The tricarboxylic acid cycle, commonly called the TCA cycle or Krebs cycle, is a series of chemical reactions inside mitochondria that breaks down fuel molecules and generates most of the energy your cells run on. But calling it just an energy-producing pathway understates its importance. The cycle also supplies the raw materials cells need to build amino acids, fatty acids, and other essential molecules, which is why researchers increasingly describe it as a metabolic hub rather than a simple wheel.
How the Cycle Actually Works
The TCA cycle takes place in the matrix of mitochondria, the compartments inside your cells that handle most energy production. The cycle begins when a two-carbon fragment called acetyl-CoA combines with a four-carbon molecule called oxaloacetate, producing a six-carbon molecule called citrate. Over the course of eight sequential reactions, that citrate is gradually rearranged, stripped of carbon atoms (released as carbon dioxide), and used to generate high-energy electron carriers. Those carriers then feed into a separate system embedded in the mitochondrial membrane, the electron transport chain, which uses their electrons to produce the bulk of your cells’ ATP.
By the end of one full turn, the cycle regenerates the oxaloacetate it started with, so the process can begin again. Each turn also produces a small amount of ATP directly, but the real payoff comes from those electron carriers. A single molecule of glucose, fully broken down through glycolysis and two turns of the TCA cycle, ultimately yields around 30 to 32 ATP molecules, with the vast majority generated downstream of the cycle itself.
Getting Fuel into the Cycle
The TCA cycle does not care where its fuel comes from. Carbohydrates, fats, and proteins all funnel into it, though they enter through different doors. This convergence is one reason the cycle sits at the center of metabolism.
Carbohydrates take the most direct route. Glucose is first split in half during glycolysis, producing pyruvate. Pyruvate then enters the mitochondria and is irreversibly converted into acetyl-CoA by an enzyme complex called pyruvate dehydrogenase. That conversion links glycolysis to the TCA cycle and represents a one-way commitment: once pyruvate becomes acetyl-CoA, the carbon is headed for oxidation or biosynthesis, not back to glucose.1JNCI: Journal of the National Cancer Institute. Therapeutic Targeting of the Pyruvate Dehydrogenase Complex/Pyruvate Dehydrogenase Kinase (PDC/PDK) Axis in Cancer
Fats arrive by a different path. Fatty acids are chopped into two-carbon units through a process called beta-oxidation, and each fragment is loaded onto CoA to form acetyl-CoA, the same molecule pyruvate dehydrogenase produces. Because a single fatty acid chain can be long, one fat molecule generates many acetyl-CoA units and therefore many turns of the cycle. This is why fats are such a dense source of energy.
Proteins contribute after their amino acids are stripped of their nitrogen-containing groups. Depending on the specific amino acid, the remaining carbon skeleton can enter the cycle at several different points, not just as acetyl-CoA but also as intermediates like alpha-ketoglutarate or oxaloacetate. The TCA cycle links the breakdown of all three major nutrient classes, promoting ATP generation and maintaining cellular energy balance.2BioPublisher. The Central Role of the Citric Acid Cycle in Energy Metabolism: From Metabolic Intermediates to Regulatory Mechanisms – Section: Abstract
The Cycle as a Supply Depot
If the TCA cycle only burned fuel, it would be important but not remarkable. What makes it a metabolic hub is that its intermediates are constantly being pulled out to serve as building blocks elsewhere. Citrate, for example, can be exported from the mitochondria into the cytoplasm via a dedicated transport protein called the citrate carrier, where it is broken down to supply the acetyl-CoA needed for fatty acid synthesis.3PubMed Central. Multiple roles played by the mitochondrial citrate carrier in cellular metabolism and physiology Alpha-ketoglutarate can be converted into the amino acid glutamate. Oxaloacetate feeds into glucose production when blood sugar drops. Succinyl-CoA is a precursor for heme, the iron-carrying component of hemoglobin.
This outflow of intermediates is called cataplerosis, and it creates a problem: if intermediates keep leaving the cycle, the pool of molecules the cycle needs to keep turning will shrink. Cells solve this through anaplerosis, a set of reactions that replenish cycle intermediates from other sources. The most common anaplerotic reaction in many tissues converts pyruvate directly into oxaloacetate, effectively topping off the pool. Together, anaplerosis and cataplerosis regulate how much raw material is available for biosynthesis and play underappreciated roles in controlling a cell’s overall energy status.4PubMed. Ins and Outs of the TCA Cycle: The Central Role of Anaplerosis
This balancing act matters in real life. During intense exercise, for instance, anaplerotic flux increases to keep the cycle running fast enough to meet energy demands. During fasting, cataplerotic flux redirects intermediates toward glucose production in the liver. The cycle is not simply spinning at one speed; it is dynamically adjusting its inputs and outputs based on what the body needs at any given moment.
How the Enzymes Are Organized
For decades, textbooks depicted TCA cycle enzymes floating freely in the mitochondrial matrix, bumping into their substrates by chance. That picture turns out to be incomplete. Research using chemical cross-linking and mass spectrometry has revealed that several of these enzymes physically associate with one another, forming a loosely organized supercomplex sometimes called a metabolon.
One study identified a complex of three sequential enzymes: malate dehydrogenase, citrate synthase, and aconitase. Surface analysis of this three-enzyme unit showed that when the proteins dock together, they create an electrostatic channel between their active sites. This channeling would allow intermediates to pass directly from one enzyme to the next without diffusing away into the surrounding matrix, which would speed up the overall reaction sequence.5PubMed. Krebs cycle metabolon: structural evidence of substrate channeling revealed by cross-linking and mass spectrometry
Separate work in bacteria found a similar story. In the model organism Bacillus subtilis, five TCA cycle enzymes that catalyze sequential reactions were found to physically interact, with citrate synthase, isocitrate dehydrogenase, and malate dehydrogenase forming a core complex, and fumarase and aconitase associating with malate dehydrogenase and each other.6PubMed. Physical interactions between tricarboxylic acid cycle enzymes in Bacillus subtilis: evidence for a metabolon The fact that this organizational principle shows up across such different organisms hints that it is ancient and functionally important, not an accident. Substrate channeling could explain why the cycle operates efficiently even when individual intermediates are present at low concentrations inside the mitochondria.
TCA Metabolites as Signals That Control Cell Fate
One of the more surprising findings in recent years is that TCA cycle intermediates do not just serve as fuel or building blocks. Some of them act as signaling molecules that influence gene expression and even determine what type of cell a stem cell becomes.
Alpha-ketoglutarate is the clearest example. It is a required cosubstrate for a large family of enzymes called alpha-ketoglutarate-dependent dioxygenases. This family includes enzymes that remove methyl marks from DNA and from histones, the proteins that DNA wraps around. Those methyl marks affect which genes are turned on or off, so by supplying or withholding alpha-ketoglutarate, a cell can shift its own gene expression landscape.7Trends in Cell Biology. What Is the TCA Cycle? A Hub for Cellular Metabolism – Section: αKG-Dependent Dioxygenases as Metabolic Mediators of Cell Fate Control
In embryonic stem cells, the ratio of alpha-ketoglutarate to succinate (another TCA intermediate) is elevated compared to more specialized cells. That high ratio promotes demethylation of histones and DNA, helping maintain the stem cell’s ability to become any cell type. Experimentally manipulating this ratio is enough to shift chromatin modifications and alter the expression of genes tied to pluripotency.8PubMed Central. Intracellular α-ketoglutarate maintains the pluripotency of embryonic stem cells In other words, the metabolic state of a cell is not just a consequence of what the cell is doing; it actively steers what the cell becomes.
Succinate and fumarate work in the opposite direction. They inhibit those same dioxygenase enzymes, meaning that when succinate or fumarate levels rise, demethylation slows down and gene silencing increases. This creates a kind of metabolic toggle: the balance among TCA intermediates can push a cell toward growth, differentiation, or even disease.
When the Cycle Goes Wrong
Given the TCA cycle’s central position in metabolism, it is not surprising that disruptions can contribute to serious disease. Two areas of active research stand out: cancer and neurodegeneration.
Cancer and Oncometabolites
Mutations in TCA cycle enzymes can cause intermediates to accumulate to abnormally high levels. The best-studied cases involve isocitrate dehydrogenase (IDH1 and IDH2), succinate dehydrogenase, and fumarate hydratase. When these enzymes are mutated, they produce or allow the buildup of metabolites, sometimes called oncometabolites, that competitively inhibit the alpha-ketoglutarate-dependent dioxygenases described above. The result is widespread changes to DNA and histone methylation patterns, which can silence tumor-suppressor genes and promote uncontrolled growth.9PubMed Central. Oncometabolites in Cancer Metabolism: Mechanistic Insights and Biomarker Potential‐ A Narrative Review
Mutant IDH enzymes are particularly interesting because instead of producing alpha-ketoglutarate normally, they generate an abnormal metabolite called 2-hydroxyglutarate. This molecule acts as a potent inhibitor of the same dioxygenase family, rewiring the cell’s epigenetic landscape. IDH mutations are found in a significant fraction of certain brain cancers and leukemias, and drugs targeting the mutant enzyme have already reached the clinic.
Neurodegeneration
The brain is the most energy-hungry organ in the body, consuming a disproportionate share of the body’s ATP. That makes neurons especially vulnerable to TCA cycle impairment. In Alzheimer’s disease, several TCA cycle enzymes are disrupted, and the resulting drop in ATP production can impair neuronal function, synaptic plasticity, and cognitive processes.10PubMed Central. Impairment of Tricarboxylic Acid Cycle (TCA) Cycle in Alzheimer’s Disease: Mechanisms, Implications, and Potential Therapies
One mechanism under investigation involves abnormal chemical modification of TCA cycle enzymes, particularly alpha-ketoglutarate dehydrogenase. In neurodegenerative conditions, reactive nitrogen species can attach to this enzyme in harmful ways, disrupting mitochondrial energy production and contributing to synapse loss.11PubMed Central. What Is the TCA Cycle? A Hub for Cellular Metabolism This is a relatively new angle of research, but it underscores how tightly the cycle’s integrity is linked to brain health.
The Glyoxylate Shunt and Why You Cannot Live on Fat Alone
Plants, bacteria, and fungi have a metabolic trick that animals lack: the glyoxylate shunt. This pathway is a modified shortcut through part of the TCA cycle that skips the two carbon-dioxide-releasing steps. By doing so, it allows organisms to convert fats into sugars, something your cells fundamentally cannot do. In plants, this capability is critical during seed germination, when stored fats must be turned into the sugars needed for growth. The TCA cycle in higher plants provides electrons for oxidative phosphorylation, intermediates for amino-acid synthesis, and oxaloacetate for gluconeogenesis from fatty-acid-derived succinate via the glyoxylate cycle.12PubMed. Evolution of the enzymes of the citric acid cycle and the glyoxylate cycle of higher plants. A case study of endosymbiotic gene transfer
The glyoxylate shunt also has medical relevance. Mycobacterium tuberculosis, the bacterium that causes tuberculosis, uses this pathway to survive inside human immune cells called macrophages. During infection, the bacterium is starved of carbohydrates and relies on host fatty acids for carbon. The glyoxylate shunt lets it assimilate those fatty acids and persist, and blocking the shunt’s key enzyme, malate synthase, disrupts the bacterium’s ability to grow on fatty acids.13PubMed Central. Glyoxylate detoxification is an essential function of malate synthase required for carbon assimilation in Mycobacterium tuberculosis Because humans do not possess this pathway, it represents a potential drug target that could hit the pathogen without affecting the host.
The absence of the glyoxylate shunt in animals is also why you cannot convert fat into glucose. Your body can break fatty acids into acetyl-CoA and burn them through the TCA cycle, but it cannot use acetyl-CoA as net input for new glucose. That biochemical constraint shapes everything from the way your body responds to starvation (it has to break down muscle protein to make glucose) to why ketogenic diets work the way they do.
A Cycle Older Than Life Itself
The TCA cycle is sometimes described as one of the oldest metabolic pathways on Earth, and recent research suggests its chemical logic may even predate biology. The reverse TCA cycle, which runs the same reactions in the opposite direction to build up carbon compounds instead of breaking them down, has long been proposed as a primordial pathway that could have operated in a prebiotic world.
Experimental work has shown that metal ions naturally present in early Earth conditions can promote multiple reactions of the reverse TCA cycle in sequence without any enzymes. In acidic aqueous solution, zinc, chromium, and metallic iron catalyzed six of the cycle’s eleven reactions in consecutive order.14PubMed Central. Metals promote sequences of the reverse Krebs cycle This does not prove the reverse TCA cycle was operating in volcanic vents billions of years ago, but it shows the chemistry is plausible without biological machinery. Iron catalysis in particular has been proposed as a key driver at the origin of life, with the closure of a complete reverse Krebs cycle eventually displacing simpler carbon-fixation pathways in many lineages.15PubMed. Iron catalysis at the origin of life
The implication is striking: the chemical skeleton of the TCA cycle may have been etched into the fabric of metabolism before there were cells or even genes. Enzymes were recruited later to make the reactions faster and more reliable, but the underlying chemistry was already there, dictated by the thermodynamics of small organic molecules interacting with mineral surfaces.
How Researchers Track the Cycle in Real Time
Studying the TCA cycle in living cells is harder than it might seem. You cannot simply measure how much of each intermediate is present and infer how fast the cycle is spinning, because pool sizes and flux rates are two different things. A large pool of citrate might mean the cycle is running fast and producing a lot, or it might mean citrate is accumulating because something downstream is blocked.
The standard approach for measuring actual flux through the cycle uses carbon-13-labeled nutrients. Researchers feed cells glucose (or another fuel) in which some carbon atoms have been replaced with carbon-13, a stable, non-radioactive isotope. As labeled carbons flow through glycolysis and into the TCA cycle, they end up in downstream metabolites and eventually in amino acids. By measuring the patterns of labeled versus unlabeled carbons in those amino acids using mass spectrometry, researchers can back-calculate the rates of individual reactions in the cycle.16Nature Protocols. 13C-based metabolic flux analysis
The catch is that these flux rates cannot be measured directly. They have to be estimated through computer models that interpret the isotope labeling patterns, which means the results depend on model assumptions.17PubMed Central. Metabolic networks in motion: 13C-based flux analysis Despite this limitation, carbon-13 flux analysis has become the go-to tool for mapping metabolic rewiring in cancer cells, understanding how immune cells shift their metabolism during activation, and identifying which nutrients feed specific biosynthetic pathways. The technique has revealed, for instance, that many cancer cells reroute TCA cycle intermediates toward biosynthesis rather than energy production, a finding that would have been invisible from snapshot measurements of metabolite concentrations alone.