GO:0006555 L-methionine metabolic process: Sulfur Amino Acid Pathway, Genes, Functions and Research Methods
Research-grade guide for scientists and biopharma professionals
Key Takeaways
• GO:0006555 describes the chemical reactions and pathways involving L-methionine, a sulfur-containing essential amino acid incorporated into proteins.
• L-methionine is not only a protein building block but also the precursor of S-adenosyl-L-methionine (SAM), the principal methyl donor in cells.
• Microbial and yeast systems have been engineered to overproduce L-methionine and SAM, revealing the pathway's industrial relevance.
• Key enzymes include MetA, MetB, MetC, MetE, MetH, MTR, and CBS, which catalyze the interconversion of homoserine, cysteine, and methionine.
• Dysregulation of methionine metabolism is linked to cancer, neurological disorders, and inflammatory cell death through SAM scarcity.
• CRISPR knockout, point mutation, knock-in, and overexpression models are essential to dissect gene function within this pathway.
Description
L-methionine metabolic process (GO:0006555) encompasses the biochemical reactions and pathways that synthesize, interconvert, and degrade L-methionine (2-amino-4-(methylthio)butanoic acid), a sulfur-containing essential amino acid found in peptide linkage in proteins. Because animals cannot synthesize methionine de novo, they must obtain it from the diet or from microbial sources, making its metabolism a central node in nutrition, redox balance, and one-carbon transfer. The pathway is also the source of S-adenosyl-L-methionine (SAM), the universal methyl donor for DNA, RNA, protein, and lipid methylation, and of homocysteine, which sits at the intersection of methionine, folate, and transsulfuration cycles. Researchers study GO:0006555 because it connects amino acid biosynthesis to epigenetics, oxidative stress, and cell fate. In bacteria and yeast, the pathway has been extensively engineered for industrial production of L-methionine and SAM, with systems metabolic engineering enabling gram-per-liter titers. In mammals, perturbations in methionine metabolism are associated with cancer, neurodegeneration, and inflammatory signaling, as exemplified by the recent finding that RIPK1 senses SAM scarcity to drive cell death and inflammation. Understanding the genes, enzymes, and regulatory loops of this pathway is therefore critical for both biotechnology and medicine. This article integrates the QuickGO definition of GO:0006555 with verified PubMed literature to provide a research-grade overview of the pathway's mechanism, key genes, disease links, and experimental models, including CRISPR-based approaches for functional dissection.
L-methionine metabolic process At A Glance
| GO ID | GO:0006555 |
|---|---|
| GO term | L-methionine metabolic process |
| Ontology | biological_process |
| Synonym | methionine and threonine metabolic process; methionine and threonine metabolism; methionine metabolism |
| Major function | Synthesis, interconversion, and utilization of L-methionine, including SAM production and homocysteine remethylation |
| Key enzymes | MetA, MetB, MetC, MetE, MetH, MTR, CBS, MAT |
| Pathway context | Aspartate family amino acid biosynthesis; one-carbon metabolism; transsulfuration |
| Industrial relevance | Microbial production of L-methionine and SAM via metabolic engineering |
What Is GO:0006555?
GO:0006555, L-methionine metabolic process, is defined by QuickGO as the chemical reactions and pathways involving L-methionine (2-amino-4-(methylthio)butanoic acid), a sulfur-containing, essential amino acid found in peptide linkage in proteins. In practice, this includes de novo biosynthesis from homoserine or cysteine, conversion of methionine to SAM and homocysteine, remethylation of homocysteine back to methionine, and catabolic routes that feed into the transsulfuration pathway.
Why Is L-methionine metabolic process Important in Cell Biology?
L-methionine metabolic process is important because it supplies the essential amino acid methionine for protein synthesis and generates SAM, the major methyl donor for epigenetic and post-translational modifications. It also controls homocysteine levels, which are linked to cardiovascular and neurological disease, and provides cysteine for glutathione synthesis. In biotechnology, engineering this pathway in Escherichia coli, Corynebacterium glutamicum, and Saccharomyces cerevisiae enables sustainable production of methionine and SAM for feed, food, and pharmaceutical applications.
• Provides L-methionine for protein synthesis in all organisms.
• Generates SAM, the universal methyl donor for DNA, RNA, protein, and lipid methylation.
• Controls homocysteine homeostasis, a risk factor for cardiovascular and neurological disorders.
• Supplies cysteine and glutathione precursors via the transsulfuration pathway.
• Serves as a target for industrial strain engineering to produce methionine and SAM.
• Links to inflammatory cell death through SAM scarcity sensing by RIPK1.
• Impacts cancer metabolism and epigenetics via one-carbon flux.
• Enables studies of essential amino acid auxotrophy and auxotrophic selection markers.
• Relevant to nutritional interventions and animal feed supplementation.
• Provides a model for systems metabolic engineering and Bayesian optimization.
What Happens During L-methionine metabolic process?
De novo biosynthesis from aspartate and cysteine
In simple terms: Cells build methionine from simpler molecules like aspartate and cysteine.
In bacteria, L-methionine biosynthesis starts from aspartate, which is converted to homoserine and then to O-succinylhomoserine or O-acetylhomoserine. MetA catalyzes the activation of homoserine, and MetB or MetC replaces the hydroxyl group with sulfur from cysteine to form cystathionine, which is then cleaved to homocysteine. Homocysteine is methylated to methionine by MetE or MetH using methyltetrahydrofolate as the methyl donor. In E. coli, module genetic perturbations in this pathway have been mapped to local metabolic responses, revealing flux control nodes.
Methionine activation to S-adenosyl-L-methionine (SAM)
In simple terms: Methionine is activated by adding an adenosyl group to become SAM, the cell's main methyl donor.
Methionine adenosyltransferase (MAT or MetK) condenses methionine with ATP to form SAM, the principal methyl donor for transmethylation reactions. SAM is essential for methylation of DNA, RNA, proteins, and lipids, and its availability is sensed by stress-responsive proteins such as RIPK1, which triggers cell death and inflammation when SAM is scarce. Metabolic engineering of SAM synthesis in S. cerevisiae and C. glutamicum has achieved enhanced titers through combinatorial pathway optimization and Bayesian optimization.
Remethylation of homocysteine to methionine
In simple terms: Homocysteine can be recycled back into methionine using methyl groups from folate or betaine.
The methionine synthase MTR (MetH in bacteria) transfers a methyl group from 5-methyltetrahydrofolate to homocysteine, regenerating methionine and tetrahydrofolate. This reaction links methionine metabolism to folate and one-carbon cycles. In engineered E. coli, overexpression of metH or metE improves methionine production by increasing remethylation flux. In yeast, combinatorial engineering of the methionine and SAM pathways enhances SAM synthesis.
Transsulfuration and catabolism
In simple terms: Excess methionine can be converted to cysteine and other sulfur-containing molecules.
Cystathionine beta-synthase (CBS) condenses homocysteine with serine to form cystathionine, which is then cleaved by cystathionine gamma-lyase to cysteine and alpha-ketobutyrate. This transsulfuration route provides cysteine for glutathione and taurine synthesis. In bacteria, the reverse reaction (MetB/MetC) is used for methionine biosynthesis, highlighting the bidirectional nature of the pathway. Catabolic enzymes also degrade methionine to methanethiol and alpha-ketobutyrate, which can be recycled or excreted.
Regulation and metabolic engineering of the pathway
In simple terms: Cells adjust methionine production based on need, and scientists can rewire these controls to make more.
Methionine biosynthesis is regulated by feedback inhibition of MetA by methionine and SAM, and by transcriptional regulators such as MetJ and MetR in E. coli. Systems metabolic engineering strategies, including promoter replacement, gene knockout, and Bayesian optimization, have been used to relieve feedback repression and increase flux to methionine and SAM. In C. glutamicum, engineering of the methionine biosynthetic operon and SAM synthetase improved SAM production. These studies demonstrate that GO:0006555 is both a fundamental metabolic process and a tractable engineering target.
Key Genes Involved in GO:0006555 L-methionine metabolic process
The following genes and enzymes are central to L-methionine metabolic process (GO:0006555) and have been experimentally characterized in bacteria, yeast, and mammalian systems.
| Gene | Major Role | Research Relevance |
|---|---|---|
| metA | Homoserine O-succinyltransferase; first committed step in bacterial methionine biosynthesis | Target for metabolic engineering to increase methionine flux |
| metB | Cystathionine gamma-synthase; converts O-succinylhomoserine to cystathionine | Key node for sulfur incorporation and flux control |
| metC | Cystathionine beta-lyase; cleaves cystathionine to homocysteine | Essential for methionine biosynthesis and transsulfuration |
| metE | Cobalamin-independent methionine synthase; methylates homocysteine | Overexpressed to enhance methionine production |
| metH | Cobalamin-dependent methionine synthase; alternative homocysteine methyltransferase | Used in engineered strains for improved SAM/methionine synthesis |
| metK | SAM synthetase; converts methionine to SAM | Overexpressed for SAM overproduction |
| MTR | Mammalian methionine synthase; remethylates homocysteine | Links methionine metabolism to folate cycle and disease |
| CBS | Cystathionine beta-synthase; initiates transsulfuration | Associated with homocystinuria and redox regulation |
| MAT1A | Methionine adenosyltransferase I alpha; hepatic SAM synthesis | Relevant to liver disease and cancer metabolism |
| MAT2A | Methionine adenosyltransferase II alpha; ubiquitous SAM synthesis | Target in cancer and epigenetic studies |
| MetJ | Transcriptional repressor of methionine regulon in E. coli | Engineered to relieve feedback repression |
| MetR | Transcriptional activator of methionine genes in E. coli | Modulated for improved flux |
| SAM2 | S. cerevisiae SAM synthetase | Overexpressed for SAM production |
| MET6 | S. cerevisiae methionine synthase | Target for yeast metabolic engineering |
| MTHFR | Methylenetetrahydrofolate reductase; supplies methyl groups for remethylation | Polymorphisms linked to homocysteine and disease |
| BHMT | Betaine-homocysteine S-methyltransferase; alternative remethylation route | Relevant to liver methionine metabolism |
| GNMT | Glycine N-methyltransferase; uses SAM to methylate glycine | Regulates SAM/SAH ratio and methylation capacity |
| AHCY | S-adenosylhomocysteine hydrolase; hydrolyzes SAH to homocysteine | Controls methylation potential and homocysteine levels |
How Is L-methionine metabolic process Regulated?
L-methionine metabolic process is regulated at multiple levels. In bacteria, MetJ represses the methionine regulon in response to SAM, while MetR activates genes under methionine limitation. Feedback inhibition of MetA by methionine and SAM controls flux into the pathway. In yeast, the methionine and SAM pathways are regulated by transcriptional and metabolic feedback, and combinatorial engineering has been used to overcome these controls. In mammals, SAM availability is sensed by RIPK1, which triggers cell death and inflammation when SAM is scarce, linking methionine metabolism to innate immune signaling. Additionally, the methionine cycle is coordinated with folate and one-carbon metabolism through MTHFR and MTR, and with transsulfuration through CBS.
L-methionine metabolic process and Human Disease
| Gene | Disease / Biology | Potential Experimental Model |
|---|---|---|
| CBS | Homocystinuria, cardiovascular risk | Knockout or point-mutation cell models to assess enzyme activity and homocysteine flux |
| MTR | Megaloblastic anemia, neurological dysfunction | Knock-in of patient variants to study remethylation defects |
| MAT1A | Liver disease, cancer metabolism | Overexpression and knockout in hepatocyte cell lines |
| MAT2A | Cancer proliferation, epigenetic regulation | CRISPR knockout and point mutation to dissect SAM synthesis |
| RIPK1 | Inflammation, cell death under SAM scarcity | Knockout and tagged knock-in to monitor SAM sensing |
Cancer and epigenetic dysregulation
Altered methionine metabolism supports cancer cell proliferation by supplying SAM for DNA and histone methylation, and by maintaining redox balance through glutathione synthesis. MAT2A and MTR are frequently upregulated in tumors, and SAM scarcity can sensitize cells to death, making the pathway a potential therapeutic target.
Inflammatory cell death and SAM scarcity
RIPK1 senses S-adenosylmethionine scarcity to drive cell death and inflammation, directly connecting GO:0006555 to inflammatory signaling and tissue damage. This mechanism suggests that modulating methionine metabolism could influence inflammatory diseases.
Neurological and cardiovascular disorders
Elevated homocysteine, a metabolite in the methionine cycle, is associated with cardiovascular disease and neurodegeneration. Polymorphisms in MTHFR and CBS affect homocysteine levels and are studied as risk factors for these conditions.
Inborn errors of metabolism
Deficiencies in CBS cause homocystinuria, characterized by elevated homocysteine and methionine, while MTR defects lead to megaloblastic anemia and neurological symptoms. These disorders highlight the essentiality of GO:0006555 for human health.
From L-methionine metabolic process-Related Genes to Experimental Models
| Research Question | Suitable Model |
|---|---|
| Does loss of metA affect methionine auxotrophy? | CRISPR knockout of metA in E. coli or C. glutamicum |
| How do point mutations in MTR alter enzyme kinetics? | Point-mutation knock-in in human cell lines |
| Can overexpression of metK increase SAM production? | Overexpression of metK in S. cerevisiae or C. glutamicum |
| What is the subcellular localization of CBS? | Tagged knock-in with fluorescent protein |
| Which genes regulate methionine flux? | CRISPR library screening targeting metabolic enzymes |
| How does SAM scarcity activate RIPK1? | Knockout and point-mutation models in mammalian cells |
How to Study the L-methionine metabolic process Process
| Method | What It Measures | Typical Application |
|---|---|---|
| LC-MS/MS metabolomics | Intracellular methionine, SAM, SAH, homocysteine levels | Quantify pathway flux in engineered strains |
| RNA-seq | Transcript levels of methionine pathway genes | Identify regulatory changes under perturbations |
| Proteomics | Protein abundance of pathway enzymes | Validate overexpression or knockout effects |
| Enzyme activity assay | Catalytic rate and feedback inhibition | Characterize MetA, MetB, MetC, MetE, MetH, MAT |
| CRISPR knockout screening | Gene essentiality and fitness in methionine-limited conditions | Discover novel pathway regulators |
| CRISPR point mutation knock-in | Effect of specific variants on enzyme function | Model human polymorphisms in MTR or CBS |
| Fluorescent tagging | Subcellular localization and dynamics | Study CBS or MAT localization |
| Bayesian optimization | Optimal combination of genetic modifications | Enhance SAM production in yeast |
Metabolic flux analysis and metabolomics
Quantifying intracellular methionine, SAM, SAH, and homocysteine by LC-MS/MS reveals pathway activity and bottlenecks. These methods have been used to evaluate engineered strains for methionine and SAM production.
Transcriptomics and proteomics
RNA-seq and proteomics identify expression changes in methionine biosynthetic genes under different conditions, such as module genetic perturbations in E. coli. These approaches help map regulatory networks and identify targets for engineering.
Enzyme activity assays
In vitro assays for MetA, MetB, MetC, MetE, MetH, and MAT measure catalytic rates and feedback inhibition, providing kinetic parameters for metabolic models.
CRISPR screening and functional genomics
Genome-wide CRISPR knockout or interference screens can identify genes that modulate methionine dependence or SAM levels, enabling discovery of novel regulators of GO:0006555.
How CRISPR Can Be Used to Study GO:0006555 L-methionine metabolic process
Knockout
CRISPR knockout of methionine pathway genes such as metA, metB, metC, metE, metH, or metK can create auxotrophic strains, enabling selection and flux analysis. In mammalian cells, knockout of MAT2A or CBS reveals essential roles in SAM production and transsulfuration.
Point Mutation
Point mutations introduced by CRISPR base editing or HDR can mimic human polymorphisms in MTR, CBS, or MTHFR, allowing functional assessment of enzyme variants and their impact on homocysteine levels.
Knock-in
Knock-in of tagged versions of pathway enzymes (e.g., GFP-MAT2A) enables live-cell imaging and interaction studies. Knock-in of patient-derived mutations in MTR or CBS provides disease models for inborn errors of metabolism.
Overexpression
CRISPR activation (CRISPRa) or plasmid-based overexpression of metK, metE, metH, or SAM2 increases flux to methionine and SAM, as demonstrated in engineered E. coli, C. glutamicum, and S. cerevisiae.
How EDITGENE Supports L-methionine metabolic process Research
Researchers studying L-methionine metabolic process-related genes often need to determine whether a candidate gene is causally involved in pathway regulation, disease, or industrial production. EDITGENE provides CRISPR-based cell model services to enable precise functional interrogation of these genes.
Contact EDITGENE today to design your custom CRISPR model for L-methionine metabolic process research.
Frequently Asked Questions About L-methionine metabolic process
What is L-methionine metabolic process (GO:0006555)?
It is the set of biochemical reactions and pathways involving L-methionine, an essential sulfur-containing amino acid, including its synthesis, conversion to SAM, remethylation of homocysteine, and catabolism.
What genes are involved in L-methionine metabolic process?
Key genes include metA, metB, metC, metE, metH, metK in bacteria, SAM2 and MET6 in yeast, and MTR, CBS, MAT1A, MAT2A, MTHFR, BHMT, GNMT, and AHCY in mammals.
Why is methionine metabolism important for cancer?
It supplies SAM for epigenetic methylation and supports redox balance; SAM scarcity can trigger RIPK1-dependent cell death, linking the pathway to cancer and inflammation.
How is L-methionine produced in industry?
Microbial fermentation using engineered E. coli, C. glutamicum, or S. cerevisiae strains that overexpress biosynthetic enzymes and relieve feedback repression.
What is the role of SAM in methionine metabolism?
SAM is the major methyl donor produced from methionine by MAT enzymes; it fuels methylation reactions and its scarcity is sensed by stress pathways.
Which diseases are linked to methionine metabolism?
Homocystinuria, cardiovascular disease, neurological disorders, liver disease, and cancer have been associated with altered methionine metabolism.
How can CRISPR be used to study methionine metabolism?
CRISPR knockout, point mutation, knock-in, and overexpression enable functional dissection of pathway genes in bacteria, yeast, and mammalian cells.
What methods measure methionine pathway activity?
LC-MS/MS metabolomics, RNA-seq, proteomics, enzyme activity assays, and CRISPR screens are commonly used.
What is the difference between methionine and SAM?
Methionine is an amino acid used in proteins; SAM is its activated derivative that donates methyl groups in cellular reactions.
Can methionine metabolism be engineered for higher yields?
Yes, systems metabolic engineering and Bayesian optimization have improved L-methionine and SAM production in microbial hosts.
Conclusion
GO:0006555, L-methionine metabolic process, is a fundamental biological pathway that bridges protein synthesis, one-carbon metabolism, and epigenetic regulation. Its enzymes and metabolites are critical for human health and disease, and they are prime targets for industrial biotechnology. CRISPR-based models and multi-omics methods continue to reveal new layers of regulation, offering opportunities for therapeutic and biotechnological innovation.
References
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- 3. Chen Z et al.. 2025. RIPK1 senses S-adenosylmethionine scarcity to drive cell death and inflammation.. Cell Metab 37(8):1732-1749.e9 PMID: 40570842
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- 6. Shen ZY et al.. 2023. Local metabolic response of Escherichia coli to the module genetic perturbations in l-methionine biosynthetic pathway.. J Biosci Bioeng 135(3):217-223 PMID: 36707399
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