GO:0043418 L-homocysteine catabolic process: Metabolism, Genes, Functions and Research Methods
Research-grade guide for scientists and biopharma professionals
Key Takeaways
• GO:0043418 describes the biochemical breakdown of L-homocysteine, a sulfur-containing amino acid linked to cardiovascular and neurological disease.
• The process is essential for maintaining low intracellular homocysteine and for recycling methionine via the remethylation and transsulfuration pathways.
• Key enzymes include S-adenosyl-L-homocysteine hydrolase (AHCY), cystathionine beta-synthase (CBS), and methionine synthase (MTR).
• Impaired homocysteine catabolism leads to hyperhomocysteinemia, a risk factor for atherosclerosis, thrombosis, and DNA methylation changes.
• Research models use CRISPR knockout, point mutation, and overexpression of AHCY, CBS, MTR, and related genes to dissect pathway flux and disease mechanisms.
• EDITGENE provides custom cell models and CRISPR library screening to study L-homocysteine catabolic process in disease contexts.
Description
L-homocysteine catabolic process (GO:0043418) is the set of biochemical reactions that degrade L-homocysteine, a non-proteinogenic amino acid intermediate in methionine metabolism. Homocysteine is produced from S-adenosyl-L-homocysteine (SAH) by SAH hydrolase and can be catabolized via remethylation to methionine or transsulfuration to cysteine. Dysregulation of this process results in hyperhomocysteinemia, which is associated with cardiovascular disease, thrombosis, and altered DNA methylation. Researchers study this pathway to understand sulfur amino acid metabolism, oxidative stress, and epigenetic regulation.
L-homocysteine catabolic process At A Glance
| GO ID | GO:0043418 |
|---|---|
| GO term | L-homocysteine catabolic process |
| Ontology | biological_process |
| Synonym | homocysteine breakdown, homocysteine catabolism, homocysteine degradation |
| Major function | Breakdown of L-homocysteine to maintain low cellular levels and supply methionine/cysteine |
| Key enzymes | AHCY, CBS, MTR, MTHFR, BHMT, CTH |
| Associated diseases | Hyperhomocysteinemia, atherosclerosis, thrombosis, neural tube defects |
| Research methods | CRISPR KO/point mutation/knock-in, metabolomics, enzyme assays, RNA-seq |
What Is GO:0043418?
According to the Gene Ontology, GO:0043418 (L-homocysteine catabolic process) encompasses the chemical reactions and pathways resulting in the breakdown of L-homocysteine, the amino acid alpha-amino-gamma-mercaptobutanoic acid. This includes enzymatic steps that convert homocysteine to methionine, cysteine, or other metabolites, thereby preventing its accumulation.
Why Is L-homocysteine catabolic process Important in Cell Biology?
L-homocysteine catabolic process is critical because homocysteine is a toxic intermediate whose accumulation damages endothelial cells, promotes oxidative stress, and interferes with methylation reactions. Proper catabolism ensures adequate methionine and cysteine supply and supports cellular methylation potential. Defects in this pathway are linked to cardiovascular disease, pregnancy complications, and neurological disorders.
• Maintains low homocysteine levels, preventing hyperhomocysteinemia.
• Supplies methionine for protein synthesis and S-adenosylmethionine (SAM) for methylation.
• Provides cysteine for glutathione synthesis and antioxidant defense.
• Dysregulation is a risk factor for atherosclerosis and thrombosis.
• Altered homocysteine metabolism affects DNA methylation and gene expression.
• Relevant to cancer, neurodegeneration, and pregnancy complications.
• Target for nutritional and pharmacological interventions (folate, vitamin B12).
• Key area for CRISPR-based functional genomics and drug discovery.
What Happens During L-homocysteine Catabolic process?
Formation of Homocysteine from SAH
In simple terms: Homocysteine is made when SAH is broken down.
S-adenosyl-L-homocysteine hydrolase (AHCY) catalyzes the reversible hydrolysis of S-adenosyl-L-homocysteine (SAH) to adenosine and L-homocysteine. This reaction is a major source of cellular homocysteine and links methylation reactions to homocysteine catabolism.
Remethylation to Methionine
In simple terms: Homocysteine can be converted back to methionine.
Methionine synthase (MTR) transfers a methyl group from 5-methyltetrahydrofolate to homocysteine, forming methionine and tetrahydrofolate. Alternatively, betaine-homocysteine S-methyltransferase (BHMT) uses betaine as a methyl donor in the liver and kidney. These reactions are vitamin B12- and folate-dependent.
Transsulfuration to Cysteine
In simple terms: Homocysteine can be turned into cysteine.
Cystathionine beta-synthase (CBS) condenses homocysteine with serine to form cystathionine, which is then cleaved by cystathionine gamma-lyase (CTH) to cysteine and alpha-ketobutyrate. This pathway is irreversible and provides cysteine for glutathione synthesis.
Regulation by SAM and Methylation Potential
In simple terms: The pathway is controlled by SAM levels.
S-adenosylmethionine (SAM) activates CBS and inhibits MTHFR, directing homocysteine toward transsulfuration when methylation potential is high. The SAM/SAH ratio is a key indicator of cellular methylation status.
Export and Clearance
In simple terms: Excess homocysteine is exported from cells.
Homocysteine can be exported into plasma, where it circulates in reduced and oxidized forms. Elevated plasma homocysteine is a biomarker for cardiovascular risk.
Key Genes Involved in GO:0043418 L-homocysteine catabolic process
The following genes encode enzymes and regulators directly involved in L-homocysteine catabolic process.
| Gene | Major Role | Research Relevance |
|---|---|---|
| AHCY | Hydrolyzes SAH to homocysteine and adenosine | Key source of homocysteine; target for hyperhomocysteinemia |
| CBS | Condenses homocysteine and serine to cystathionine | Defects cause homocystinuria; cardiovascular risk |
| MTR | Remethylates homocysteine to methionine | Vitamin B12-dependent; linked to neural tube defects |
| MTHFR | Produces 5-methyltetrahydrofolate for remethylation | Common polymorphisms affect homocysteine levels |
| BHMT | Remethylates homocysteine using betaine | Liver-specific; alternative remethylation route |
| CTH | Cleaves cystathionine to cysteine | Provides cysteine for glutathione |
| MAT1A | Synthesizes SAM from methionine | Regulates methylation potential and CBS activity |
| GNMT | Glycine N-methyltransferase; uses SAM | Links methylation to homocysteine production |
| MTRR | Regenerates methionine synthase | Supports remethylation |
| SLC7A11 | Cystine/glutamate antiporter | Affects cysteine availability and redox |
| GCLC | Glutamate-cysteine ligase | Glutathione synthesis downstream of cysteine |
| GCLM | Modulatory subunit of GCLC | Regulates glutathione synthesis |
| NOS3 | Endothelial nitric oxide synthase | Homocysteine-induced endothelial dysfunction |
| MTHFD1 | Folate metabolism | Provides methyl groups for remethylation |
| DHFR | Dihydrofolate reductase | Folate cycle support |
| SLC19A1 | Folate transporter | Uptake of folate for remethylation |
| PEMT | Phosphatidylethanolamine N-methyltransferase | Consumes SAM, affects homocysteine |
How Is L-homocysteine catabolic process Regulated?
L-homocysteine catabolic process is regulated by the cellular methylation potential, primarily through the SAM/SAH ratio. SAM allosterically activates CBS and inhibits MTHFR, shifting homocysteine toward transsulfuration when methionine is abundant. Conversely, low SAM favors remethylation to conserve methionine. Hormonal and nutritional factors, including folate, vitamin B12, and betaine, also modulate pathway flux. Oxidative stress can affect enzyme activities and homocysteine export.
L-homocysteine catabolic process and Human Disease
| Gene | Disease / Biology | Potential Experimental Model |
|---|---|---|
| CBS | Homocystinuria, atherosclerosis | CRISPR knockout in hepatocytes or iPSCs |
| MTHFR | Neural tube defects, cardiovascular risk | Point mutation knock-in in cell lines |
| AHCY | Hyperhomocysteinemia, methylation defects | Overexpression and knockout in HEK293 |
| MTR | Vitamin B12-responsive disorders | Knockout in fibroblasts |
| NOS3 | Endothelial dysfunction | Knock-in of polymorphic variants |
Cardiovascular Disease and Atherosclerosis
Elevated homocysteine is an independent risk factor for atherosclerosis, thrombosis, and endothelial dysfunction. Homocysteine promotes oxidative stress, impairs nitric oxide bioavailability, and stimulates vascular smooth muscle proliferation. Catabolic defects, such as CBS deficiency, cause severe hyperhomocysteinemia and premature vascular disease.
Neurological and Neurodegenerative Disorders
Homocysteine catabolism is crucial in the brain, where methylation reactions are essential for neurotransmitter synthesis and myelin maintenance. Hyperhomocysteinemia is associated with cognitive decline, Alzheimer's disease, and neural tube defects. The SAM/SAH ratio in cerebrospinal fluid reflects brain methylation status.
Cancer and Epigenetics
Altered homocysteine metabolism affects DNA methylation and gene expression, contributing to cancer development. Folate deficiency and MTHFR polymorphisms influence homocysteine levels and epigenetic stability. Targeting homocysteine catabolic enzymes is explored in cancer therapy.
Psoriasis and Inflammatory Conditions
Hyperhomocysteinemia is linked to psoriasis and systemic inflammation, potentially through DNA methylation changes and cardiovascular risk. Homocysteine may serve as a common route for cardiovascular risk and epigenetic alterations in psoriasis.
From L-homocysteine catabolic process-Related Genes to Experimental Models
| Research Question | Suitable Model |
|---|---|
| Does loss of CBS increase homocysteine? | CBS knockout cell line (HepG2 or iPSC-derived hepatocytes) |
| Does MTHFR polymorphism affect remethylation? | MTHFR point mutation knock-in (e.g., C677T) |
| Can AHCY overexpression alter SAH levels? | AHCY overexpression in HEK293 |
| Does tagged CBS localize to mitochondria? | CBS knock-in with FLAG or GFP tag |
| What genes modify homocysteine toxicity? | Genome-wide CRISPR library screening |
| Does homocysteine affect endothelial function? | NOS3 knockout or knock-in in HUVECs |
How to Study the L-homocysteine catabolic process Process
| Method | What It Measures | Typical Application |
|---|---|---|
| LC-MS/MS | Homocysteine, methionine, SAM, SAH levels | Metabolic profiling |
| Enzyme activity assay | CBS, MTR, AHCY catalytic activity | Functional validation |
| CRISPR knockout screen | Gene essentiality and modifiers | Pathway discovery |
| RNA-seq | Transcriptional changes | Expression analysis |
| Proteomics | Protein abundance and modifications | Mechanistic studies |
| Western blot | Protein expression | Validation of KO/overexpression |
| Immunofluorescence | Subcellular localization | Enzyme trafficking |
| Methylation-specific PCR | DNA methylation status | Epigenetic effects |
Metabolomics and Enzyme Assays
Quantification of homocysteine, methionine, cysteine, SAM, and SAH by LC-MS/MS or HPLC is essential to measure pathway flux. Enzyme activity assays for CBS, MTR, and AHCY provide direct functional readouts.
CRISPR Screening and Functional Genomics
Genome-wide CRISPR knockout or activation screens can identify genes that modulate homocysteine sensitivity or metabolite levels. These screens link candidate genes to L-homocysteine catabolic process.
Transcriptomics and Proteomics
RNA-seq and proteomics reveal expression changes in homocysteine catabolic enzymes under different conditions. Post-translational modifications and protein interactions can be studied by mass spectrometry.
Imaging and Reporter Assays
Fluorescent or luminescent reporters for SAM/SAH ratio or homocysteine can monitor pathway activity in live cells. Subcellular localization of enzymes like CBS and AHCY can be visualized with tagged proteins.
How CRISPR Can Be Used to Study GO:0043418 L-homocysteine catabolic process
Knockout
CRISPR knockout of CBS, MTR, AHCY, or MTHFR creates cell models to study loss-of-function effects on homocysteine levels and downstream phenotypes. These models are valuable for dissecting pathway contribution to disease.
Point Mutation
Introducing disease-associated point mutations (e.g., MTHFR C677T, CBS T833C) via CRISPR base editing or HDR allows precise modeling of altered enzyme activity and homocysteine metabolism.
Knock-in
Knock-in of tagged versions (e.g., GFP-AHCY, FLAG-CBS) enables live-cell imaging, immunoprecipitation, and proteomic analysis of homocysteine catabolic enzymes.
Overexpression
CRISPR activation or lentiviral overexpression of CBS, MTR, or BHMT can increase pathway flux and reduce homocysteine levels, providing gain-of-function models for therapeutic target validation.
How EDITGENE Supports L-homocysteine catabolic process Research
Researchers studying L-homocysteine catabolic process-related genes often need to determine whether a candidate gene is causally involved in homocysteine clearance, methylation balance, or disease phenotypes. EDITGENE provides custom CRISPR cell models and screening services to accelerate this functional validation.
Contact EDITGENE today to design your custom CRISPR model for L-homocysteine catabolic process research.
Frequently Asked Questions About L-homocysteine catabolic process
What is L-homocysteine catabolic process?
It is the biochemical breakdown of L-homocysteine, a sulfur amino acid, via remethylation to methionine or transsulfuration to cysteine.
What genes are involved in L-homocysteine catabolic process?
Key genes include AHCY, CBS, MTR, MTHFR, BHMT, and CTH.
What is the GO ID for L-homocysteine catabolic process?
The Gene Ontology ID is GO:0043418.
Why is homocysteine catabolism important?
It prevents toxic homocysteine accumulation and supports methionine and cysteine supply.
What diseases are linked to defects in homocysteine catabolism?
Hyperhomocysteinemia, atherosclerosis, thrombosis, neural tube defects, and cognitive decline.
How is homocysteine catabolism regulated?
By the SAM/SAH ratio, which allosterically regulates CBS and MTHFR.
What methods study L-homocysteine catabolic process?
LC-MS/MS metabolomics, enzyme assays, CRISPR screens, RNA-seq, and proteomics.
Can CRISPR be used to study homocysteine catabolism?
Yes, CRISPR knockout, point mutation, knock-in, and overexpression models are widely used.
What is the role of AHCY in homocysteine catabolism?
AHCY hydrolyzes SAH to homocysteine and adenosine, a major source of homocysteine.
What is the connection between homocysteine and DNA methylation?
Homocysteine catabolism affects SAM/SAH ratio, which influences DNA methylation.
Conclusion
L-homocysteine catabolic process (GO:0043418) is a central metabolic pathway that maintains low homocysteine levels and supports methylation and antioxidant defense. Its dysregulation is implicated in cardiovascular, neurological, and inflammatory diseases. CRISPR-based models and multi-omics approaches are powerful tools to dissect this pathway and identify therapeutic targets.
References
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- 4. Weir DG et al.. 1992. Correlation of the ratio of S-adenosyl-L-methionine to S-adenosyl-L-homocysteine in the brain and cerebrospinal fluid of the pig: implications for the determination of this methylation ratio in human brain.. Clin Sci (Lond) 82(1):93-7 PMID: 1310924
- 5. Brzezinski K. 2020. S-adenosyl-l-homocysteine Hydrolase: A Structural Perspective on the Enzyme with Two Rossmann-Fold Domains.. Biomolecules 10(12) PMID: 33339190
- 6. Wang WM et al.. 2017. Homocysteine: A Potential Common Route for Cardiovascular Risk and DNA Methylation in Psoriasis.. Chin Med J (Engl) 130(16):1980-1986 PMID: 28776552
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