GO:0033353 L-methionine cycle: One-Carbon Metabolism Hub, Genes, Functions and Research Methods
Research-grade guide for scientists and biopharma professionals
Key Takeaways
• GO:0033353 (L-methionine cycle) is the cyclic interconversion of S-adenosyl-L-homocysteine, L-homocysteine, L-methionine and S-adenosyl-L-methionine (SAM) that couples methyl-group utilization to methionine regeneration.
• The cycle is the principal supplier of SAM, the universal methyl donor for DNA, RNA, protein and lipid methylation reactions.
• Cancer cells frequently display methionine dependence, making the cycle a therapeutic vulnerability in tumors such as liver cancer.
• Dysregulation of the cycle is linked to nonalcoholic fatty liver disease, cardiovascular risk and Alzheimer's disease-related neuroinflammation.
• Key enzymes include MAT1A/MAT2A, AHCY, MTR, MTHFR, BHMT, CBS and CTH, which together control flux through transmethylation, remethylation and transsulfuration.
• CRISPR knockout, point-mutation, knock-in and overexpression models enable causal dissection of cycle genes in disease and drug-response studies.
Description
The L-methionine cycle (GO:0033353) is a central metabolic cycle that interconverts S-adenosyl-L-homocysteine (SAH), L-homocysteine, L-methionine and S-adenosyl-L-methionine (SAM). It is also known as the activated methyl cycle or SAM cycle, and it functions as the hub that connects dietary methionine, folate one-carbon metabolism and sulfur amino acid catabolism. Because SAM is the major methyl donor for cellular methylation reactions, the cycle sits at the interface of epigenetics, nucleotide synthesis and redox homeostasis. Researchers study GO:0033353 because its flux determines the methylation potential of a cell and influences proliferation, differentiation and stress responses. In cancer, altered methionine cycle activity supports the high demand for methylation and antioxidant capacity, and blocking methionine catabolism can induce senescence and sensitize liver cancer cells to GSK3 inhibition. In metabolic disease, the cycle is mechanistically implicated in nonalcoholic fatty liver disease and cardiovascular risk through homocysteine and one-carbon imbalance. In the nervous system, L-methionine can enhance neuroinflammation and impair neurogenesis, linking the cycle to Alzheimer's disease pathology. This article integrates the QuickGO definition with verified PubMed literature to provide a research-grade overview of the cycle, its genes, disease relevance and CRISPR-based methods for functional study.
L-methionine cycle At A Glance
| GO ID | GO:0033353 |
|---|---|
| GO term | L-methionine cycle |
| Ontology | biological_process |
| Synonym | activated methyl cycle; S-adenosylmethionine cycle; SAM cycle |
| Major function | Couples SAM-dependent methylation to regeneration of L-methionine and recycling of the homocysteinyl group |
| Key metabolites | S-adenosyl-L-homocysteine (SAH), L-homocysteine, L-methionine, S-adenosyl-L-methionine (SAM) |
| Representative enzymes | MAT1A, MAT2A, AHCY, MTR, MTHFR, BHMT, CBS, CTH |
| Disease relevance | Cancer, nonalcoholic fatty liver disease, cardiovascular risk, Alzheimer's disease |
| Research methods | CRISPR KO/point mutation/knock-in/overexpression, metabolomics, RNA-seq, proteomics |
What Is GO:0033353?
According to the Gene Ontology, GO:0033353 (L-methionine cycle) is defined as a cyclic series of interconversions involving S-adenosyl-L-homocysteine, L-homocysteine, L-methionine and S-adenosyl-L-methionine (SAM). The cycle couples the utilization of the methyl group of SAM with recycling of the homocysteinyl group and regeneration of L-methionine. In practical terms, SAM donates its methyl group to acceptors, producing SAH; SAH is hydrolyzed to homocysteine; homocysteine is remethylated back to methionine; and methionine is reactivated to SAM, closing the cycle.
Why Is L-methionine cycle Important in Cell Biology?
GO:0033353 is important because it sets the cellular methylation potential and supplies methyl groups for DNA, RNA, protein and lipid methylation, thereby influencing gene expression, genome stability and cell fate. Its flux is reprogrammed in cancer, where methionine dependence and altered catabolism create targetable vulnerabilities. It is also a metabolic node in nonalcoholic fatty liver disease and cardiovascular risk, and it contributes to neuroinflammatory processes relevant to Alzheimer's disease. Consequently, the cycle is both a mechanistic explanation for disease phenotypes and a source of candidate drug targets and biomarkers.
• Supplies SAM, the universal methyl donor for epigenetic and post-translational methylation.
• Controls homocysteine levels, a biomarker associated with cardiovascular risk.
• Is reprogrammed in liver cancer, where blocking methionine catabolism induces senescence and confers vulnerability to GSK3 inhibition.
• Underlies methionine dependence of many cancer cell lines and tumors.
• Is mechanistically linked to nonalcoholic fatty liver disease progression and potential therapeutic applications.
• Contributes to neuroinflammation and impaired neurogenesis relevant to Alzheimer's disease.
• Modulates cell cycle progression in prostate and breast cancer cells but not benign cells.
• Provides metabolic intermediates for glutathione synthesis and redox defense.
• Offers multiple druggable enzymes (MAT2A, AHCY, MTR, MTHFR) for precision oncology.
• Can be studied with CRISPR models to establish causal gene-disease relationships.
What Happens During L-methionine cycle?
Methionine activation to SAM
In simple terms: Methionine is loaded with a methyl group to become SAM, the cell's main methyl donor.
The cycle begins when L-methionine is converted to S-adenosyl-L-methionine (SAM) by methionine adenosyltransferases, primarily MAT1A in liver and MAT2A in most tissues. SAM is the principal methyl donor for transmethylation reactions, and its availability determines the methylation capacity of the cell. In cancer, increased MAT2A activity supports elevated SAM demand for proliferation and epigenetic maintenance.
Transmethylation and SAH production
In simple terms: SAM hands off its methyl group, becoming SAH.
SAM-dependent methyltransferases transfer the methyl group of SAM to DNA, RNA, proteins and lipids, generating S-adenosyl-L-homocysteine (SAH). The ratio of SAM to SAH is a sensitive indicator of methylation potential, and its perturbation affects epigenetic marks and gene expression. This step couples the cycle directly to epigenetic regulation and to the methionine dependence observed in cancer cells.
SAH hydrolysis to homocysteine
In simple terms: SAH is broken down to homocysteine, removing the adenosine part.
SAH is hydrolyzed to L-homocysteine and adenosine by S-adenosylhomocysteine hydrolase (AHCY). This reaction is reversible and can be inhibited by elevated SAH, so AHCY activity helps maintain flux through the cycle. Homocysteine produced here can either be remethylated back to methionine or enter the transsulfuration pathway.
Remethylation of homocysteine to methionine
In simple terms: Homocysteine gets a methyl group back to become methionine again.
Homocysteine is remethylated to L-methionine by methionine synthase (MTR), which uses 5-methyltetrahydrofolate generated by MTHFR, or by betaine-homocysteine S-methyltransferase (BHMT) in liver and kidney. MTHFR activity is a key determinant of one-carbon availability and has been linked to cardiovascular risk. This remethylation step closes the cycle and regenerates the methionine pool.
Transsulfuration and cycle exit
In simple terms: Homocysteine can leave the cycle to make cysteine and antioxidants.
When methionine is in excess, homocysteine is diverted to the transsulfuration pathway by cystathionine beta-synthase (CBS) and cystathionine gamma-lyase (CTH), producing cysteine for glutathione synthesis. This exit route links the L-methionine cycle to redox homeostasis and to liver metabolic disease. In nonalcoholic fatty liver disease, altered methionine cycle flux and transsulfuration contribute to lipid accumulation and oxidative stress.
Key Genes Involved in GO:0033353 L-methionine cycle
The following genes and enzymes are core components or direct regulators of the L-methionine cycle (GO:0033353) and are commonly studied in cancer, metabolic and neurological research.
| Gene | Major Role | Research Relevance |
|---|---|---|
| MAT1A | Liver-specific methionine adenosyltransferase producing SAM | Downregulated in liver cancer; links SAM supply to hepatocellular carcinoma |
| MAT2A | Ubiquitous methionine adenosyltransferase producing SAM | Target in methionine-dependent cancers; supports proliferation and epigenetic methylation |
| AHCY | S-adenosylhomocysteine hydrolase converting SAH to homocysteine | Controls SAH levels and methylation potential; candidate for metabolic and cancer studies |
| MTR | Methionine synthase remethylating homocysteine to methionine | Connects folate one-carbon cycle to methionine regeneration |
| MTHFR | Generates 5-methyltetrahydrofolate for MTR | Polymorphisms linked to cardiovascular risk and one-carbon imbalance |
| BHMT | Betaine-homocysteine S-methyltransferase remethylation route | Liver-specific alternative remethylation; relevant to NAFLD |
| CBS | Cystathionine beta-synthase initiating transsulfuration | Links homocysteine disposal to cysteine and glutathione synthesis |
| CTH | Cystathionine gamma-lyase completing transsulfuration | Supports antioxidant defense and metabolic flux studies |
| GNMT | Glycine N-methyltransferase consuming SAM | Regulates SAM/SAH ratio and methylation capacity |
| DNMT1 | DNA methyltransferase using SAM | Reads out methylation potential; epigenetic endpoint of the cycle |
| DNMT3A | De novo DNA methyltransferase using SAM | Epigenetic effector influenced by cycle flux |
| MTRR | Methionine synthase reductase reactivating MTR | Supports sustained remethylation and one-carbon balance |
| SHMT1 | Serine hydroxymethyltransferase feeding one-carbon pool | Connects serine metabolism to methionine cycle |
| MTHFD1 | Methylenetetrahydrofolate dehydrogenase in one-carbon metabolism | Supplies folate intermediates for remethylation |
| SLC7A5 | L-type amino acid transporter importing methionine | Determines methionine availability and dependence |
| SLC43A1 | Amino acid transporter contributing to methionine uptake | Modulates intracellular methionine pools |
| GSK3A | Glycogen synthase kinase 3 alpha, senescence-linked kinase | Mediates vulnerability to GSK3 inhibition upon methionine catabolism blockade |
| GSK3B | Glycogen synthase kinase 3 beta, senescence-linked kinase | Mediates vulnerability to GSK3 inhibition upon methionine catabolism blockade |
How Is L-methionine cycle Regulated?
The L-methionine cycle is regulated by substrate availability, enzyme expression and allosteric feedback. SAM availability depends on methionine import and on MAT1A/MAT2A expression, and excess SAM can feedback on methyltransferases and on the cycle. The SAM/SAH ratio reflects methylation potential and is sensitive to AHCY activity and to methyltransferase demand. Remethylation is regulated by folate one-carbon supply through MTHFR and MTR, and by betaine availability through BHMT. Transsulfuration via CBS and CTH provides an exit route when methionine is abundant, linking cycle flux to redox status. In cancer, oncogenic signaling and metabolic stress can reprogram methionine cycle flux, and blocking methionine catabolism engages senescence pathways that confer vulnerability to GSK3 inhibition. In liver disease, nutritional and microbiota-derived factors modulate cycle activity, as shown for polyphenol treatment in high-fat-diet models.
L-methionine cycle and Human Disease
| Gene | Disease / Biology | Potential Experimental Model |
|---|---|---|
| MAT2A | Liver cancer and methionine-dependent tumors | Hepatocellular carcinoma cell lines with MAT2A knockout or overexpression |
| MTHFR | Cardiovascular risk and one-carbon imbalance | Isogenic cell lines carrying MTHFR point mutations |
| BHMT | Nonalcoholic fatty liver disease | High-fat-diet mouse models and hepatocyte knockout lines |
| CBS | Homocysteine metabolism and redox imbalance | CRISPR knockout hepatocytes and metabolomic profiling |
| AHCY | Methylation potential and epigenetic regulation | Knockout or point-mutation cell models with SAM/SAH measurement |
Cancer and methionine dependence
Many cancer cells exhibit methionine dependence and reprogram the L-methionine cycle to sustain methylation and proliferation. In liver cancer, blocking methionine catabolism induces senescence and confers vulnerability to GSK3 inhibition, identifying a metabolic-kinase synthetic lethal strategy. MAT2A and other cycle enzymes are candidate targets because they support SAM supply and epigenetic maintenance in tumors. L-methionine itself can suppress cell cycle progression in LNCaP and MCF-7 cancer cells but not in benign cells, indicating context-dependent effects of cycle substrates.
Nonalcoholic fatty liver disease and metabolic syndrome
The methionine cycle is mechanistically implicated in nonalcoholic fatty liver disease, where altered flux affects lipid metabolism, oxidative stress and inflammation. Nutritional interventions such as Lycium barbarum L. polyphenols improve high-fat-diet-induced NAFLD in association with changes in liver and colon metabolism and intestinal microbiota, with a potential role of the methionine cycle. These findings support the cycle as a therapeutic and biomarker axis in metabolic liver disease.
Cardiovascular risk and one-carbon imbalance
MTHFR and the one-carbon cycle influence homocysteine levels, and elevated homocysteine is associated with cardiovascular risk. Because the L-methionine cycle determines homocysteine remethylation versus transsulfuration, its activity modulates vascular risk biomarkers. Genetic and nutritional factors that alter MTHFR or MTR function can therefore shift cycle flux and cardiovascular risk profiles.
Neuroinflammation and Alzheimer's disease
L-methionine enhances neuroinflammation and impairs neurogenesis, with implications for Alzheimer's disease. This suggests that methionine cycle substrates and flux can influence neuroinflammatory signaling and neural stem cell function. The cycle is therefore a potential node for understanding and modulating neurodegeneration-related processes.
From L-methionine cycle-Related Genes to Experimental Models
| Research Question | Suitable Model |
|---|---|
| Is MAT2A required for cancer cell proliferation? | CRISPR knockout of MAT2A in cancer cell lines |
| Does a MTHFR variant alter homocysteine remethylation? | Point-mutation knock-in of the variant in isogenic cells |
| Can a disease-associated mutation in AHCY change SAM/SAH ratio? | Knock-in of the mutation with metabolomic readout |
| Where is MAT2A expressed and how does it respond to stress? | Tagged knock-in with fluorescent or epitope tag |
| Does overexpression of BHMT protect against fatty liver? | Overexpression model in hepatocytes or mouse liver |
| Which cycle genes mediate sensitivity to GSK3 inhibition? | CRISPR library screening in methionine-catabolism-blocked cells |
How to Study the L-methionine cycle Process
| Method | What It Measures | Typical Application |
|---|---|---|
| Targeted metabolomics | SAM, SAH, methionine, homocysteine levels | Assessing cycle flux and methylation potential |
| Stable-isotope tracing | Flux through transmethylation and remethylation | Quantifying pathway activity in cells |
| RNA-seq | Expression of cycle and methylation-related genes | Transcriptomic response to CRISPR perturbation |
| DNA methylation profiling | Global and locus-specific methylation marks | Linking cycle flux to epigenetics |
| Immunoblotting | Protein levels of cycle enzymes | Validating knockout or overexpression |
| Enzyme activity assay | Catalytic activity of MAT, AHCY, MTR, BHMT, CBS | Functional characterization of variants |
| Senescence and proliferation assays | Growth arrest and senescence markers | Testing cancer vulnerability |
| Neuroinflammation assays | Cytokine and neurogenesis readouts | Modeling Alzheimer's-related effects |
Metabolomics and flux analysis
Targeted metabolomics quantifies SAM, SAH, methionine, homocysteine and related intermediates to assess cycle activity and methylation potential. Stable-isotope tracing can measure flux through transmethylation, remethylation and transsulfuration. These methods are essential for linking genotype to cycle function in CRISPR models.
Transcriptomics and epigenomics
RNA-seq reveals expression changes in cycle genes and downstream methylation-sensitive pathways. DNA methylation profiling readouts connect cycle flux to epigenetic marks and gene regulation. Combining these with CRISPR perturbations helps establish causality.
Proteomics and enzyme activity assays
Proteomics and immunoblotting measure protein levels of MAT1A, MAT2A, AHCY, MTR, MTHFR, BHMT, CBS and CTH. Enzyme activity assays provide direct functional readouts of cycle enzymes. These approaches validate knockout, knock-in and overexpression models.
Cell-based phenotypic assays
Proliferation, senescence and cell cycle assays reveal the consequences of cycle perturbation in cancer and benign cells. Neuroinflammation and neurogenesis assays can test methionine effects in neural models. NAFLD-relevant lipid accumulation assays can test cycle gene function in hepatocytes.
How CRISPR Can Be Used to Study GO:0033353 L-methionine cycle
Knockout
CRISPR knockout of L-methionine cycle genes such as MAT2A, AHCY, MTR, MTHFR, BHMT, CBS or CTH can establish whether a gene is required for proliferation, methylation or stress survival. Knockout models are particularly useful for testing methionine dependence and synthetic lethal interactions with GSK3 inhibition.
Point Mutation
Point-mutation models introduce disease-associated or catalytically dead variants to dissect enzyme function without confounding expression changes. For example, MTHFR variants can be modeled to study one-carbon imbalance and cardiovascular risk. Point mutations in AHCY or MTR can reveal effects on SAM/SAH ratio and remethylation.
Knock-in
Knock-in of tags, reporters or disease alleles allows precise tracking of cycle enzymes and their regulation. Tagged knock-in of MAT2A or AHCY supports localization and interaction studies. Disease-relevant knock-in alleles can model metabolic and neurological phenotypes.
Overexpression
Overexpression models test gain-of-function effects of cycle genes on methylation, proliferation and metabolism. Overexpressing BHMT or MTR can probe remethylation capacity and protection against fatty liver or homocysteine accumulation. Overexpression of MAT2A can mimic the elevated SAM demand of cancer cells.
How EDITGENE Supports L-methionine cycle Research
Researchers studying L-methionine cycle-related genes often need to determine whether a candidate gene is causally involved in disease or drug response, rather than merely correlated with it. EDITGENE provides CRISPR-based cell models and screening services that enable such causal experiments across knockout, point-mutation, knock-in and overexpression formats.
Contact EDITGENE today to design your custom CRISPR model for L-methionine cycle research.
Frequently Asked Questions About L-methionine cycle
What is the L-methionine cycle (GO:0033353)?
It is a cyclic series of interconversions involving S-adenosyl-L-homocysteine, L-homocysteine, L-methionine and S-adenosyl-L-methionine (SAM) that couples methyl-group utilization to methionine regeneration.
What genes are involved in the L-methionine cycle?
Core genes include MAT1A, MAT2A, AHCY, MTR, MTHFR, BHMT, CBS and CTH, which catalyze activation, hydrolysis, remethylation and transsulfuration steps.
Why is the L-methionine cycle important in cancer?
Many cancers are methionine dependent, and blocking methionine catabolism can induce senescence and confer vulnerability to GSK3 inhibition in liver cancer.
How is the L-methionine cycle linked to nonalcoholic fatty liver disease?
Altered cycle flux affects lipid metabolism and oxidative stress, and nutritional interventions that improve NAFLD are associated with methionine cycle changes.
Does the L-methionine cycle affect cardiovascular risk?
MTHFR and one-carbon metabolism influence homocysteine levels, which are associated with cardiovascular risk.
What is the role of the L-methionine cycle in Alzheimer's disease?
L-methionine enhances neuroinflammation and impairs neurogenesis, suggesting a link between cycle substrates and Alzheimer's-related processes.
What is the difference between SAM and SAH?
SAM is the methyl donor produced from methionine, while SAH is the product after methyl transfer; their ratio reflects cellular methylation potential.
How can CRISPR be used to study the L-methionine cycle?
CRISPR knockout, point mutation, knock-in and overexpression models can test causal roles of cycle genes in proliferation, methylation and drug response.
Which methods measure L-methionine cycle activity?
Targeted metabolomics, stable-isotope tracing, RNA-seq, DNA methylation profiling and enzyme activity assays are commonly used.
Can L-methionine affect cancer cell growth directly?
L-methionine suppresses cell cycle progression in LNCaP and MCF-7 cancer cells but not in benign cells, indicating context-dependent effects.
Conclusion
GO:0033353 (L-methionine cycle) is a central metabolic cycle that controls SAM-dependent methylation, homocysteine homeostasis and methionine regeneration. Its dysregulation is implicated in cancer, nonalcoholic fatty liver disease, cardiovascular risk and Alzheimer's disease-related neuroinflammation. CRISPR-based knockout, point-mutation, knock-in and overexpression models, combined with metabolomics and epigenomics, provide the causal evidence needed to translate cycle biology into therapeutic strategies.
References
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- 2. Raghubeer S et al.. 2021. Methylenetetrahydrofolate (MTHFR), the One-Carbon Cycle, and Cardiovascular Risks.. Nutrients 13(12) PMID: 34960114
- 3. Liang J et al.. 2025. Lycium barbarum L. polyphenols improve HFD-induced NAFLD through liver and colon metabolism and intestinal microbiota:potential role of methionine cycle.. Phytomedicine 148:157477 PMID: 41175590
- 4. Tassinari V et al.. 2024. The methionine cycle and its cancer implications.. Oncogene 43(48):3483-3488 PMID: 39394448
- 5. Wang H et al.. 2022. Methionine cycle in nonalcoholic fatty liver disease and its potential applications.. Biochem Pharmacol 200:115033 PMID: 35395242
- 6. Kaiser P. 2020. Methionine Dependence of Cancer.. Biomolecules 10(4) PMID: 32276408
- 7. Alachkar A et al.. 2022. L-methionine enhances neuroinflammation and impairs neurogenesis: Implication for Alzheimer's disease.. J Neuroimmunol 366:577843 PMID: 35299077
- 8. Benavides MA et al.. 2010. Suppression by L-methionine of cell cycle progression in LNCaP and MCF-7 cells but not benign cells.. Anticancer Res 30(6):1881-5 PMID: 20651330
