The TCA cycle is a series of eight chemical reactions that form a continuous loop inside your cells, breaking down nutrients to produce the raw materials your body uses to generate energy. Also called the citric acid cycle or the Krebs cycle, it takes place in the mitochondria and serves as the central hub of metabolism, connecting the breakdown of carbohydrates, fats, and proteins to the production of usable cellular fuel.
How Nutrients Enter the Cycle
Before anything can enter the TCA cycle, it first needs to be converted into a small two-carbon molecule called acetyl-CoA. For carbohydrates, this happens through a bridge reaction: after glucose is broken down into pyruvate during glycolysis (which occurs in the main body of the cell), pyruvate is transported into the mitochondria. There, a large enzyme complex strips off one carbon (released as CO2) and attaches what remains to a helper molecule, producing acetyl-CoA. This reaction is the primary entry point for carbohydrates into the cycle.
Fats and proteins can also feed into the cycle. Fatty acids are broken into two-carbon units that become acetyl-CoA directly, while certain amino acids are converted into various cycle intermediates. This flexibility is what makes the TCA cycle a metabolic crossroads rather than a one-track pathway.
What Happens in Each Turn
The cycle begins when acetyl-CoA (two carbons) combines with a four-carbon molecule called oxaloacetate to form citrate, a six-carbon molecule. This is the reaction that gives the cycle its alternate name, the citric acid cycle. From there, citrate is rearranged into a closely related molecule called isocitrate through a two-step process involving dehydration and rehydration.
The next two steps are where the cycle sheds carbon. Isocitrate loses one carbon as CO2 and generates a carrier molecule loaded with electrons (NADH), producing a five-carbon compound. That five-carbon compound then loses another carbon as CO2, generating a second NADH and forming succinyl-CoA, a four-carbon molecule with a high-energy bond.
Succinyl-CoA’s high-energy bond is used to directly produce one GTP (essentially equivalent to one ATP), converting succinyl-CoA into succinate. Succinate is then oxidized to fumarate, generating one FADH2, another electron carrier. Fumarate picks up a water molecule to become malate, and finally malate is oxidized back into oxaloacetate, producing a third NADH. With oxaloacetate regenerated, the cycle is ready to accept another acetyl-CoA and start again.
Energy Output Per Turn
Each complete turn of the TCA cycle produces three NADH molecules, one FADH2, one GTP, and two molecules of CO2. The CO2 is the waste product you eventually exhale. The NADH and FADH2 are the real energy payoff: they carry high-energy electrons to the next stage of energy production, the electron transport chain, where the vast majority of ATP is actually made.
To put this in perspective, glycolysis (the initial breakdown of glucose) produces a net of two ATP per glucose molecule and doesn’t require oxygen. The TCA cycle itself generates only one GTP directly, but the NADH and FADH2 it produces go on to drive the creation of roughly 9 to 10 additional ATP per turn through the electron transport chain, which does require oxygen. Since each glucose molecule generates two acetyl-CoA (and therefore two turns of the cycle), the TCA cycle’s contribution to total energy yield far exceeds what glycolysis provides on its own.
How the Cycle Regulates Itself
Your cells don’t run the TCA cycle at full speed all the time. Three key enzymes act as control points, adjusting the cycle’s pace based on the cell’s energy status. All three are slowed down when NADH levels are high, which signals that the cell already has plenty of electron carriers waiting to be processed.
The first checkpoint is at the very beginning, where acetyl-CoA and oxaloacetate combine. This enzyme is also inhibited by succinyl-CoA, one of the cycle’s own intermediates, creating a built-in feedback brake. The second checkpoint, where isocitrate is converted and CO2 is released, responds to the ratio of ATP to ADP: when ATP is abundant relative to ADP, the enzyme slows down because the cell doesn’t need more energy. The third checkpoint, at the next carbon-releasing step, is also slowed by its own product, succinyl-CoA. Together, these controls ensure the cycle speeds up when energy demand is high and throttles back when the cell is well supplied.
More Than Just Energy
The TCA cycle isn’t only about burning fuel. Several of its intermediate molecules serve as raw materials for building other things your body needs. Citrate can be exported from the mitochondria and used for fat synthesis, a process that is particularly active in the liver. Succinyl-CoA provides the backbone for heme, the iron-containing component of hemoglobin in red blood cells. Other intermediates feed into the production of amino acids like aspartate, linking the cycle to protein metabolism.
Because these “exit ramps” constantly pull intermediates out of the cycle, the cell needs ways to refill them. These replenishment reactions are called anaplerotic pathways. One of the most important converts pyruvate directly into oxaloacetate, bypassing acetyl-CoA entirely. Without these refueling mechanisms, the cycle would stall as its components were siphoned off for biosynthesis.
What Happens When the Cycle Goes Wrong
Genetic defects in TCA cycle enzymes are rare but can be severe. Mutations in the enzyme that converts fumarate to malate cause fumaric aciduria when both copies of the gene are affected, resulting in severe neurological problems and developmental delays that appear in infancy. When only one copy is mutated, the consequences are different: these individuals are predisposed to noncancerous tumors of the skin and uterus, and to an aggressive form of kidney cancer.
Mutations in the enzyme that converts succinate to fumarate have been linked to a range of tumors, including paragangliomas and pheochromocytomas (tumors of nerve tissue and the adrenal glands), as well as gastrointestinal stromal tumors, kidney cancers, and thyroid tumors. The connection between TCA cycle defects and cancer appears to involve a buildup of cycle intermediates that can alter how genes are expressed and how cells respond to oxygen levels, pushing cells toward uncontrolled growth.
These discoveries have shifted the understanding of the TCA cycle from a purely energy-generating pathway to one that plays a direct role in cell signaling and disease. Intermediates that accumulate when the cycle is disrupted don’t just represent a metabolic bottleneck. They act as chemical signals that can reprogram cell behavior in ways that promote tumor formation.