Citric Acid Cycle: Steps, Products, and Regulation

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-19

The citric acid cycle is an eight-step mitochondrial pathway that oxidizes one acetyl-CoA to two molecules of carbon dioxide while reducing three NAD+ to NADH, one FAD to FADH2, and phosphorylating one GDP to GTP. It is also called the tricarboxylic acid cycle, the TCA cycle, or the Krebs cycle, and all four names describe the same set of reactions.

The cycle matters because it is the final common oxidation route for carbohydrates, fats, and most amino acids. Glycolysis and fatty acid beta-oxidation both deliver acetyl-CoA to the same entry point, so the pathway sits at the center of aerobic energy metabolism in nearly every cell type that has mitochondria. Beyond energy, its intermediates are drawn off as biosynthetic precursors, which is why the pathway is described as amphibolic, meaning it serves both catabolic and anabolic roles at once [1]. Flux through the cycle is not constant. It rises and falls with the cell's energy charge, with substrate supply, and with calcium signals that report contractile or secretory demand [2][3].

The Eight Reactions in Order

Each turn of the cycle begins with a two-carbon acetyl group and ends by regenerating the four-carbon acceptor oxaloacetate. Two carbons leave as CO2, but they are not the two carbons that entered on that turn, a point that trips up many students. The table below lists the reaction, enzyme, product, and the main regulator for each step.

Step 1: Citrate Synthase

Citrate synthase condenses the two-carbon acetyl group of acetyl-CoA with the four-carbon oxaloacetate to form six-carbon citrate. The reaction is irreversible and is the committed step, so it is the first place the cell checks whether it actually needs more cycle activity. ATP, NADH, succinyl-CoA, and citrate itself all inhibit the enzyme, which is a direct readout of high energy charge. Because oxaloacetate is usually scarce, the reaction is also limited by how much acceptor is available.

Step 2: Aconitase

Aconitase isomerizes citrate to isocitrate through a dehydration and rehydration sequence that passes through the intermediate cis-aconitate. The enzyme contains an iron-sulfur cluster, and its activity is sensitive to oxidative damage, which is one reason the cycle can slow under oxidative stress [4]. The reaction is reversible and near equilibrium, so it does not act as a major control point.

Step 3: Isocitrate Dehydrogenase

Isocitrate dehydrogenase performs the first oxidative decarboxylation of the cycle. It converts isocitrate to alpha-ketoglutarate while reducing NAD+ to NADH and releasing one CO2. This is a rate-limiting step and one of the two strongest regulatory checkpoints in the pathway. ADP and calcium activate the enzyme, while ATP and NADH inhibit it [1]. Calcium activation links cycle rate to muscle contraction and other calcium-dependent processes.

Step 4: Alpha-Ketoglutarate Dehydrogenase

The alpha-ketoglutarate dehydrogenase complex carries out the second oxidative decarboxylation, converting alpha-ketoglutarate to succinyl-CoA, reducing NAD+ to NADH, and releasing the second CO2. It is a large multienzyme complex that requires thiamine pyrophosphate, lipoic acid, CoA, FAD, and NAD+ as cofactors. Like isocitrate dehydrogenase, it is inhibited by ATP, NADH, and succinyl-CoA and activated by calcium [3]. This enzyme is a major source of mitochondrial hydrogen peroxide, so it also participates in redox signaling beyond simple energy conversion [5][3].

Step 5: Succinyl-CoA Synthetase

Succinyl-CoA synthetase cleaves the high-energy thioester bond of succinyl-CoA and uses the released energy to phosphorylate GDP to GTP. This is the only substrate-level phosphorylation in the cycle. The GTP can be converted to ATP by nucleoside diphosphate kinase, which is why the cycle is often credited with one ATP equivalent per turn. In plants and some bacteria the enzyme uses ADP and produces ATP directly.

Step 6: Succinate Dehydrogenase

Succinate dehydrogenase oxidizes succinate to fumarate and reduces FAD to FADH2. This enzyme is unusual because it is embedded in the inner mitochondrial membrane and is also known as complex II of the electron transport chain. That dual identity means the FADH2 it produces feeds electrons directly into the respiratory chain at complex II, bypassing complex I. Succinate dehydrogenase is inhibited by oxaloacetate and by malonate, a classic competitive inhibitor used in teaching laboratories.

Step 7: Fumarase

Fumarase adds water across the double bond of fumarate to produce malate. The reaction is reversible and runs close to equilibrium, so it is not a regulated step. Fumarase is also found in the cytosol and in some bacteria plays a role in DNA repair, but the mitochondrial form is the one that serves the cycle.

Step 8: Malate Dehydrogenase

Malate dehydrogenase oxidizes malate to oxaloacetate while reducing NAD+ to NADH. The reaction is strongly endergonic as written, but it proceeds because oxaloacetate is continuously consumed by citrate synthase in step 1. This coupling keeps the cycle moving even though the final step is thermodynamically unfavorable on its own. Malate dehydrogenase activity is elevated in some inflammatory states, and it has been measured as a marker of tissue metabolism in oral samples [6].

Energy Yield per Acetyl-CoA

One turn of the cycle, starting from one acetyl-CoA, produces the following:

• 3 NADH (from isocitrate dehydrogenase, alpha-ketoglutarate dehydrogenase, and malate dehydrogenase)
• 1 FADH2 (from succinate dehydrogenase)
• 1 GTP (from succinyl-CoA synthetase, equivalent to 1 ATP)
• 2 CO2 (from the two oxidative decarboxylations)

The NADH and FADH2 are not ATP yet. They carry electrons to the electron transport chain, where oxidative phosphorylation converts them to ATP. Using the modern estimate of about 2.5 ATP per NADH and 1.5 ATP per FADH2, one acetyl-CoA yields roughly 10 ATP equivalents in total. The exact number depends on the P/O ratios used, and older textbooks cite 12 ATP because they assumed 3 ATP per NADH and 2 ATP per FADH2.

The important point for exams and for bench work is that the cycle itself makes only one GTP directly. Everything else is deferred to the respiratory chain. This is why cells with defective oxidative phosphorylation can still run the cycle for biosynthesis but cannot harvest its full energy content.

Regulation at Three Checkpoints

The cycle is regulated primarily at citrate synthase, isocitrate dehydrogenase, and alpha-ketoglutarate dehydrogenase. These three enzymes share a common logic: they are inhibited when the cell has plenty of energy and activated when it needs more.

ATP and NADH as Inhibitors

ATP and NADH are the products of energy metabolism. When their concentrations rise, the cell is well supplied and the cycle slows. All three checkpoint enzymes are inhibited by ATP and by NADH, and alpha-ketoglutarate dehydrogenase is also inhibited by its own product, succinyl-CoA [1][3]. This feedback keeps the cycle from overproducing NADH that the respiratory chain cannot handle, which would otherwise drive excessive reactive oxygen species formation [2].

Calcium as an Activator

Calcium activates isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase. When a muscle contracts or a cell receives a calcium signal, the cycle speeds up to match the increased demand for ATP. This is a feed-forward mechanism, not a feedback one, and it explains why exercise increases cycle flux in skeletal muscle. Calcium works alongside ADP, which also activates isocitrate dehydrogenase by signaling a low energy charge.

Substrate Availability

The cycle also responds to substrate supply. Acetyl-CoA availability depends on glycolysis and fatty acid oxidation, and oxaloacetate availability depends on anaplerotic reactions. When oxaloacetate is low, citrate synthase cannot run even if ATP and NADH are low, so the cycle stalls at the first step. This is why anaplerosis matters as much as the checkpoint enzymes.

Anaplerotic Reactions That Replenish Oxaloacetate

The cycle constantly loses intermediates to biosynthesis. Amino acid synthesis consumes alpha-ketoglutarate and oxaloacetate, heme synthesis consumes succinyl-CoA, and gluconeogenesis consumes oxaloacetate. Without replacement, the cycle would run down. Anaplerotic reactions, from the Greek for "filling up," replenish these pools.

The most important anaplerotic reaction in most cells is the carboxylation of pyruvate to oxaloacetate by pyruvate carboxylase. This enzyme requires biotin and ATP and is activated by acetyl-CoA, which signals that the cycle has plenty of acetyl groups but not enough acceptor. In bacteria, phosphoenolpyruvate carboxylase serves a similar role, and it is a regulated node. In Escherichia coli, the small RNA OmrA inhibits translation of phosphoenolpyruvate carboxylase, reducing cycle flux, and growth can be restored by supplementing glutamate, glutamine, or downstream cycle metabolites [7].

Other anaplerotic routes include the conversion of glutamate to alpha-ketoglutarate by glutamate dehydrogenase, the conversion of aspartate to oxaloacetate by transamination, and the breakdown of isoleucine, valine, and methionine to succinyl-CoA. Malic enzyme, which converts malate to pyruvate while producing NADPH, can also contribute to cycle balance. In stem cells exiting naive pluripotency, pyruvate carboxylase and malic enzyme create a cyclical carbon flow that supports both metabolism and histone acetylation [8]. This shows that anaplerosis is not just a maintenance function. It shapes cell fate.

The Cycle Is Amphibolic

The term amphibolic means the pathway serves both breakdown and building. Catabolically, it oxidizes acetyl-CoA to CO2 and feeds electrons to the respiratory chain. Anabolically, its intermediates are starting materials for other pathways.

• Oxaloacetate is a precursor for gluconeogenesis and for aspartate.
• Alpha-ketoglutarate is a precursor for glutamate, glutamine, and other amino acids.
• Succinyl-CoA is a precursor for heme synthesis.
• Citrate is exported to the cytosol for fatty acid synthesis and for the production of acetyl-CoA for histone acetylation.
• Malate can be converted to pyruvate to generate NADPH for reductive biosynthesis.

Because of these dual roles, the cycle cannot be understood as a simple energy generator. It is a distribution hub. The reverse tricarboxylic acid cycle, used by some anaerobic bacteria and archaea to fix CO2, is a related but distinct pathway that runs the same reactions in the opposite direction [9][10][11]. The forward oxidative cycle and the reverse reductive cycle share enzymes but serve opposite metabolic goals, which is a useful reminder that pathway direction is set by the cell's needs, not by the chemistry alone.

How the Cycle Is Studied in Practice

Several laboratory methods measure cycle activity directly. Stable isotope labeling with 13C-labeled substrates is the standard approach. Cells or tissues are incubated with 13C-glucose, 13C-glutamine, or 13C-acetate, and mass spectrometry tracks which carbons end up in which intermediates. This technique has been used to show that sperm increase flux through the cycle after capacitation, with glycolytically derived pyruvate feeding the mitochondria to maximize energy yield [12]. The same approach revealed that glutamine becomes the dominant carbon source for the cycle and for histone acetylation as stem cells exit naive pluripotency [8].

Breath tests offer a less invasive option. Administering 13C-labeled 2-oxoglutarate or 13C-labeled citrate and measuring 13C in exhaled CO2 provides an index of cycle flux, and these substrates have been evaluated as potential probes in animal models [13].

Enzyme activity assays measure individual steps. Malate dehydrogenase and triosephosphate isomerase activities were elevated in samples from participants with gingivitis, and treatment with a stannous fluoride dentifrice reduced those activities, showing that cycle enzyme activity can be tracked in clinical samples [6]. Dehydrogenase-like nanozymes have also been engineered to convert alpha-ketoglutarate to succinate and disrupt the cycle in tumor cells, which is a therapeutic application of the same chemistry [14].

Common Mistakes and Limitations

Students mix up the carbon accounting. The two CO2 molecules released per turn come from the carbons that entered the cycle in a previous turn, not from the acetyl-CoA that just entered. The acetyl carbons are still in the cycle intermediates at the end of the turn.

Students also confuse the cycle with oxidative phosphorylation. The cycle produces NADH and FADH2, which are electron carriers. It does not produce most of the ATP directly. Only one GTP is made per turn.

Another frequent error is treating the cycle as a closed loop that only makes energy. In reality, intermediates leave constantly, and anaplerotic reactions must replace them. A cell with high biosynthetic demand can run the cycle at high flux while exporting most of the carbon.

The regulation is often oversimplified as "ATP inhibits, ADP activates." Calcium is an equally important activator at two of the three checkpoints, and substrate availability can override the nucleotide signals entirely.

Finally, the pathway is not identical in all organisms. Bacteria, plants, and fungi have variations in enzyme isoforms, cofactor use, and anaplerotic routes [15][16][17]. The version taught in most courses is the mammalian mitochondrial one, and it should not be assumed to apply without modification to every species.

Quick Review

• Eight reactions convert one acetyl-CoA to two CO2, three NADH, one FADH2, and one GTP.
• Citrate synthase, isocitrate dehydrogenase, and alpha-ketoglutarate dehydrogenase are the regulated steps.
• ATP and NADH inhibit. ADP and calcium activate.
• Anaplerotic reactions, especially pyruvate carboxylase, replenish oxaloacetate.
• The cycle is amphibolic, supplying both energy and biosynthetic precursors.
• Succinate dehydrogenase is also complex II of the electron transport chain.
• The cycle is a hub, not an isolated loop.

Frequently Asked Questions

What is the net yield of one turn of the citric acid cycle?

One turn produces 3 NADH, 1 FADH2, 1 GTP, and 2 CO2 from one acetyl-CoA. The NADH and FADH2 are later converted to ATP by oxidative phosphorylation.

Why is the citric acid cycle called amphibolic?

It is amphibolic because it serves both catabolism and anabolism. It oxidizes acetyl-CoA for energy and also supplies intermediates for biosynthesis.

What are the three main regulatory enzymes of the cycle?

Citrate synthase, isocitrate dehydrogenase, and alpha-ketoglutarate dehydrogenase. All three are inhibited by ATP and NADH, and the latter two are activated by calcium.

What is an anaplerotic reaction?

An anaplerotic reaction replenishes cycle intermediates that have been removed for biosynthesis. Pyruvate carboxylase, which converts pyruvate to oxaloacetate, is the classic example.

Why does the cycle need oxygen even though it does not use oxygen directly?

The cycle produces NADH and FADH2, which must be reoxidized by the electron transport chain. Without oxygen as the final electron acceptor, NAD+ and FAD cannot be regenerated, and the cycle stops.

Is the citric acid cycle the same as the Krebs cycle?

Yes. Citric acid cycle, tricarboxylic acid cycle, TCA cycle, and Krebs cycle are all names for the same pathway.

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