GO:0006099 tricarboxylic acid cycle: Central Metabolic Hub, Genes, Functions and Research Methods
Research-grade guide for scientists and biopharma professionals
Key Takeaways
• GO:0006099 (tricarboxylic acid cycle, TCA cycle) is a nearly universal metabolic pathway that oxidizes the acetyl group of acetyl-CoA to two CO2 molecules and transfers four pairs of electrons to coenzymes.
• In eukaryotes, the TCA cycle is confined to the mitochondria, where it supplies reducing equivalents (NADH, FADH2) to the electron transport chain and provides biosynthetic precursors.
• Dysregulation of TCA cycle enzymes and metabolites is implicated in cancer, neurodegeneration, viral infections, aging, and immune cell polarization [3, 4, 5, 6, 2].
• Key TCA cycle genes include citrate synthase (CS), aconitase (ACO2), isocitrate dehydrogenase (IDH2/IDH3), 2-oxoglutarate dehydrogenase (OGDH), succinate dehydrogenase (SDHA-D), fumarate hydratase (FH), and malate dehydrogenase (MDH2) [1, 7].
• Mutations in TCA cycle genes cause human diseases such as SDH-deficient tumors, fumarate hydratase deficiency, and 2-oxoglutarate dehydrogenase deficiency.
• CRISPR knockout, point mutation, knock-in, and overexpression models are essential for dissecting the causal roles of TCA cycle genes in health and disease [1, 3].
Description
The tricarboxylic acid (TCA) cycle, also known as the citric acid cycle or Krebs cycle, is a central metabolic pathway that oxidizes acetyl-CoA to CO2 while generating reducing equivalents and biosynthetic precursors. This pathway is nearly universal across aerobic organisms and is confined to the mitochondria in eukaryotes. The TCA cycle is not only a bioenergetic hub but also a signaling node that influences immunity, cancer, aging, and infection [3, 4, 5, 2]. Researchers study GO:0006099 to understand how metabolic flux is reprogrammed in disease and to identify therapeutic targets [1, 6].
tricarboxylic acid cycle At A Glance
| GO ID | GO:0006099 |
|---|---|
| GO term | tricarboxylic acid cycle |
| Ontology | biological_process |
| Synonym | citric acid cycle, Krebs cycle, oxidative TCA cycle, TCA cycle |
| Major function | Oxidation of acetyl-CoA to CO2, generation of NADH/FADH2, and provision of biosynthetic precursors |
| Subcellular location | Mitochondria in eukaryotes |
| Key enzymes | CS, ACO2, IDH2/IDH3, OGDH, SDHA-D, FH, MDH2 |
| Related pathway | Glyoxylate cycle (see also) |
What Is GO:0006099?
GO:0006099 (tricarboxylic acid cycle) is defined as a nearly universal metabolic pathway in which the acetyl group of acetyl coenzyme A is effectively oxidized to two CO2 molecules and four pairs of electrons are transferred to coenzymes. The acetyl group combines with oxaloacetate to form citrate, which undergoes successive transformations to isocitrate, 2-oxoglutarate, succinyl-CoA, succinate, fumarate, malate, and oxaloacetate again, thus completing the cycle. In eukaryotes, the TCA cycle is confined to the mitochondria.
Why Is tricarboxylic acid cycle Important in Cell Biology?
The TCA cycle is essential for cellular energy production and biosynthesis, and its dysfunction is linked to a wide range of human diseases, including cancer, neurodegeneration, and immune disorders [1, 3, 6]. Understanding its regulation and genetic control is critical for developing targeted therapies and for interpreting metabolic phenotypes in research [1, 5].
• Central to oxidative metabolism and ATP production in mitochondria.
• Provides precursors for amino acids, lipids, and nucleotides.
• Dysregulated in cancer, contributing to tumor growth and immune evasion.
• Involved in viral infections by modulating host immune responses.
• Plays a role in aging and metabolic interventions.
• Impaired in Alzheimer's disease and other neurodegenerative conditions.
• Mutations in TCA cycle genes cause inherited metabolic disorders and tumors.
• Essential for Plasmodium metabolism, offering antimalarial targets.
• Modulates macrophage polarization and inflammation.
• Serves as a hub for metabolic reprogramming in immunity and cancer.
What Happens During tricarboxylic acid cycle?
Formation of Citrate from Acetyl-CoA and Oxaloacetate
In simple terms: The cycle starts by joining a two-carbon acetyl group with a four-carbon molecule to make a six-carbon citrate.
The TCA cycle begins with the condensation of acetyl-CoA and oxaloacetate to form citrate, catalyzed by citrate synthase (CS). This reaction is irreversible and commits the acetyl group to oxidation.
Isomerization and Oxidative Decarboxylations
In simple terms: Citrate is rearranged and then two carbon atoms are removed as CO2, generating energy-rich molecules.
Citrate is isomerized to isocitrate by aconitase (ACO2), which is then oxidatively decarboxylated by isocitrate dehydrogenase (IDH2/IDH3) to 2-oxoglutarate, producing NADH and CO2. 2-Oxoglutarate is further decarboxylated by the 2-oxoglutarate dehydrogenase complex (OGDH) to succinyl-CoA, generating another NADH and CO2.
Substrate-Level Phosphorylation and Oxidation to Oxaloacetate
In simple terms: The cycle converts succinyl-CoA to succinate, then oxidizes it stepwise back to oxaloacetate, capturing electrons.
Succinyl-CoA is converted to succinate by succinyl-CoA synthetase, generating GTP (or ATP) via substrate-level phosphorylation. Succinate is oxidized to fumarate by succinate dehydrogenase (SDHA-D), which also feeds electrons to the respiratory chain. Fumarate is hydrated to malate by fumarate hydratase (FH), and malate is oxidized to oxaloacetate by malate dehydrogenase (MDH2), producing NADH.
Electron Transfer and Energy Yield
In simple terms: The cycle harvests electrons in the form of NADH and FADH2, which are used to make ATP.
Each turn of the TCA cycle yields three NADH, one FADH2, one GTP, and two CO2. The reduced coenzymes donate electrons to the electron transport chain, driving oxidative phosphorylation and ATP synthesis.
Anaplerosis and Biosynthetic Roles
In simple terms: The cycle also provides building blocks for other molecules and can be replenished by side reactions.
TCA cycle intermediates are used for biosynthesis of amino acids, lipids, and heme, and anaplerotic reactions replenish cycle intermediates. This dual role makes the TCA cycle a central metabolic hub.
Key Genes Involved in GO:0006099 tricarboxylic acid cycle
The following genes encode core enzymes and regulators of the tricarboxylic acid cycle, and their study is essential for understanding metabolic control in health and disease.
| Gene | Major Role | Research Relevance |
|---|---|---|
| CS | Condenses acetyl-CoA and oxaloacetate to citrate | Rate-limiting step; target for metabolic flux analysis |
| ACO2 | Isomerizes citrate to isocitrate | Mutations linked to neurodegeneration and mitochondrial dysfunction |
| IDH2 | Oxidative decarboxylation of isocitrate to 2-oxoglutarate | Mutations in cancer and leukemia |
| IDH3A | Catalytic subunit of isocitrate dehydrogenase | Regulates TCA flux and NADH production |
| OGDH | Decarboxylates 2-oxoglutarate to succinyl-CoA | Deficiency causes metabolic disorders |
| SDHA | Oxidizes succinate to fumarate | Mutations in paraganglioma and pheochromocytoma |
| SDHB | Electron transfer subunit of succinate dehydrogenase | Tumor suppressor; mutations in cancers |
| SDHC | Membrane anchor subunit of succinate dehydrogenase | Associated with hereditary paraganglioma |
| SDHD | Small subunit of succinate dehydrogenase | Implicated in familial tumors |
| FH | Hydrates fumarate to malate | Mutations cause fumarate hydratase deficiency and leiomyomatosis |
| MDH2 | Oxidizes malate to oxaloacetate | Regulates NADH/NAD+ ratio |
| DLST | Dihydrolipoamide succinyltransferase component of OGDH complex | Deficiency linked to neurological disorders |
| DLD | Dihydrolipoamide dehydrogenase component of OGDH complex | Mutations cause E3 deficiency |
| SUCLA2 | Succinyl-CoA synthetase beta subunit | Mutations cause mitochondrial DNA depletion syndrome |
| SUCLG1 | Succinyl-CoA synthetase alpha subunit | Deficiency leads to fatal infantile lactic acidosis |
| PC | Pyruvate carboxylase, anaplerotic enzyme | Supplies oxaloacetate for TCA cycle |
| GOT2 | Aspartate aminotransferase, links amino acid metabolism to TCA | Regulates malate-aspartate shuttle |
How Is tricarboxylic acid cycle Regulated?
The TCA cycle is regulated at multiple levels, including allosteric control by NADH/NAD+ and ATP/ADP ratios, calcium signaling, and transcriptional regulation of enzyme genes. Post-translational modifications such as acetylation and phosphorylation also modulate enzyme activity. In cancer and immune cells, oncogenic signaling and hypoxia-inducible factors reprogram TCA cycle flux [3, 2].
tricarboxylic acid cycle and Human Disease
| Gene | Disease / Biology | Potential Experimental Model |
|---|---|---|
| SDHB | Paraganglioma, pheochromocytoma | Knockout in chromaffin cells |
| FH | Hereditary leiomyomatosis and renal cell cancer | Point mutation knock-in in renal cells |
| IDH2 | Acute myeloid leukemia, glioma | Overexpression of mutant IDH2 in hematopoietic cells |
| OGDH | Neurodegeneration, metabolic disorders | Knockout in neurons |
| ACO2 | Neurodegeneration, mitochondrial dysfunction | Knock-in of patient mutations in iPSC-derived neurons |
Cancer
Mutations in TCA cycle genes such as SDH subunits and FH lead to accumulation of oncometabolites (succinate, fumarate) that inhibit 2-oxoglutarate-dependent dioxygenases, promoting tumorigenesis [3, 7]. IDH mutations produce 2-hydroxyglutarate, which alters epigenetic landscapes.
Neurodegeneration
Impaired TCA cycle activity is observed in Alzheimer's disease and other neurodegenerative disorders, contributing to energy deficits and oxidative stress. Mutations in ACO2 and OGDH are associated with neurological phenotypes.
Infections and Immunity
TCA cycle metabolites modulate macrophage polarization and inflammatory responses. Viruses can hijack TCA cycle intermediates to support replication. In Plasmodium, the TCA cycle is essential for survival, making it a drug target.
Aging
The TCA cycle acts as a central regulator of aging rate, and metabolic interventions targeting it can extend lifespan in model organisms.
From tricarboxylic acid cycle-Related Genes to Experimental Models
| Research Question | Suitable Model |
|---|---|
| Does loss of SDHB drive tumorigenesis? | SDHB knockout cell line |
| How does IDH2 mutation affect metabolism? | IDH2 point mutation knock-in |
| Can wild-type FH rescue fumarate accumulation? | FH overexpression |
| What is the role of OGDH in neuronal survival? | OGDH knockout in primary neurons |
| Does ACO2 mutation impair mitochondrial function? | ACO2 knock-in in iPSCs |
| How does CS level affect flux? | CS overexpression or knockout |
How to Study the tricarboxylic acid cycle Process
| Method | What It Measures | Typical Application |
|---|---|---|
| LC-MS metabolomics | Levels of TCA intermediates | Profiling metabolic reprogramming |
| 13C isotope tracing | Flux through TCA cycle | Determining pathway activity |
| RNA-seq | Gene expression changes | Identifying regulators of TCA genes |
| CRISPR knockout screens | Essentiality of TCA genes | Cancer dependency mapping |
| Proteomics | Protein abundance and modifications | Studying enzyme regulation |
| Seahorse respirometry | Oxygen consumption rate | Measuring mitochondrial function |
| Western blot | Protein expression | Validating knockout or overexpression |
Metabolomics and Flux Analysis
Mass spectrometry-based metabolomics and isotope tracing are used to quantify TCA cycle intermediates and flux.
Genomic and Transcriptomic Profiling
RNA-seq and CRISPR screens identify genes that regulate TCA cycle activity and dependencies [1, 3].
Proteomics and Post-Translational Modifications
Proteomics reveals enzyme abundance and modifications such as acetylation that affect TCA cycle function.
Imaging and Functional Assays
Seahorse respirometry and live-cell imaging measure mitochondrial respiration and TCA cycle-dependent oxygen consumption.
How CRISPR Can Be Used to Study GO:0006099 tricarboxylic acid cycle
Knockout
CRISPR knockout of TCA cycle genes such as SDHB or FH is used to model loss-of-function and study metabolic consequences [1, 3].
Point Mutation
Point mutation knock-in models, such as IDH2 R140Q or FH mutations, recapitulate disease-associated alleles for mechanistic studies.
Knock-in
Tagged knock-in of TCA enzymes enables live-cell imaging and interaction studies.
Overexpression
Overexpression of CS or MDH2 is used to test sufficiency in driving TCA flux and biosynthetic output.
How EDITGENE Supports tricarboxylic acid cycle Research
Researchers studying tricarboxylic acid cycle-related genes often need to determine whether a candidate gene is causally involved in metabolic regulation, disease progression, or therapeutic response. Precise genetic models are essential to move from correlation to causation.
Contact EDITGENE today to design your custom CRISPR model for tricarboxylic acid cycle research.
Frequently Asked Questions About tricarboxylic acid cycle
What is the tricarboxylic acid cycle?
The tricarboxylic acid cycle (TCA cycle), also known as the citric acid cycle or Krebs cycle, is a metabolic pathway that oxidizes acetyl-CoA to CO2 and generates NADH, FADH2, and GTP.
What genes are involved in the tricarboxylic acid cycle?
Key genes include CS, ACO2, IDH2, IDH3A, OGDH, SDHA, SDHB, SDHC, SDHD, FH, and MDH2.
Where does the tricarboxylic acid cycle occur?
In eukaryotes, the TCA cycle occurs in the mitochondria.
What is the function of the TCA cycle?
It generates energy and biosynthetic precursors, and it is central to oxidative metabolism.
How is the TCA cycle regulated?
It is regulated by substrate availability, allosteric effectors, calcium signaling, and post-translational modifications.
What diseases are associated with TCA cycle dysfunction?
Cancer, neurodegeneration, metabolic disorders, and infections are linked to TCA cycle dysfunction [3, 6, 7].
What are TCA cycle metabolites?
They include citrate, isocitrate, 2-oxoglutarate, succinyl-CoA, succinate, fumarate, malate, and oxaloacetate.
How can CRISPR be used to study the TCA cycle?
CRISPR knockout, point mutation, knock-in, and overexpression models allow functional dissection of TCA cycle genes [1, 3].
What is the role of the TCA cycle in cancer?
Mutations in TCA enzymes can lead to oncometabolite accumulation and tumorigenesis [3, 7].
What is the role of the TCA cycle in immunity?
TCA cycle metabolites modulate immune cell function and inflammation.
Conclusion
The tricarboxylic acid cycle (GO:0006099) is a fundamental metabolic pathway with broad implications for health and disease. Its genetic and biochemical dissection continues to reveal new therapeutic opportunities. Precise CRISPR models are indispensable for advancing this field.
References
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- 2. Yang Y et al.. 2024. Tricarboxylic acid cycle metabolites: new players in macrophage.. Inflamm Res 73(4):531-539 PMID: 38498178
- 3. Scagliola A et al.. 2020. The Tricarboxylic Acid Cycle at the Crossroad Between Cancer and Immunity.. Antioxid Redox Signal 32(12):834-852 PMID: 31847530
- 4. Sánchez-García FJ et al.. 2021. The Role of Tricarboxylic Acid Cycle Metabolites in Viral Infections.. Front Cell Infect Microbiol 11:725043 PMID: 34595133
- 5. Borkum JM. 2023. The Tricarboxylic Acid Cycle as a Central Regulator of the Rate of Aging: Implications for Metabolic Interventions.. Adv Biol (Weinh) 7(7):e2300095 PMID: 37132059
- 6. Mohan GS et al.. 2025. Impairment of Tricarboxylic Acid Cycle (TCA) Cycle in Alzheimer's Disease: Mechanisms, Implications, and Potential Therapies.. Aging Dis 16(5):2553-2574 PMID: 40479573
- 7. Brière JJ et al.. 2006. Tricarboxylic acid cycle dysfunction as a cause of human diseases and tumor formation.. Am J Physiol Cell Physiol 291(6):C1114-20 PMID: 16760265
- 8. Suryavanshi A et al.. 2025. Metabolic Flexibility and Essentiality of the Tricarboxylic Acid Cycle in Plasmodium.. ACS Infect Dis 11(2):335-349 PMID: 39869313
