glucokinase regulatory protein in hepatocytes.

2. 2

   Glucose-6-phosphate → fructose-6-phosphate

   Phosphoglucose isomerase 5.3.1.9 ST-0002 Reversible
   In Glucose-6-phosphate

   Out Fructose-6-phosphate
   › Notes

   Phosphoglucose isomerase catalyzes reversible aldose–ketose isomerization through an enediol intermediate; no nucleotide cofactor is consumed. This reversible reaction prepares the molecule for symmetrical cleavage after a second phosphorylation.

3. 3

   Fructose-6-phosphate + ATP -> fructose-1,6-bisphosphate + ADP

   Phosphofructokinase-1 (PFK-1) 2.7.1.11 ST-0003 Irreversible rate-limiting ATP Mg2+
   In Fructose-6-phosphate

   Out Fructose-1,6-bisphosphate
   › Notes

   Phosphofructokinase-1 (PFK-1) uses ATP and Mg2+. This is the principal rate-limiting, committed, and irreversible step of glycolysis; fructose-1,6-bisphosphate is committed to glycolytic cleavage rather than glucose-6-phosphate-dependent alternative pathways.

4. 4

   Fructose-1,6-bisphosphate → glyceraldehyde-3-phosphate + dihydroxyacetone phosphate

   Fructose-bisphosphate aldolase (Aldolase A/B/C) 4.1.2.13 ST-0004 Reversible
   In Fructose-1,6-bisphosphate

   Out Glyceraldehyde-3-phosphate Dihydroxyacetone phosphate
   › Notes

   Fructose-bisphosphate aldolase reversibly cleaves the six-carbon ketose bisphosphate into two phosphorylated trioses. No redox cofactor or ATP is required.

5. 5

   Dihydroxyacetone phosphate → glyceraldehyde-3-phosphate

   Triose phosphate isomerase 5.3.1.1 ST-0005 Reversible
   In Dihydroxyacetone phosphate

   Out Glyceraldehyde-3-phosphate
   › Notes

   Triose phosphate isomerase reversibly interconverts the two trioses through an enediol intermediate. Only glyceraldehyde-3-phosphate continues directly through the payoff reactions, so one glucose yields two glyceraldehyde-3-phosphate molecules from this point onward.

6. 6

   Glyceraldehyde-3-phosphate + inorganic phosphate + NAD+ → 1,3-bisphosphoglycerate + NADH + H+

   Glyceraldehyde-3-phosphate dehydrogenase 1.2.1.12 ST-0006 Reversible NAD+ NADH Pi
   In Glyceraldehyde-3-phosphate Pi NAD+

   Out 1,3-Bisphosphoglycerate NADH H+
   › Notes

   Glyceraldehyde-3-phosphate dehydrogenase oxidizes the aldehyde and captures the energy in an acyl-phosphate bond; it requires NAD+ and free inorganic phosphate (Pi). The reaction is reversible. Because it occurs twice per glucose, it generates two cytosolic NADH.

7. 7

   1,3-Bisphosphoglycerate + ADP → 3-phosphoglycerate + ATP

   Phosphoglycerate kinase 2.7.2.3 ST-0007 Reversible ATP Mg2+
   In 1,3-Bisphosphoglycerate ADP

   Out 3-Phosphoglycerate ATP
   › Notes

   Phosphoglycerate kinase requires Mg2+ and catalyzes reversible substrate-level phosphorylation. Occurring twice per glucose, this first payoff reaction produces two ATP and thereby repays the two ATP invested in steps 1 and 3.

8. 8

   3-Phosphoglycerate → 2-phosphoglycerate

   Phosphoglycerate mutase 5.4.2.11 ST-0008 Reversible
   In 3-Phosphoglycerate

   Out 2-Phosphoglycerate
   › Notes

   Phosphoglycerate mutase reversibly shifts the phosphate from carbon 3 to carbon 2; the 2,3-bisphosphoglycerate-dependent isoenzyme requires a catalytic phosphohistidine and 2,3-bisphosphoglycerate for priming. No ATP is consumed.

9. 9

   2-Phosphoglycerate → phosphoenolpyruvate + H2O

   Enolase 4.2.1.11 ST-0009 Reversible Mg2+ Mn2+ H2O
   In 2-Phosphoglycerate

   Out Phosphoenolpyruvate H2O
   › Notes

   Enolase requires Mg2+ (or Mn2+) and reversibly dehydrates 2-phosphoglycerate to form phosphoenolpyruvate (PEP), a compound with high phosphoryl-transfer potential. Fluoride inhibits enolase and is used to limit glycolysis in some blood collection tubes.

10. 10

    Phosphoenolpyruvate + ADP → pyruvate + ATP

    Pyruvate kinase 2.7.1.40 ST-0010 Irreversible ATP Mg2+ Mn2+ K+
    In Phosphoenolpyruvate ADP

    Out Pyruvate ATP
    › Notes

    Pyruvate kinase requires K+ and Mg2+ (or Mn2+) and performs the second substrate-level phosphorylation. This step is irreversible; in liver it is an important regulatory point and is activated feed-forward by fructose-1,6-bisphosphate. It occurs twice per glucose and yields two additional ATP.

Showing all 10 steps.

Overview

Glycolysis is the cytosolic sequence that converts one molecule of glucose to two molecules of pyruvate (or, when mitochondrial oxidation is limited, lactate), conserving energy as ATP and NADH. It supplies rapid ATP in every cell and is indispensable in cells without mitochondria, notably erythrocytes; its intermediates also feed biosynthetic pathways. The pathway has an energy-investment phase (steps 1–5) and an energy-payoff phase (steps 6–10).

Cellular location

All ten reactions occur in the cytosol. Glycolytic flux is especially high in exercising skeletal muscle, erythrocytes, renal medulla, brain, leukocytes, and many proliferating cells; hepatocytes use glycolysis prominently in the fed, insulinized state. Cytosolic NADH produced by glycolysis is reoxidized through the malate–aspartate or glycerol 3-phosphate shuttle under aerobic conditions, or by lactate dehydrogenase under anaerobic conditions.

Net energetics

Per glucose converted to two pyruvate, glycolysis consumes 2 ATP (steps 1 and 3) and forms 4 ATP (steps 7 and 10), for net 2 ATP by substrate-level phosphorylation. It also produces 2 NADH + 2 H+ and 2 H2O; no NADPH, FADH2, or GTP is produced. The net reaction is: glucose + 2 ADP + 2 Pi + 2 NAD+ → 2 pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O; under anaerobic conditions, pyruvate + NADH → lactate + NAD+ regenerates NAD+ without additional ATP.

Clinical significance

Glycolysis is the sole ATP-producing pathway available to mature erythrocytes and a major rapid-energy source in hypoxic or intensely exercising tissue. Aerobic tumor cells frequently exhibit high glycolytic flux and lactate formation despite oxygen availability (the Warburg effect), which supports anabolic precursor generation and is the basis of fluorodeoxyglucose positron-emission tomography. Pyruvate kinase deficiency reduces erythrocyte ATP, causing chronic nonspherocytic hemolytic anemia; phosphofructokinase deficiency (Tarui disease) causes exercise intolerance and hemolysis.