GO:0035999 folate cycle: One-Carbon Metabolism, Genes, Functions and Research Methods
Research-grade guide for scientists and biopharma professionals
Key Takeaways
• GO:0035999 (folate cycle) describes the cyclic interconversion of tetrahydrofolate (THF)-bound one-carbon units among 10-formyl-THF, 5,10-methenyl-THF, 5,10-methylene-THF and 5-methyl-THF.
• The cycle couples one-carbon acquisition from serine, glycine or formate to biosynthetic transfer reactions and to regeneration of THF.
• Key enzymes include SHMT1/2, MTHFD1/2, MTHFR, MTR, MTHFD1L, TYMS, DHFR and MTHFS, which collectively maintain nucleotide synthesis and methionine remethylation.
• Disruption of folate cycle flux is linked to neural tube defects, cardiovascular risk, cancer, and autism spectrum disorder.
• Both folate deficiency and excess folate can perturb one-carbon homeostasis, making quantitative flux analysis essential.
• CRISPR knockout, point-mutation, knock-in and overexpression models enable causal testing of folate cycle gene variants in isogenic backgrounds.
Description
The folate cycle (GO:0035999) is a central metabolic hub that distributes one-carbon units carried by tetrahydrofolate (THF) to nucleotide biosynthesis, methionine remethylation, and other methylation reactions. It is defined by the reversible interconversion of THF-bound one-carbon species, including 10-formyl-THF, 5,10-methenyl-THF, 5,10-methylene-THF and 5-methyl-THF, which differ in oxidation state and biosynthetic destination. Because these interconversions are coupled to serine, glycine and formate metabolism, the cycle integrates amino acid catabolism with DNA synthesis and epigenetic maintenance. Researchers study GO:0035999 to understand how metabolic flux is rewired in cancer, how inherited variants influence cardiovascular and neurodevelopmental risk, and how folate status modulates drug response. The pathway is also a paradigm for compartmentalized metabolism, since mitochondrial and cytosolic folate enzymes perform distinct but interconnected reactions.
folate cycle At A Glance
| GO ID | GO:0035999 |
|---|---|
| GO term | folate cycle |
| Ontology | biological_process |
| Synonym | folate-mediated one-carbon metabolism; folate transformations; folic acid cycle; tetrahydrofolate interconversion |
| Major function | Cyclic interconversion of THF-bound one-carbon units for nucleotide synthesis, methionine remethylation and methylation reactions |
| Key intermediates | 10-formyl-THF, 5,10-methenyl-THF, 5,10-methylene-THF, 5-methyl-THF |
| One-carbon donors | Serine, glycine, formate |
| Subcellular locations | Cytosol and mitochondria |
| Representative enzymes | SHMT1/2, MTHFD1/2, MTHFR, MTR, MTHFD1L, TYMS, DHFR, MTHFS |
What Is GO:0035999?
In our own words, GO:0035999 (folate cycle) is a biological process in which tetrahydrofolate (THF) accepts one-carbon units from donors such as serine, glycine or formate and then cycles through a series of THF-bound intermediates, including 10-formyl-THF, 5,10-methenyl-THF, 5,10-methylene-THF and 5-methyl-THF. These interconversions allow the one-carbon unit to be delivered in the appropriate oxidation state to biosynthetic acceptors, such as thymidylate synthase for dTMP synthesis or methionine synthase for homocysteine remethylation, while regenerating THF to sustain the cycle. The process is therefore both a donor-processing and an acceptor-supplying system that links amino acid metabolism to nucleotide synthesis and methylation.
Why Is folate cycle Important in Cell Biology?
The folate cycle is essential because it supplies one-carbon units for de novo purine and thymidylate synthesis, supports methionine regeneration from homocysteine, and influences the global methylation potential of the cell. Perturbations in this cycle are associated with neural tube defects, cardiovascular disease, cancer and neurodevelopmental conditions, and folate status is a modifiable factor in public health. Understanding GO:0035999 therefore has direct implications for nutrition, pharmacogenomics and oncology.
• Provides one-carbon units for de novo purine and thymidylate synthesis, which are required for DNA replication and repair.
• Regenerates methionine from homocysteine via methionine synthase, linking folate status to the methionine remethylation cycle.
• Maintains S-adenosylmethionine (SAM) levels and therefore supports DNA and histone methylation reactions.
• Genetic variants such as MTHFR C677T influence homocysteine levels and cardiovascular risk.
• Disrupted folate metabolism is a metabolic hallmark of many cancers and a target for antifolate chemotherapy.
• Folate deficiency remains a global micronutrient problem associated with adverse pregnancy outcomes.
• Excess folate intake can also perturb one-carbon homeostasis and may have unintended effects.
• Folate cycle enzymes are compartmentalized between cytosol and mitochondria, enabling metabolic flexibility.
• The cycle intersects with serine/glycine metabolism, making it responsive to nutrient availability.
• It is a model system for studying gene-environment interactions in human disease.
What Happens During folate cycle?
One-carbon acquisition from serine and glycine
In simple terms: The cycle starts by taking a one-carbon unit from serine or glycine.
Serine hydroxymethyltransferase (SHMT1 in cytosol, SHMT2 in mitochondria) transfers a one-carbon unit from serine to THF, producing 5,10-methylene-THF and glycine. This reaction is a major entry point for one-carbon units into the folate cycle and is reversible, allowing the cycle to respond to metabolic demand. In mitochondria, the glycine cleavage system can also contribute one-carbon units, and formate can enter the cytosolic cycle after transport.
Interconversion of THF-bound one-carbon species
In simple terms: The one-carbon unit changes its chemical form as it moves through the cycle.
5,10-methylene-THF can be oxidized to 5,10-methenyl-THF and then to 10-formyl-THF by the bifunctional enzyme MTHFD1 (cytosol) or MTHFD2/MTHFD2L (mitochondria). These interconversions adjust the oxidation state of the one-carbon unit so it can be used for different biosynthetic reactions. 10-formyl-THF is required for purine synthesis, while 5,10-methylene-THF is used for thymidylate synthesis.
Thymidylate synthesis and THF regeneration
In simple terms: The cycle helps make thymine, a DNA building block, and recycles THF.
Thymidylate synthase (TYMS) transfers the methylene group from 5,10-methylene-THF to dUMP, forming dTMP and dihydrofolate (DHF). DHF is then reduced back to THF by dihydrofolate reductase (DHFR), completing the regeneration step that allows the cycle to continue. This reaction is a key target of antifolate drugs such as methotrexate.
Methionine remethylation and the methylation cycle
In simple terms: The cycle also helps convert homocysteine back to methionine.
5-methyl-THF, produced by MTHFR, donates its methyl group to homocysteine in a reaction catalyzed by methionine synthase (MTR), yielding methionine and regenerating THF. This reaction links the folate cycle to the methionine remethylation cycle and to SAM-dependent methylation reactions. Vitamin B12 is a required cofactor for MTR, and its deficiency can trap folate as 5-methyl-THF.
Compartmentalization and formate overflow
In simple terms: The cycle operates in both mitochondria and cytosol, with formate as a carrier.
Mitochondrial one-carbon metabolism generates formate, which can be exported to the cytosol to support cytosolic folate cycle reactions. MTHFD1L in mitochondria produces formate from 10-formyl-THF, and this formate is a major source of cytosolic one-carbon units. This compartmentalization allows the cycle to adapt to different cellular needs and is often rewired in cancer.
Key Genes Involved in GO:0035999 folate cycle
The folate cycle is executed by a set of conserved enzymes and transporters whose expression and activity determine flux through the pathway.
| Gene | Major Role | Research Relevance |
|---|---|---|
| SHMT1 | Cytosolic serine hydroxymethyltransferase; produces 5,10-methylene-THF | Target for studying cytosolic one-carbon flux and nucleotide synthesis |
| SHMT2 | Mitochondrial serine hydroxymethyltransferase; produces glycine and one-carbon units | Frequently upregulated in cancer; linked to mitochondrial one-carbon metabolism |
| MTHFD1 | Bifunctional 5,10-methylene-THF dehydrogenase/cyclohydrolase in cytosol | Genetic variants associated with folate-related disorders |
| MTHFD2 | Mitochondrial methylenetetrahydrofolate dehydrogenase 2 | Overexpressed in many cancers; target for metabolic inhibitors |
| MTHFD1L | Mitochondrial 10-formyl-THF synthetase; produces formate | Key enzyme for mitochondrial formate production |
| MTHFR | Reduces 5,10-methylene-THF to 5-methyl-THF | Common C677T variant affects homocysteine and cardiovascular risk |
| MTR | Methionine synthase; remethylates homocysteine using 5-methyl-THF | Requires vitamin B12; links folate to methionine cycle |
| MTHFS | Methenyltetrahydrofolate synthetase; interconverts 5-formyl-THF and 5,10-methenyl-THF | Regulates available one-carbon pools |
| TYMS | Thymidylate synthase; synthesizes dTMP from dUMP | Target of 5-fluorouracil and other antifolates |
| DHFR | Dihydrofolate reductase; regenerates THF from DHF | Target of methotrexate; essential for cycle maintenance |
| GART | Phosphoribosylglycinamide formyltransferase; uses 10-formyl-THF for purine synthesis | Connects folate cycle to de novo purine synthesis |
| ATIC | AICAR transformylase/IMP cyclohydrolase; uses 10-formyl-THF | Links folate cycle to purine biosynthesis |
| SLC19A1 | Reduced folate carrier; transports folates into cells | Determines cellular folate uptake and drug response |
| FOLR1 | Folate receptor alpha; mediates folate transport | Target for folate-conjugated therapeutics |
| MTHFD2L | Mitochondrial methylenetetrahydrofolate dehydrogenase 2-like | Contributes to mitochondrial one-carbon metabolism |
| ALDH1L1 | 10-formyltetrahydrofolate dehydrogenase; regulates 10-formyl-THF levels | Tumor suppressor-like role in folate metabolism |
| MTRR | Methionine synthase reductase; maintains MTR activity | Supports methionine remethylation and folate cycle |
| CBS | Cystathionine beta-synthase; transsulfuration of homocysteine | Connects methionine cycle to cysteine synthesis |
How Is folate cycle Regulated?
The folate cycle is regulated at multiple levels. Enzyme expression is cell-cycle dependent, with peaks in S phase to meet nucleotide demand. Allosteric and post-translational regulation of MTHFR, SHMT and MTHFD enzymes adjusts flux in response to one-carbon availability. The cycle is also responsive to nutrient status, including folate, vitamin B12, serine and glycine levels. In cancer, oncogenic signaling can reprogram one-carbon metabolism to support proliferation.
folate cycle and Human Disease
| Gene | Disease / Biology | Potential Experimental Model |
|---|---|---|
| MTHFR | Cardiovascular risk, hyperhomocysteinemia | MTHFR C677T knock-in cell line; homocysteine measurement |
| MTR | Methionine remethylation defects, vitamin B12-related disorders | MTR knockout or point-mutation HAP1 cells; methionine synthesis assay |
| SHMT2 | Cancer metabolic reprogramming | SHMT2 knockout cancer cell lines; serine/glycine flux analysis |
| MTHFD2 | Cancer proliferation and redox balance | MTHFD2 overexpression and knockout models; nucleotide synthesis assays |
| TYMS | Chemotherapy response, thymidylate synthesis | TYMS knockout or point-mutation models; 5-FU sensitivity testing |
Folate cycle defects and cardiovascular disease
Reduced MTHFR activity, often due to the common C677T variant, leads to decreased 5-methyl-THF and elevated homocysteine, which is associated with increased cardiovascular risk. Folate supplementation can lower homocysteine, but the clinical benefit remains debated. The folate cycle is therefore a key interface between genetics, nutrition and vascular health.
Folate metabolism in cancer
Cancer cells frequently upregulate folate cycle enzymes, including SHMT2, MTHFD2 and MTHFD1L, to sustain nucleotide synthesis and redox balance. Antifolates such as methotrexate and 5-fluorouracil target DHFR and TYMS, respectively, and remain important chemotherapeutic agents. Understanding pathway rewiring can inform resistance mechanisms and new drug combinations.
Neurodevelopmental and psychiatric associations
Folate status is critical for neural tube closure, and periconceptional folic acid supplementation reduces neural tube defect risk. Disruptions in the folate-methionine cycle have also been reported in autism spectrum disorder, although causality and intervention effects require further study. These findings highlight the importance of one-carbon metabolism in brain development.
Excess folate and unintended consequences
While folate deficiency is harmful, excessive folate intake may also perturb one-carbon homeostasis and has been linked to altered immune function and potential adverse effects. The balance between deficiency and excess is therefore important for public health recommendations.
From folate cycle-Related Genes to Experimental Models
| Research Question | Suitable Model |
|---|---|
| Does loss of MTHFR alter homocysteine levels? | MTHFR knockout cell line (e.g., HAP1 or HEK293) |
| Does the MTHFR C677T variant reduce enzyme activity? | MTHFR C677T point-mutation knock-in cell line |
| Does SHMT2 overexpression increase one-carbon flux? | SHMT2 overexpression cell line; metabolic flux analysis |
| Does MTHFD2 localize to mitochondria? | MTHFD2 tagged knock-in with fluorescent tag; imaging |
| Does MTR require vitamin B12 for activity? | MTR knockout with rescue by wild-type or mutant MTR |
| Can folate cycle gene knockouts be rescued by formate? | Formate supplementation in knockout cell lines |
How to Study the folate cycle Process
| Method | What It Measures | Typical Application |
|---|---|---|
| 13C-serine tracing + LC-MS | One-carbon flux into nucleotides and methionine | Cancer cell metabolic reprogramming |
| RNA-seq | Expression of folate cycle genes | Response to folate deprivation or oncogenic signals |
| CRISPR knockout screen | Genes required for growth under folate stress | Identify synthetic lethal targets |
| Enzyme activity assay | MTHFR or SHMT catalytic activity | Functional validation of variants |
| Western blot | Protein levels of folate enzymes | Assess expression changes |
| Immunofluorescence | Subcellular localization of enzymes | Mitochondrial vs cytosolic distribution |
| Homocysteine assay | Methionine remethylation capacity | Cardiovascular risk studies |
| Targeted metabolomics | THF intermediates and related metabolites | Quantify folate cycle pool sizes |
Metabolic flux analysis
Stable isotope tracing with 13C-serine, 13C-glycine or 13C-formate coupled to mass spectrometry measures one-carbon flux through the folate cycle. This approach quantifies labeling of nucleotides, methionine and other metabolites, revealing pathway activity and compartmentalization.
Genomic and transcriptomic profiling
RNA-seq and targeted gene expression panels can assess folate cycle enzyme expression across conditions. CRISPR screens with metabolic readouts can identify genes that modify folate dependency.
Protein and post-translational analysis
Western blotting, immunoprecipitation and proteomics can measure enzyme levels and modifications, such as phosphorylation of SHMT1 or MTHFR. Activity assays using recombinant enzymes or cell lysates provide functional validation.
Imaging and subcellular localization
Fluorescence microscopy of tagged folate cycle enzymes (e.g., MTHFD2-GFP) reveals mitochondrial versus cytosolic distribution. Live-cell imaging can track dynamic changes in response to nutrient availability.
How CRISPR Can Be Used to Study GO:0035999 folate cycle
Knockout
CRISPR knockout of folate cycle genes such as MTHFR, SHMT2 or MTHFD2 can reveal essentiality and metabolic dependencies. Knockout cell lines are useful for testing whether a gene is required for proliferation under different folate conditions.
Point Mutation
Introducing disease-associated point mutations, such as MTHFR C677T, into isogenic cell lines allows precise assessment of variant effects on enzyme activity and homocysteine levels. Point-mutation models avoid confounding effects of different genetic backgrounds.
Knock-in
Knock-in of tagged versions of folate enzymes (e.g., GFP or HA tags) enables localization and interaction studies. Knock-in of reporter cassettes can also monitor pathway activity in real time.
Overexpression
Overexpression of genes like SHMT2 or MTHFD2 can model the metabolic rewiring seen in cancer and test whether increased flux promotes proliferation or drug resistance. Overexpression models are also useful for biochemical purification of enzymes.
How EDITGENE Supports folate cycle Research
Researchers studying folate cycle-related genes often need to determine whether a candidate gene is causally involved in metabolic flux, disease risk or drug response. Isogenic CRISPR models provide a rigorous way to test these hypotheses by introducing precise genetic changes into relevant cell types.
Contact EDITGENE today to design your custom CRISPR model for folate cycle research.
Frequently Asked Questions About folate cycle
What is the folate cycle (GO:0035999)?
The folate cycle is a biological process in which tetrahydrofolate carries one-carbon units through a series of interconversions to support nucleotide synthesis, methionine remethylation and methylation reactions.
What genes are involved in the folate cycle?
Key genes include SHMT1, SHMT2, MTHFD1, MTHFD2, MTHFD1L, MTHFR, MTR, MTHFS, TYMS, DHFR, GART and ATIC, among others.
What are the main one-carbon donors for the folate cycle?
Serine, glycine and formate are the principal one-carbon donors that feed the folate cycle.
How is the folate cycle linked to methionine metabolism?
5-methyl-THF donates a methyl group to homocysteine via methionine synthase (MTR), regenerating methionine and linking folate to the methionine remethylation cycle.
What diseases are associated with folate cycle defects?
Folate cycle defects are associated with neural tube defects, cardiovascular disease, cancer and some neurodevelopmental conditions.
What is the role of MTHFR in the folate cycle?
MTHFR reduces 5,10-methylene-THF to 5-methyl-THF, the substrate for methionine synthase; common variants like C677T reduce activity and can elevate homocysteine.
How can I study folate cycle genes with CRISPR?
CRISPR knockout, point-mutation knock-in and overexpression models allow causal testing of gene function in isogenic backgrounds.
What methods measure folate cycle activity?
Stable isotope tracing, targeted metabolomics, enzyme activity assays and homocysteine measurements are commonly used.
Is folate supplementation always beneficial?
Folate supplementation prevents deficiency and reduces neural tube defect risk, but excess folate may have unintended effects, so balance is important.
Why is the folate cycle important in cancer?
Cancer cells often upregulate folate cycle enzymes to support proliferation, and antifolates remain important chemotherapeutic agents.
Conclusion
The folate cycle (GO:0035999) is a fundamental metabolic process that integrates one-carbon acquisition, nucleotide synthesis and methylation. Its dysregulation contributes to a wide range of human diseases, from cardiovascular disorders to cancer and neurodevelopmental conditions. Advances in CRISPR modeling and metabolic flux analysis now allow researchers to dissect the causal roles of individual folate cycle genes and variants, paving the way for targeted interventions.
References
- 1. Froese DS et al.. 2019. Vitamin B(12) , folate, and the methionine remethylation cycle-biochemistry, pathways, and regulation.. J Inherit Metab Dis 42(4):673-685 PMID: 30693532
- 2. Roufael M et al.. 2023. Folate-Methionine Cycle Disruptions in ASD Patients and Possible Interventions: A Systematic Review.. Genes (Basel) 14(3) PMID: 36980981
- 3. Lan X et al.. 2018. Cell cycle regulation of folate-mediated one-carbon metabolism.. Wiley Interdiscip Rev Syst Biol Med 10(6):e1426 PMID: 29889360
- 4. Lee Y et al.. 2024. Cycling back to folate metabolism in cancer.. Nat Cancer 5(5):701-715 PMID: 38698089
- 5. Fowler B. 2001. The folate cycle and disease in humans.. Kidney Int Suppl 78:S221-9 PMID: 11169015
- 6. Raghubeer S et al.. 2021. Methylenetetrahydrofolate (MTHFR), the One-Carbon Cycle, and Cardiovascular Risks.. Nutrients 13(12) PMID: 34960114
- 7. Fardous AM et al.. 2023. Uncovering the Hidden Dangers and Molecular Mechanisms of Excess Folate: A Narrative Review.. Nutrients 15(21) PMID: 37960352
- 8. Bailey RL et al.. 2015. The epidemiology of global micronutrient deficiencies.. Ann Nutr Metab 66 Suppl 2:22-33 PMID: 26045325
