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Introduction
Glycolysis is the breakdown of glucose to form energy for cellular processes.
Glucose is broken down into two pyruvate molecules. If oxygen and mitochondria are present, these pyruvate enter the citric acid cycle (TCA cycle) and subsequently the electron transport chain (ETC), generating a large amount of energy, mainly ATP, up to 32 ATP per glucose.
Unlike the TCA cycle and ETC, which occur in the mitochondria, glycolysis occurs in the cytoplasm. This reflects its ancient origin, predating mitochondria. It is also anaerobic, and in many tissues acts as an emergency backup source of ATP.¹
Clinical relevance: cells without mitochondria
Red blood cells and corneal cells completely lack mitochondria; other cells, such as retinal, kidney medulla, leukocytes, and testicular cells, have few. Therefore, glycolysis is the only way to produce ATP in these tissues. 1
Sources of glucose
Glucose is required for the process of glycolysis and can be sourced from three places:
1. Diet: absorbed via the gut and delivered through the portal vein to the liver
2. Glycogenolysis: glycogen is essentially a storage form of glucose, and can be broken down to release glucose
3. Gluconeogenesis: the liver (and kidneys) can make glucose from non-carbohydrate sources like amino acids, glycerol, and lactate
Glycolysis
Phase 1
1. Glucose → glucose-6-phosphate
- This is a phosphorylation reaction catalysed by the enzyme hexokinase. This reaction traps glucose in the cell and encourages more glucose to travel down its concentration gradient.
- This uses one ATP
- Remember the prefix ‘hex’ is six – glucose has six carbons
2. Glucose-6-phosphate → fructose-6-phosphate
- An isomerisation reaction catalysed by the enzyme phosphoglucose isomerase
- Fructose is an isomer of glucose (same formula – both are six-carbon molecules, but have different structures)
3. Fructose-6-phosphate → fructose-1,6-bisphosphate
- A phosphorylation reaction catalysed by the enzyme phosphofructokinase (PFK)
- This uses one ATP
4. Fructose-1,6-bisphosphate → dihydroxyacetone phosphate (DHAP) + glyceraldehyde-3-phosphate (G3P)
- A cleavage reaction catalysed by fructose bisphosphate aldolase, a lyase, or a splitter
5. DHAP → G3P
- Catalysed by the enzyme triose phosphate isomerase
- DHAP is inconvertible and only G3P continues through glycolysis, so two G3P are produced per glucose
In phase 1, two ATP are used, and G3P molecules are produced.
Phase 2
This will examine what happens to one G3P. As two G3P are produced, steps 6-10 will occur twice per glucose.
6. G3P → 1,3-bisphosphoglycerate
- An oxidation reaction (loss of electrons) catalysed by the enzyme glyceraldehyde phosphate dehydrogenase
- This reaction requires inorganic phosphate (Pi) and produces NADH from NAD+
7. 1,3-bisphosphoglycerate → 3-phosphoglycerate
- A dephosphorylation reaction catalysed by phosphoglycerate kinase
- This produces one ATP
8. 3-phosphoglycerate → 2-phosphoglycerate
- A phosphate group rearrangement catalysed by the enzyme phosphoglycerate mutase
9. 2-phosphoglycerate → phosphoenolpyruvate (PEP)
- A dehydration reaction catalysed by enolase
10. Phosphoenolpyruvate (PEP) → pyruvate
- A substrate-level phosphorylation catalysed by pyruvate kinase
- This produces one ATP
Clinical relevance: pyruvate kinase deficiency
Pyruvate kinase deficiency (PKD) is the most common glycolytic enzyme deficiency.
Pyruvate kinase catalyses the phosphorylation in step 10. As red blood cells depend solely on glycolysis, PKD means ATP production stops, resulting in the Na⁺/K⁺ ATPase pump failing. This causes the electrochemical gradient across the cell membrane to equalise and the cell to lose its bioconcave shape and burst, resulting in haemolytic anaemia.2
Output
In summary, for one molecule of glucose, glycolysis produces:
1. Pyruvate: two pyruvate molecules, which are utilised in either aerobic or anaerobic conditions
2. ATP: glycolysis uses two ATP and produces four ATP, resulting in a net gain of two ATP per glucose molecule
3. NADH: one NADH is formed from NAD⁺. This high-energy electron carrier can later be utilised by the ETC for further ATP generation.
Fate of pyruvate
Glycolysis produces two pyruvate molecules per glucose molecule. The fate of pyruvate is dependent on the availability of oxygen:
- Aerobic conditions: pyruvate enters the mitochondria and is utilised within the citric acid cycle to produce NADH, which then enters the electron transport chain to produce the maximum ATP
- Anaerobic conditions: pyruvate is converted to lactate, which enters the blood, and is used in the Cori cycle (lactate shuttle) within the liver to produce a small amount of ATP
Clinical relevance: arsenic poisoning
Amongst other enzymes, arsenic inhibits pyruvate dehydrogenase, which breaks down pyruvate in aerobic conditions. Therefore, arsenic inhibits ATP production.3
Clinical relevance: lactic acidosis
Lactic acid is constantly made by tissues and subsequently broken down in the liver. Increased production (e.g. hypoxia, shock) or decreased breakdown (e.g. liver dysfunction) can lead to accumulation and lactic acidosis.
Lactate production is a consequence of glycolysis in anaerobic conditions. It is an emergency way of generating ATP, and its presence in the blood at elevated levels is a bad sign.4
Key points
- Glycolysis is an evolutionarily ancient metabolic process
- It’s vital in cells that lack mitochondria or during low oxygen delivery, as it is an anaerobic process
- It involves the breakdown of glucose to pyruvate
- Byproducts of glycolysis include a net gain of two ATP and one NADH
- The fate of pyruvate depends on the presence or absence of oxygen and mitochondria
Editor
Dr Jamie Scriven
References
- Devlin TM. Textbook of Biochemistry With Clinical Correlations. John Wiley & Sons. 2006.
- Fattizzo B, Cavallaro F, Marcello APML, et al. Pyruvate Kinase Deficiency: Current Challenges and Future Prospects. Journal of Blood Medicine. 2022. Available from: [LINK].
- Hughes MF. Arsenic toxicity and potential mechanisms of action. Toxicology Letters. 2022. Available from: [LINK].
- Baddam S, Tubben RE. Lactic Acidosis. StatPearls. Treasure Island. 2025. Available from: [LINK].
Image references
- Figure 2. Phi-Gastrein. Pompe sodium/potassium. Available from: [LINK]. License: [CC BY-SA 3.0].
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