Anaerobic Glycolysis / Lactic Acid Fermentation
- Compartment
- Cytosol
- Main tissue
- RBC, exercising muscle, hypoxic tissue, tumours
- Rate-limiting
- Phosphofructokinase-1 (PFK-1)
- Steps
- 11
Reaction steps
In source order, 11 total
-
1
Glucose + ATP → glucose-6-phosphate + ADP
› Notes
Required cofactors: ATP and Mg2+. This reaction is essentially irreversible and traps glucose inside the cell; hexokinase is inhibited by glucose-6-phosphate, whereas glucokinase has a high Km and is not inhibited by glucose-6-phosphate.
-
2
Glucose-6-phosphate ⇌ fructose-6-phosphate
› Notes
Required cofactor: none. The reaction is reversible and prepares the substrate for phosphorylation and symmetric cleavage.
-
3
Fructose-6-phosphate + ATP → fructose-1,6-bisphosphate + ADP
› Notes
Required cofactors: ATP and Mg2+. This is the pathway’s essentially irreversible, rate-limiting, and committed step of glycolysis; once fructose-1,6-bisphosphate is formed, the carbon is committed to glycolytic cleavage rather than glycogen synthesis or pentose phosphate flux.
-
4
Fructose-1,6-bisphosphate ⇌ glyceraldehyde-3-phosphate + dihydroxyacetone phosphate
› Notes
Required cofactor: none in the human class I aldolase mechanism, which uses a Schiff-base lysine intermediate. The reaction is reversible.
-
5
Dihydroxyacetone phosphate ⇌ glyceraldehyde-3-phosphate
› Notes
Required cofactor: none. This reversible step ensures that both three-carbon products proceed as glyceraldehyde-3-phosphate; from this point, steps 6–10 occur twice per glucose.
-
6
Glyceraldehyde-3-phosphate + Pi + NAD+ ⇌ 1,3-bisphosphoglycerate + NADH + H+
› Notes
Required cofactors/substrates: NAD+ and inorganic phosphate; the enzyme uses an active-site cysteine. This reversible redox step creates the cytosolic NADH that must be reoxidized by lactate dehydrogenase under anaerobic conditions.
-
7
1,3-Bisphosphoglycerate + ADP ⇌ 3-phosphoglycerate + ATP
› Notes
Required cofactors: ADP and Mg2+. This reversible substrate-level phosphorylation step produces the first ATP-generating reaction of glycolysis; because it occurs twice per glucose, it replaces the two ATP invested earlier.
-
8
3-Phosphoglycerate ⇌ 2-phosphoglycerate
› Notes
Required cofactor: 2,3-bisphosphoglycerate as a catalytic priming cofactor in the major human enzyme. The reaction is reversible.
-
9
2-Phosphoglycerate ⇌ phosphoenolpyruvate + H2O
› Notes
Required cofactors: Mg2+. The reaction is reversible; fluoride inhibits enolase and is used in some blood-collection tubes to limit ex vivo glycolysis.
-
10
Phosphoenolpyruvate + ADP → pyruvate + ATP
› Notes
Required cofactors: ADP, Mg2+, and K+. This is essentially irreversible and a major regulatory step; it is activated feed-forward by fructose-1,6-bisphosphate in many tissues. The second substrate-level phosphorylation generates two ATP per glucose at this stage.
-
11
Pyruvate + NADH + H+ ⇌ lactate + NAD+
› Notes
Required cofactor: NADH/NAD+. This reversible reaction is the defining fermentation step; it produces no additional ATP but is indispensable for maintaining the NAD+ required by glyceraldehyde-3-phosphate dehydrogenase when cytosolic NADH cannot be reoxidized sufficiently through mitochondrial shuttles and the ETC.
Showing all 11 steps.
Regulation
What speeds each enzyme up and what slows it down
Hexokinase / glucokinase
Hexokinase: glucose availability; glucokinase: high portal glucose and glucokinase regulatory protein release
Hexokinase: glucose-6-phosphate; glucokinase: sequestration by glucokinase regulatory protein in low glucose/fructose-6-phosphate conditions
Insulin induces hepatic glucokinase expression; glucagon decreases its expression in fasting
Phosphofructokinase-1 (PFK-1)
AMP, ADP, Pi, fructose-2,6-bisphosphate
ATP, citrate, low pH (H+) in muscle
Insulin activates hepatic PFK-2 activity, raising fructose-2,6-bisphosphate and PFK-1 flux; glucagon via PKA lowers fructose-2,6-bisphosphate in liver and inhibits glycolysis
Pyruvate kinase
Fructose-1,6-bisphosphate; ADP
ATP, alanine; hepatic phosphorylation inhibits the L isoform
Insulin promotes dephosphorylation/activation and expression of liver pyruvate kinase; glucagon/epinephrine phosphorylate and inhibit hepatic pyruvate kinase
Lactate dehydrogenase
Pyruvate and NADH availability; high cytosolic NADH/NAD+ ratio favors lactate formation
Product/thermodynamic limitation by high lactate and NAD+; no primary allosteric inhibitor of central control
No dominant acute direct hormonal switch; hormonal changes in glycolytic flux and perfusion indirectly affect lactate production
Overview
Anaerobic glycolysis converts glucose to pyruvate and then reduces pyruvate to lactate, regenerating cytosolic NAD+ so glycolytic ATP production can continue when mitochondrial oxidative capacity or oxygen delivery is insufficient. In humans, this pathway is essential in erythrocytes, which lack mitochondria, and becomes prominent in vigorously contracting skeletal muscle, hypoperfused tissue, and other settings of high glycolytic flux. The term “lactic acid fermentation” conventionally describes the pathway, although at physiological pH the product exists predominantly as lactate plus H+ rather than undissociated lactic acid.
Cellular location
All reactions of glycolysis and lactate dehydrogenase occur in the cytosol. Mature erythrocytes rely entirely on anaerobic glycolysis for ATP; fast-twitch skeletal muscle fibers use it during intense exercise, and tissues with limited oxygen delivery or mitochondrial dysfunction may increase lactate production. Lactate can be exported through monocarboxylate transporters and subsequently oxidized by heart/oxidative muscle or converted to glucose by liver in the Cori cycle.
Net energetics
Per glucose converted to two lactate, anaerobic glycolysis consumes 2 ATP in the investment phase and produces 4 ATP by substrate-level phosphorylation, for a net yield of 2 ATP. It generates 2 NADH at glyceraldehyde-3-phosphate dehydrogenase but consumes exactly 2 NADH in the LDH reaction, so the net yield is 0 NADH, 0 FADH2, 0 GTP, and 2 lactate; the overall reaction is commonly written as glucose + 2 ADP + 2 Pi → 2 lactate + 2 ATP + 2 H2O. No oxidative phosphorylation occurs in this anaerobic route. By contrast, if glucose-derived pyruvate is oxidized aerobically through PDC, TCA cycle, ETC, and oxidative phosphorylation, total yield is approximately 30–32 ATP per glucose with modern P/O ratios; older texts often list approximately 36–38 ATP.
Clinical significance
Rapid anaerobic glycolysis provides ATP at a high rate but low yield, enabling short-duration intense muscular work and sustaining erythrocyte membrane pumps and redox metabolism. Lactate is not simply a waste product: it is a circulating fuel and carbon carrier, used by oxidative tissues and by liver for gluconeogenesis in the Cori cycle. Type A lactic acidosis arises from tissue hypoxia or hypoperfusion (for example, shock or severe anemia), whereas type B lactic acidosis may occur with mitochondrial disease, drugs/toxins, malignancy, or impaired hepatic clearance. Defects of pyruvate kinase cause chronic nonspherocytic hemolytic anemia because erythrocytes depend exclusively on glycolytic ATP; impaired mitochondrial pyruvate oxidation also drives compensatory lactate formation. The following standard biochemistry textbooks provide additional depth, alternative pedagogical framing, and expanded clinical correlations for the pathways summarized in this reference.
Recent literature
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