Glycolysis is the universal, oxygen-independent metabolic pathway that catabolises one molecule of Glucose (a six-carbon sugar) into two molecules of pyruvate (three carbons each), yielding a net gain of 2 ATP and 2 NADH per glucose consumed. The name derives from the Greek glykys (sweet) and lysis (splitting). Every living cell on Earth possesses this pathway, underscoring its ancient evolutionary origin -- it almost certainly predates the appearance of atmospheric oxygen and therefore mitochondrial Oxidative Phosphorylation. All ten enzymatic steps occur in the cytoplasm, making glycolysis the one energy-producing pathway that requires neither mitochondria nor oxygen, which is why red blood cells (which lack mitochondria entirely) depend on it exclusively for ATP production.
The fate of pyruvate at the end of glycolysis represents a critical metabolic branch point. Under aerobic conditions and when mitochondrial capacity is sufficient, pyruvate enters the mitochondrial matrix, where pyruvate dehydrogenase (PDH) converts it to acetyl-CoA for oxidation through the citric acid (Krebs/TCA) cycle and subsequent Oxidative Phosphorylation, ultimately yielding approximately 30-32 ATP per glucose. Under anaerobic conditions -- or when the cell deliberately chooses speed over efficiency -- pyruvate is instead reduced to Lactic acid by lactate dehydrogenase (LDH), regenerating the NAD+ required to keep the glycolytic pathway running. This anaerobic branch produces ATP far less efficiently (only 2 per glucose) but operates at much higher speed, which matters enormously for cells under metabolic stress or acute activation.
In Immunometabolism, glycolysis holds a position of central importance because it is the pathway that activated leukocytes preferentially upregulate during immune responses. M1 macrophages, effector T cells (Th1, Th2, Th17), activated dendritic cells, and proliferating NK cells all dramatically increase glycolytic flux -- even when oxygen is plentiful -- in a phenomenon known as the Warburg Effect or Aerobic Glycolysis. This metabolic reprogramming is not a deficiency but a deliberate strategy: rapid ATP production, generation of biosynthetic intermediates for proliferation and cytokine synthesis, and diversion of glucose carbons into the pentose phosphate pathway for NADPH and nucleotide production. Understanding glycolysis is therefore indispensable for understanding how the immune system fuels itself during infection, wound healing, and chronic inflammation.
Net yield: 2 ATP + 2 NADH + 2 Pyruvate per Glucose. Key regulatory enzyme: PFK-1 (activated by AMP, F-2,6-BP; inhibited by ATP, Citrate).
The first half of glycolysis is sometimes called the "investment phase" because the cell must spend 2 ATP to phosphorylate and rearrange the glucose molecule before any energy can be recovered. This initial investment destabilises the glucose carbon skeleton, trapping it inside the cell and preparing it for cleavage into two three-carbon fragments.
Step 1 -- Hexokinase (HK): Glucose to Glucose-6-Phosphate. The pathway begins the moment Glucose crosses the plasma membrane via a glucose transporter (GLUT1, GLUT4, or others depending on cell type). Hexokinase (or glucokinase in hepatocytes and pancreatic beta cells) immediately phosphorylates glucose at the C-6 position, consuming one ATP and producing glucose-6-phosphate (G6P). This phosphorylation is effectively irreversible under physiological conditions and serves two purposes: the negatively charged phosphate group prevents G6P from diffusing back out through glucose transporters, thereby trapping glucose inside the cell; and it destabilises the molecule for subsequent reactions. G6P sits at a major metabolic crossroads -- it can proceed through glycolysis, enter the pentose phosphate pathway via G6PD (producing NADPH and ribose-5-phosphate), or be stored as glycogen. In activated leukocytes, hexokinase II (HK-II) is markedly upregulated and physically associates with the outer mitochondrial membrane, gaining preferential access to mitochondrially generated ATP and making this step extremely efficient during immune responses.
Step 2 -- Phosphoglucose Isomerase (PGI): G6P to Fructose-6-Phosphate. Phosphoglucose isomerase catalyses the reversible isomerisation of glucose-6-phosphate (an aldose) to fructose-6-phosphate (a ketose). This rearrangement from a six-membered pyranose ring to a five-membered furanose ring is necessary because the next enzyme, phosphofructokinase-1, specifically requires a fructose substrate. The reaction is freely reversible and near equilibrium, making it a non-regulatory step. Interestingly, PGI has a completely different extracellular function -- secreted PGI acts as a cytokine called autocrine motility factor (AMF), involved in Cancer cell migration and metastasis.
Step 3 -- Phosphofructokinase-1 (PFK-1): F6P to Fructose-1,6-Bisphosphate. This is the committed, rate-limiting, and most tightly regulated step of the entire glycolytic pathway. PFK-1 phosphorylates fructose-6-phosphate at C-1, consuming a second ATP and producing fructose-1,6-bisphosphate (F1,6BP). Because this reaction is essentially irreversible, PFK-1 serves as the primary gatekeeper controlling glycolytic flux. PFK-1 is an allosteric enzyme exquisitely sensitive to the cell's energy status: it is activated by AMP (signalling low energy), ADP, fructose-2,6-bisphosphate (the most potent activator, produced by PFK-2/FBPase-2), and inorganic phosphate; it is inhibited by ATP (signalling energy sufficiency), Citrate (signalling that the TCA cycle is well-fed and does not need more input), and low pH. The allosteric activation by fructose-2,6-bisphosphate is particularly important in the context of Immunometabolism: HIF-1alpha directly upregulates the enzyme PFKFB3 (a PFK-2 isoform), increasing fructose-2,6-bisphosphate levels and thereby powerfully driving glycolytic flux in activated leukocytes and Cancer cells. Insulin signalling also stimulates PFKFB activity, linking hormonal regulation to glycolytic control.
Step 4 -- Aldolase: F1,6BP to DHAP + G3P. Aldolase cleaves the six-carbon fructose-1,6-bisphosphate into two different three-carbon molecules: dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P). This is the "lysis" in glycolysis -- the splitting of the sugar. The reaction is thermodynamically unfavourable under standard conditions but is pulled forward by the rapid consumption of products in subsequent steps. Only G3P continues directly through glycolysis; DHAP must first be converted. DHAP also serves as a precursor for glycerol-3-phosphate, linking glycolysis to lipid and triglyceride synthesis -- a connection relevant to Adipocytes and lipid storage in metabolic syndrome.
Step 5 -- Triose Phosphate Isomerase (TPI): DHAP to G3P. Triose phosphate isomerase rapidly and reversibly interconverts DHAP and G3P, ensuring that both three-carbon products from the aldolase reaction can proceed through the remaining glycolytic steps. TPI is one of the most catalytically efficient enzymes known (operating near the diffusion-controlled limit). From this point forward, all reactions occur twice per original glucose molecule, since each glucose has now been split into two G3P molecules.
The second half of glycolysis is the "payoff phase," in which each of the two G3P molecules is oxidised and rearranged to produce pyruvate, generating 4 ATP and 2 NADH in total. Since 2 ATP were consumed in the investment phase, the net yield is 2 ATP and 2 NADH per glucose.
Step 6 -- Glyceraldehyde-3-Phosphate Dehydrogenase (GAPDH): G3P to 1,3-Bisphosphoglycerate. GAPDH catalyses the first oxidation in glycolysis, coupling the oxidation of G3P's aldehyde group to the reduction of NAD+ to NADH and the addition of an inorganic phosphate to form 1,3-bisphosphoglycerate (1,3-BPG). This is a critical step because it generates the high-energy acyl-phosphate bond that will be used in the next step to produce ATP via substrate-level phosphorylation. GAPDH requires NAD+ as a cofactor, and the availability of NAD+ can become rate-limiting -- this is precisely why Anaerobic Glycolysis (pyruvate to Lactic acid via LDH) is essential: it regenerates NAD+ to keep GAPDH and therefore the entire pathway running when mitochondrial NADH reoxidation is insufficient. GAPDH has also been recognised as a moonlighting protein with roles in apoptosis, DNA repair, and gene expression, and it binds to mTOR to regulate cellular growth signalling.
Step 7 -- Phosphoglycerate Kinase (PGK): 1,3-BPG to 3-Phosphoglycerate. PGK transfers the high-energy phosphate group from 1,3-BPG to ADP, producing ATP and 3-phosphoglycerate (3-PG). This is the first substrate-level phosphorylation in glycolysis. Because this step occurs twice per glucose (once for each G3P), it generates 2 ATP -- exactly recouping the 2 ATP invested in steps 1 and 3. A side reaction of clinical interest: 1,3-BPG can be converted to 2,3-bisphosphoglycerate (2,3-BPG) by bisphosphoglycerate mutase, particularly in red blood cells; 2,3-BPG decreases haemoglobin's oxygen affinity, facilitating oxygen release to tissues -- a direct link between glycolysis and oxygen delivery.
Step 8 -- Phosphoglycerate Mutase: 3-PG to 2-Phosphoglycerate. Phosphoglycerate mutase catalyses the reversible shift of the phosphate group from the C-3 to the C-2 position, converting 3-phosphoglycerate to 2-phosphoglycerate. This rearrangement is necessary to position the phosphate group for the dehydration reaction in the next step that creates the high-energy phosphoenolpyruvate.
Step 9 -- Enolase: 2-PG to Phosphoenolpyruvate (PEP). Enolase catalyses the dehydration of 2-phosphoglycerate, removing a water molecule to form phosphoenolpyruvate (PEP). This dehydration dramatically raises the free energy of hydrolysis of the phosphate bond, making PEP one of the highest-energy phosphorylated compounds in the cell (with a standard free energy of hydrolysis of approximately -61.9 kJ/mol, far exceeding that of ATP). Enolase is inhibited by fluoride ions, which is why fluoride is added to blood collection tubes for glucose measurement (it prevents glycolysis from consuming glucose ex vivo). Enolase-alpha is expressed on the surface of many cell types and has been implicated as an autoantigen in certain autoimmune disease conditions.
Step 10 -- Pyruvate Kinase (PK): PEP to Pyruvate. Pyruvate kinase catalyses the final, irreversible step of glycolysis: the transfer of the high-energy phosphate from PEP to ADP, producing ATP and pyruvate. This is the second substrate-level phosphorylation (occurring twice per glucose, generating 2 ATP total in this step). Pyruvate kinase exists in multiple isoforms; the PKM2 isoform is especially relevant in Immunometabolism and Cancer because it can be allosterically regulated to slow down at this step, causing upstream glycolytic intermediates to accumulate and be diverted into biosynthetic branch pathways (the pentose phosphate pathway, serine biosynthesis, and hexosamine pathway). In activated M1 macrophages and Cancer cells, PKM2 is often found in its less active dimeric form, which also translocates to the nucleus where it functions as a transcription co-activator for HIF-1alpha, further reinforcing the glycolytic programme. This is a remarkable example of a metabolic enzyme moonlighting as a transcriptional regulator.
After glycolysis produces pyruvate, the cell faces its most consequential metabolic decision:
Aerobic fate -- Pyruvate Dehydrogenase (PDH): Pyruvate to Acetyl-CoA. When oxygen is available and mitochondrial function is intact, pyruvate enters the mitochondrial matrix and is oxidatively decarboxylated by the pyruvate dehydrogenase complex (PDH), producing acetyl-CoA, CO2, and NADH. Acetyl-CoA then enters the TCA (Krebs) cycle, where its two carbons are fully oxidised, generating further NADH and FADH2 that feed the electron transport chain and Oxidative Phosphorylation. The complete aerobic oxidation of one glucose through glycolysis, PDH, TCA cycle, and Oxidative Phosphorylation yields approximately 30-32 ATP total. PDH is regulated by phosphorylation: pyruvate dehydrogenase kinase (PDK) inactivates PDH, and HIF-1alpha upregulates PDK1, diverting pyruvate away from mitochondria toward Lactic acid production -- a key mechanism by which HIF-1alpha enforces the glycolytic phenotype in hypoxic or inflammatory conditions.
Anaerobic fate -- Lactate Dehydrogenase (LDH): Pyruvate to Lactate. Under anaerobic conditions -- or deliberately during Aerobic Glycolysis in activated leukocytes -- LDH reduces pyruvate to Lactic acid (lactate), simultaneously reoxidising NADH back to NAD+. This NAD+ regeneration is the essential purpose of the reaction: without it, GAPDH (step 6) would stall, halting the entire pathway. Anaerobic Glycolysis thus sacrifices energetic efficiency (2 ATP per glucose instead of ~32) for speed and NAD+ recycling. Lactate is exported from the cell via monocarboxylate transporters (MCTs) and can be taken up by other tissues (the Liver reconverts it to glucose via the Cori cycle; the heart and slow-twitch muscle oxidise it as fuel; the brain can use it as an alternative substrate).
Per molecule of glucose, glycolysis yields:
If pyruvate proceeds aerobically through PDH, TCA cycle, and Oxidative Phosphorylation, the complete oxidation of one glucose yields approximately 30-32 ATP total (including the 2 from glycolysis and the NADH equivalents). If pyruvate is converted to Lactic acid, the cell gains only the 2 ATP from glycolysis, but gains it very quickly -- an important trade-off in the biology of immune responses and Cancer.
The single most important clinical connection of glycolysis in cPNI is the Warburg Effect -- the observation that activated immune cells and cancer cells preferentially use glycolysis and produce Lactic acid even when oxygen is abundant, a phenomenon originally described by Otto Warburg in the 1920s in tumour cells. In the context of Immunometabolism, this metabolic reprogramming occurs within hours of immune activation and is not a sign of dysfunction but a deliberate, signalling-driven metabolic strategy.
When M1 macrophages are activated by LPS or IFN-gamma, they undergo a dramatic metabolic shift: glucose uptake increases 10-fold or more (driven by upregulation of GLUT1), glycolytic enzyme expression surges (driven by HIF-1alpha and NF-kappaB), and mitochondrial Oxidative Phosphorylation is actively suppressed (through PDK1-mediated inactivation of PDH and through Succinate-mediated stabilisation of HIF-1alpha). Similarly, when naive T cells are activated through their T-cell receptor and co-stimulatory signals, they rapidly upregulate GLUT1, switch to Aerobic Glycolysis, and engage the mTOR signalling pathway. This shift is required for effector function -- blocking glycolysis pharmacologically impairs cytokine production and proliferation.
Why do activated immune cells choose this seemingly wasteful strategy? Several reasons converge:
Speed over efficiency. Glycolysis produces ATP approximately 100 times faster than Oxidative Phosphorylation, which matters enormously when immune cells must mount a rapid response to infection. The immune system is a "selfish" organ that commandeers metabolic resources when threatened, and fast glycolytic ATP supports this emergency response.
Biosynthetic intermediates. Glycolytic intermediates feed critical biosynthetic pathways. Glucose-6-phosphate enters the pentose phosphate pathway (via G6PD) to generate NADPH (for Reactive Oxygen Species production by NADPH oxidase and for reductive biosynthesis) and ribose-5-phosphate (for nucleotide synthesis required by proliferating lymphocytes). Fructose-6-phosphate feeds the hexosamine biosynthetic pathway (for protein glycosylation). 3-phosphoglycerate feeds serine and glycine biosynthesis (for one-carbon metabolism and nucleotide synthesis). DHAP feeds glycerol-3-phosphate synthesis (for membrane phospholipid production). The glycolytic pathway is therefore not merely an energy pathway but a metabolic hub supplying building blocks for the activated immune cell.
Signalling functions. Glycolytic intermediates and enzymes have direct signalling roles. Succinate, which accumulates when the TCA cycle is broken at specific points in M1 macrophages, stabilises HIF-1alpha and directly drives IL-1beta transcription. PKM2, the glycolytic enzyme from step 10, translocates to the nucleus and co-activates HIF-1alpha target genes. Lactic acid itself, long dismissed as a waste product, is now recognised as a potent signalling molecule with immunomodulatory properties.
Redox control. By shifting to glycolysis and reducing mitochondrial electron transport chain activity, activated immune cells can control Reactive Oxygen Species production, using NADPH oxidase-derived ROS precisely for pathogen killing while limiting damaging mitochondrial ROS leak.
The lactate produced by glycolysis in activated immune cells is far more than a waste product. Lactate exported into the tissue microenvironment has profound immunological effects: it suppresses the activation and proliferation of effector T cells, inhibits NK cells, reduces dendritic cell maturation, and promotes the polarisation of macrophages toward the anti-inflammatory M2 macrophages phenotype via histone lactylation -- a recently discovered epigenetic modification. In this sense, lactate functions as an endogenous brake on inflammation, contributing to inflammatory resolution. However, in the tumour microenvironment, this same immunosuppressive property of lactate shields Cancer cells from immune attack, representing one of the key mechanisms of tumour immune evasion.
HIF-1alpha (hypoxia-inducible factor 1-alpha) is the transcription factor most directly responsible for upregulating the glycolytic programme. Under normoxia, HIF-1alpha is continuously synthesised but rapidly degraded (hydroxylated by prolyl hydroxylases using O2, iron, and 2-Oxoglutarate, then ubiquitinated by VHL and sent to the proteasome). Under hypoxia, the hydroxylases cannot function, HIF-1alpha accumulates, and it translocates to the nucleus where it drives expression of virtually every glycolytic enzyme (hexokinase II, PFK-1, PFKFB3, aldolase, GAPDH, enolase, pyruvate kinase, LDH-A) as well as GLUT1 and PDK1. Critically, HIF-1alpha can also be stabilised under normoxic conditions by inflammatory signals: Succinate accumulation in M1 macrophages inhibits the prolyl hydroxylases, and NF-kappaB directly transcribes the HIF1A gene. This explains why activated immune cells exhibit the Warburg Effect even with ample oxygen -- it is an inflammation-driven, not hypoxia-driven, phenomenon (though true tissue hypoxia at sites of infection and wound healing compounds the effect).
As the rate-limiting enzyme, PFK-1 is the most important regulatory node in glycolysis. Its allosteric regulation integrates multiple metabolic signals:
The regulation by fructose-2,6-bisphosphate deserves particular emphasis. This molecule is not a glycolytic intermediate but a dedicated regulatory signal produced by the bifunctional enzyme PFK-2/FBPase-2. Different isoforms exist in different tissues (PFKFB1 in liver, PFKFB2 in heart, PFKFB3 in brain and immune cells, PFKFB4 in testes). PFKFB3, the isoform dominant in leukocytes and Cancer cells, has an exceptionally high kinase-to-phosphatase ratio, meaning it produces large amounts of fructose-2,6-bisphosphate and thus drives powerful glycolytic activation. HIF-1alpha directly upregulates PFKFB3 expression, and Insulin signalling activates PFKFB2 -- two pathways that converge on maximising glycolytic flux.
Otto Warburg's original observation in the 1920s was that Cancer cells consume enormous amounts of Glucose and produce Lactic acid even in the presence of oxygen. He initially hypothesised that cancer cells had defective mitochondria, but we now understand that the mitochondria of most cancer cells are functional -- the Warburg effect is an actively maintained metabolic programme driven by oncogenes (c-Myc, Ras, Akt) and transcription factors (HIF-1alpha, mTOR). The glycolytic phenotype gives cancer cells the same advantages it gives activated immune cells: rapid ATP, biosynthetic precursors, and an acidic microenvironment (from exported lactate) that promotes invasion, suppresses immune attack, and selects for acid-resistant tumour cells. The clinical exploitation of this metabolic phenotype is the basis of FDG-PET (fluorodeoxyglucose positron emission tomography) scanning, which detects tumours by their avid glucose uptake. Glycolytic inhibitors (2-deoxyglucose, lonidamine) and metabolic modulators (Metformin, dichloroacetate) are under investigation as anti-cancer strategies.
In cPNI, the concept of Metabolic flexibility -- the ability to switch smoothly between glycolysis and Oxidative Phosphorylation, between glucose and fatty acid oxidation -- is central to health. A healthy cell, and a healthy organism, can upregulate glycolysis acutely when needed (immune activation, exercise, hypoxia) and then return to efficient oxidative metabolism when the challenge has passed. chronic low-grade inflammation, Insulin resistance, obesity, and Type 2 diabetes are all characterised by a loss of this flexibility: immune cells become locked in a glycolytic, pro-inflammatory state (trained immunity), skeletal muscle cannot switch efficiently between fuel sources, and the liver overproduces glucose via Gluconeogenesis even when blood glucose is already elevated. Interventions that restore metabolic flexibility -- Intermittent fasting, physical activity, cold exposure, Omega-3 fatty acids, ketogenic diet approaches -- are therefore central to cPNI therapeutic strategy.