GO:0004637 phosphoribosylamine-glycine ligase activity: Mechanism, Genes and Research Methods
Research-grade guide for scientists and biopharma professionals
Key Takeaways
• GO:0004637 (phosphoribosylamine-glycine ligase activity) catalyzes the ATP-dependent ligation of 5-phospho-D-ribosylamine and glycine to form N(1)-(5-phospho-D-ribosyl)glycinamide, a committed step in de novo purine biosynthesis.
• The enzyme is also known as GAR synthetase (GARS) and is encoded by the GART gene in humans, which produces a trifunctional protein harboring GARS, AIRS, and GART activities.
• In eukaryotes, GARS is part of a purinosome metabolon that channels substrates between sequential enzymes, enhancing pathway efficiency.
• GART/GARS is overexpressed in several cancers and has been linked to tumor stemness and chemoresistance, making it a potential therapeutic target.
• Post-translational modifications and allosteric regulation fine-tune GARS activity in response to cellular purine demand.
• CRISPR-based knockout, knock-in, and overexpression models are essential for dissecting GARS function in purine metabolism and disease.
Description
Phosphoribosylamine-glycine ligase activity (GO:0004637) is a molecular function that catalyzes the second committed step of de novo purine biosynthesis, the ATP-dependent ligation of 5-phospho-D-ribosylamine (PRA) with glycine to produce N(1)-(5-phospho-D-ribosyl)glycinamide (GAR), ADP, phosphate, and protons. This reaction is essential for the synthesis of purine nucleotides, which are required for DNA and RNA synthesis, energy metabolism, and cell signaling. The enzyme responsible for this activity is known as glycinamide ribonucleotide synthetase (GARS) or phosphoribosylglycinamide synthetase, and in humans it is part of a trifunctional protein encoded by the GART gene. Researchers study GO:0004637 because it represents a key node in purine metabolism, with implications for cancer, developmental disorders, and metabolic diseases. The enzyme is highly conserved across evolution, from Drosophila to humans, and its dysfunction can lead to purine imbalances that affect cell proliferation and survival. Moreover, recent evidence indicates that GARS participates in non-canonical functions, such as methyltransferase activity in cancer stemness, expanding its biological significance beyond purine synthesis. Understanding the regulation and structure of GARS is critical for developing targeted therapies. The enzyme is subject to post-translational modifications and participates in a multi-enzyme complex called the purinosome, which dynamically assembles in response to purine demand. This article provides a comprehensive overview of GO:0004637, covering its definition, mechanism, key genes, disease associations, and research methodologies, with a focus on CRISPR-based models for functional studies.
phosphoribosylamine-glycine ligase activity At A Glance
| GO ID | GO:0004637 |
|---|---|
| GO term | phosphoribosylamine-glycine ligase activity |
| Ontology | molecular_function |
| Synonym | GAR synthetase activity; glycinamide ribonucleotide synthetase activity; phosphoribosylglycinamide synthetase activity |
| Major function | Catalyzes the second step of de novo purine biosynthesis, forming GAR from PRA and glycine |
| Reaction | 5-phospho-D-ribosylamine + ATP + glycine = N(1)-(5-phospho-D-ribosyl)glycinamide + ADP + 2 H+ + phosphate |
| Cofactors | ATP, Mg2+ (implied by ATP-dependent ligase mechanism) |
| Subcellular location | Cytoplasm; purinosome complex |
| Pathway | De novo purine biosynthesis (IMP biosynthesis) |
What Is GO:0004637?
Phosphoribosylamine-glycine ligase activity (GO:0004637) is defined as the catalysis of the reaction: 5-phospho-D-ribosylamine + ATP + glycine = N(1)-(5-phospho-D-ribosyl)glycinamide + ADP + 2 H+ + phosphate. In simpler terms, it is an enzyme activity that joins a ribose-phosphate molecule with the amino acid glycine, using ATP as an energy source, to build a precursor for purine nucleotides. This activity is synonymous with GAR synthetase, GARS activity, and glycinamide ribonucleotide synthetase activity, among other names.
Why Is phosphoribosylamine-glycine ligase activity Important in Cell Biology?
GO:0004637 is critical because it catalyzes a rate-limiting step in de novo purine biosynthesis, a pathway essential for nucleotide supply in proliferating cells. Dysregulation of this activity is associated with cancer, as tumor cells often upregulate purine synthesis to support rapid growth. Moreover, the enzyme is part of a dynamic metabolon, the purinosome, which allows efficient substrate channeling and is a target for antimetabolite drugs. Understanding its mechanism and regulation can inform therapeutic strategies for cancer and metabolic disorders.
• Provides precursors for purine nucleotides, essential for DNA/RNA synthesis and cell proliferation.
• Dysregulated in multiple cancers, including colorectal cancer, where GART promotes stemness.
• Target of antifolate and antimetabolite drugs (e.g., methotrexate, pemetrexed) that inhibit purine synthesis.
• Participates in the purinosome, a metabolon that enhances pathway flux and is a potential drug target.
• Mutations in purine synthesis enzymes cause inborn errors of metabolism, such as AICA-ribosiduria and Lesch-Nyhan syndrome (though not directly GARS).
• Post-translational modifications regulate GARS activity, linking it to cellular signaling.
• Evolutionarily conserved from Drosophila to humans, facilitating genetic studies.
• Non-canonical roles in methyltransferase signaling and β-catenin pathway in cancer.
• Essential for nucleotide salvage and homeostasis in non-dividing cells.
• Potential biomarker for cancer diagnosis and prognosis.
Molecular Mechanism of phosphoribosylamine-glycine ligase activity
Substrate Binding and Catalysis
In simple terms: The enzyme grabs two molecules, PRA and glycine, and uses ATP to glue them together.
The catalytic mechanism of GARS involves the ordered binding of substrates: ATP first, then 5-phospho-D-ribosylamine (PRA), followed by glycine. ATP is used to activate the carboxyl group of glycine, forming an acyl phosphate intermediate, which is then attacked by the amino group of PRA to form GAR, ADP, and phosphate. The reaction is reversible but favors GAR synthesis under physiological conditions. Structural studies of the trifunctional human GART protein reveal that the GARS domain adopts a typical ATP-grasp fold, with conserved residues coordinating ATP and Mg2+.
Purinosome Assembly and Substrate Channeling
In simple terms: The enzyme teams up with other purine enzymes to form a tiny factory that passes molecules directly from one to the next.
In eukaryotes, GARS is part of a multi-enzyme complex called the purinosome, which includes other de novo purine biosynthetic enzymes such as PPAT, GART, PFAS, PAICS, ADSL, and ATIC. Fluorescence microscopy studies have shown that purinosomes assemble dynamically in response to purine depletion and disassemble when purines are abundant. This metabolon facilitates substrate channeling, allowing PRA produced by PPAT to be directly transferred to GARS, minimizing diffusion and enhancing pathway efficiency. The interaction between PPAT and GARS is transient and regulated by cellular purine levels.
Cofactors and Metal Requirements
In simple terms: The enzyme needs magnesium ions to help ATP do its job.
GARS requires divalent metal ions, typically Mg2+, for catalysis. ATP binds as a Mg2+-ATP complex, which is essential for the phosphorylation of glycine. The enzyme also requires monovalent cations (e.g., K+) for optimal activity, as shown in studies of chicken liver GARS. The reaction produces protons, which may affect local pH and enzyme activity.
Regulation by Post-Translational Modifications
In simple terms: Chemical tags can be added to the enzyme to turn its activity up or down.
Post-translational modifications (PTMs) of de novo purine biosynthetic enzymes, including GARS, have been mapped by mass spectrometry. Phosphorylation, acetylation, and ubiquitination sites have been identified, suggesting that GARS activity is regulated by cellular signaling pathways. For example, phosphorylation of GART may affect purinosome assembly or catalytic efficiency. These modifications provide a means to rapidly adjust purine synthesis in response to growth signals or stress.
Non-Canonical Functions
In simple terms: The enzyme can also do other jobs outside of purine synthesis, like helping cancer cells stay stem-like.
Recent studies have revealed that GART, the human trifunctional protein containing GARS activity, can act as a methyltransferase in the RUVBL1/β-catenin signaling pathway, promoting tumor stemness in colorectal cancer. This non-canonical function is independent of its role in purine biosynthesis and highlights the multifunctional nature of the protein. The methyltransferase activity targets specific proteins, though the exact substrates remain under investigation.
Key Genes Involved in GO:0004637 phosphoribosylamine-glycine ligase activity
The following genes and proteins are directly involved in or regulate phosphoribosylamine-glycine ligase activity (GO:0004637) and its associated pathways.
| Gene | Major Role | Research Relevance |
|---|---|---|
| GART | Encodes trifunctional enzyme with GARS, AIRS, and GART activities; catalyzes second, third, and fifth steps of purine synthesis | Overexpressed in cancers; target for antifolates; mutations cause purine-related disorders |
| PPAT | Catalyzes first step of purine synthesis, producing PRA; interacts with GARS in purinosome | Regulates substrate supply for GARS; potential target for metabolic inhibitors |
| PFAS | Phosphoribosylformylglycinamidine synthase; third enzyme in pathway | Part of purinosome; mutations cause developmental delay |
| PAICS | Bifunctional enzyme with AIRS and CAIRS activities | Purinosome component; potential cancer target |
| ADSL | Adenylosuccinate lyase; catalyzes two steps in purine synthesis | Deficiency causes succinylpurinemic autism |
| ATIC | Bifunctional enzyme with AICAR transformylase and IMP cyclohydrolase activities | Purinosome component; target of methotrexate |
| GARS (Drosophila) | Encodes GARS activity in Drosophila; part of trifunctional locus | Model for genetic studies of purine synthesis |
| RUVBL1 | AAA+ ATPase; interacts with GART in non-canonical methyltransferase pathway | Regulates β-catenin signaling and stemness |
| β-catenin | Transcription co-activator; downstream of GART methyltransferase activity | Promotes tumor stemness in colorectal cancer |
| PRPS1 | Phosphoribosyl pyrophosphate synthetase 1; supplies PRPP for purine synthesis | Mutations cause gout and deafness |
| PRPS2 | Phosphoribosyl pyrophosphate synthetase 2; isoform of PRPS1 | Regulated by PTMs; potential cancer target |
| GMPS | GMP synthase; converts XMP to GMP in purine salvage | Not directly in de novo pathway but affects purine pools |
| IMPDH1 | Inosine monophosphate dehydrogenase 1; rate-limiting for guanine synthesis | Target of mycophenolic acid; regulated by PTMs |
| IMPDH2 | Inosine monophosphate dehydrogenase 2; isoform | Overexpressed in cancers; target for chemotherapy |
| HPRT1 | Hypoxanthine phosphoribosyltransferase 1; salvage enzyme | Deficiency causes Lesch-Nyhan syndrome; cross-talk with de novo pathway |
| APRT | Adenine phosphoribosyltransferase; salvage enzyme | Deficiency causes kidney stones; affects purine balance |
| MTHFD1 | Methylenetetrahydrofolate dehydrogenase; supplies one-carbon units for purine synthesis | Polymorphisms linked to cancer and birth defects |
| SHMT2 | Serine hydroxymethyltransferase 2; provides glycine for purine synthesis | Mitochondrial enzyme; links serine metabolism to purines |
How Is phosphoribosylamine-glycine ligase activity Regulated?
Phosphoribosylamine-glycine ligase activity is regulated at multiple levels. Transcriptionally, the GART gene is controlled by the transcription factor E2F1, linking its expression to cell cycle progression. Allosterically, purine nucleotides such as AMP, GMP, and IMP feedback-inhibit the pathway, though direct inhibition of GARS is not well characterized. Post-translational modifications, including phosphorylation and acetylation, modulate GARS activity and purinosome assembly. Additionally, the purinosome dynamically assembles in response to purine depletion, a process regulated by mTOR signaling and casein kinase 2. These regulatory mechanisms ensure that purine synthesis matches cellular demand.
phosphoribosylamine-glycine ligase activity and Human Disease
| Gene | Disease / Biology | Potential Experimental Model |
|---|---|---|
| GART | Colorectal cancer; tumor stemness | Knockout and overexpression in HCT116 and SW480 cell lines |
| PPAT | Purine synthesis defects; developmental delay | Knockout in HEK293T cells; rescue with wild-type or mutant PPAT |
| PFAS | Developmental delay; AICA-ribosiduria | Patient-derived fibroblasts; CRISPR correction |
| ADSL | Succinylpurinemic autism | Knockout in neuroblastoma cells; metabolomics |
| ATIC | Cancer; methotrexate resistance | Point mutations in ATIC to study drug binding |
Cancer
GART, the human protein containing GARS activity, is overexpressed in several cancers, including colorectal cancer, where it promotes tumor stemness through a non-canonical methyltransferase function in the RUVBL1/β-catenin pathway. Knockdown of GART reduces tumor growth and stem cell markers, suggesting that GARS activity or its non-canonical role is required for cancer cell proliferation. Additionally, purine synthesis is upregulated in rapidly dividing tumor cells, making GARS a potential target for antimetabolite drugs.
Inborn Errors of Purine Metabolism
Deficiencies in de novo purine synthesis enzymes cause severe neurological and developmental disorders. While GARS deficiency has not been reported in humans, mutations in other pathway enzymes such as PPAT, PFAS, and ADSL lead to AICA-ribosiduria, developmental delay, and autism. These conditions highlight the importance of the pathway in normal development. Mouse models with disrupted purine synthesis exhibit embryonic lethality, underscoring the essential role of GARS.
Neurodegeneration
Purine metabolism is critical for brain function, as purines act as neurotransmitters and neuromodulators. Dysregulation of purine synthesis has been implicated in neurodegenerative diseases such as Alzheimer's and Parkinson's, though direct links to GARS are not established. However, the purinosome is expressed in neurons, and its dysfunction may contribute to neuronal vulnerability.
From phosphoribosylamine-glycine ligase activity-Related Genes to Experimental Models
| Research Question | Suitable Model |
|---|---|
| Does GART knockout reduce cancer cell proliferation? | CRISPR knockout in colorectal cancer cell lines (e.g., HCT116) |
| What is the effect of a catalytic-dead GARS mutation on purine synthesis? | Point mutation (e.g., D123A) knock-in in HEK293T cells |
| How does GART overexpression affect tumor stemness? | Overexpression of GART in patient-derived organoids |
| Can we visualize purinosome assembly in live cells? | Tagged knock-in of GART with GFP in HeLa cells |
| Does a disease-associated mutation in GART affect enzyme activity? | Knock-in of patient mutations in iPSCs followed by differentiation |
| What are the off-target effects of antifolate drugs on GARS? | CRISPR library screening for resistance mutations |
How to Study the phosphoribosylamine-glycine ligase activity Process
| Method | What It Measures | Typical Application |
|---|---|---|
| Fluorescence microscopy | Purinosome assembly and localization | Live-cell imaging of GART-GFP in HeLa cells |
| Enzymatic assay | GARS catalytic activity | Kinetic analysis of purified enzyme or lysates |
| Mass spectrometry | Post-translational modifications | Mapping phosphorylation sites on GARS |
| CRISPR knockout | Gene function and pathway dependence | Knockout of GART in cancer cell lines |
| CRISPR knock-in | Effect of specific mutations | Introduction of catalytic-dead GARS mutation |
| RNA-seq | Transcriptional changes upon GARS perturbation | Knockdown of GART followed by RNA-seq |
| Metabolomics | Purine nucleotide levels | LC-MS quantification of ATP, GTP, etc. |
| Proteomics | Protein-protein interactions | Immunoprecipitation of GARS followed by MS |
Fluorescence Microscopy for Purinosome Detection
Purinosome assembly can be visualized by tagging purine biosynthetic enzymes, including GARS, with fluorescent proteins such as GFP. Cells are grown in purine-depleted medium to induce purinosome formation, and fluorescence microscopy reveals punctate structures. This method allows real-time monitoring of metabolon dynamics and can be combined with live-cell imaging to study regulation.
Enzymatic Assays for GARS Activity
GARS activity is measured using a coupled spectrophotometric assay that monitors the formation of GAR or the consumption of ATP. Typically, purified enzyme or cell lysates are incubated with PRA, glycine, and ATP, and the reaction is followed by detecting ADP production via a NADH-coupled system. Substrate specificity can be probed with analogs of PRA or glycine.
Mass Spectrometry for Post-Translational Modifications
Post-translational modifications of GARS and other purine enzymes can be mapped by mass spectrometry. Immunoprecipitation of GARS followed by trypsin digestion and LC-MS/MS identifies phosphorylation, acetylation, and ubiquitination sites. This approach reveals regulatory mechanisms and crosstalk with signaling pathways.
CRISPR Screening for Pathway Regulators
Genome-wide CRISPR knockout screens can identify genes that modulate sensitivity to purine synthesis inhibitors or affect GARS function. Cells are transduced with a lentiviral sgRNA library, treated with a drug (e.g., methotrexate), and sgRNA abundance is measured by next-generation sequencing. Hits are validated by individual knockout and enzymatic assays.
How CRISPR Can Be Used to Study GO:0004637 phosphoribosylamine-glycine ligase activity
Knockout
CRISPR knockout of GART or other purine synthesis genes is used to study their essentiality in cell proliferation and metabolism. For example, knockout of GART in colorectal cancer cell lines reduces tumor growth and stemness markers. Knockout models also help identify compensatory pathways and drug sensitivity.
Point Mutation
Point mutations in the GARS domain of GART can be introduced by CRISPR-mediated homology-directed repair to dissect catalytic residues. For instance, mutation of the ATP-binding lysine abolishes GARS activity, allowing separation of catalytic and non-canonical functions. Such models are valuable for understanding mechanism and drug resistance.
Knock-in
Knock-in of tagged GART (e.g., GFP or FLAG) enables visualization and purification of the enzyme for interaction studies. Knock-in of patient-derived mutations can model disease phenotypes in iPSCs or organoids. These models are essential for studying purinosome dynamics and PTMs.
Overexpression
Overexpression of wild-type or mutant GART in cell lines is used to study gain-of-function effects, such as increased purine synthesis and tumor stemness. Overexpression models can also test drug resistance and identify downstream signaling changes.
How EDITGENE Supports phosphoribosylamine-glycine ligase activity Research
Researchers studying phosphoribosylamine-glycine ligase activity-related genes often need to determine whether a candidate gene is causally involved in purine metabolism, cancer, or developmental disorders. EDITGENE provides comprehensive CRISPR-based services to generate precise cell models for such investigations.
Contact EDITGENE today to design your custom CRISPR model for phosphoribosylamine-glycine ligase activity research.
Frequently Asked Questions About phosphoribosylamine-glycine ligase activity
What is phosphoribosylamine-glycine ligase activity?
It is an enzyme activity (GO:0004637) that catalyzes the ATP-dependent formation of N(1)-(5-phospho-D-ribosyl)glycinamide from 5-phospho-D-ribosylamine and glycine, a key step in de novo purine biosynthesis.
What genes are involved in phosphoribosylamine-glycine ligase activity?
The primary gene is GART in humans, which encodes a trifunctional enzyme with GARS, AIRS, and GART activities. Other genes in the pathway include PPAT, PFAS, PAICS, ADSL, and ATIC.
What is the role of GARS in purine synthesis?
GARS (glycinamide ribonucleotide synthetase) catalyzes the second step of purine synthesis, converting PRA and glycine to GAR, which is then further processed to IMP.
How is phosphoribosylamine-glycine ligase activity regulated?
It is regulated by transcription (E2F1), allosteric feedback by purine nucleotides, post-translational modifications, and purinosome assembly in response to purine demand.
What diseases are associated with GARS mutations?
No human disease is directly linked to GARS mutations, but GART overexpression is implicated in colorectal cancer and other malignancies. Deficiencies in other purine enzymes cause developmental disorders.
What is the purinosome?
The purinosome is a multi-enzyme complex of de novo purine biosynthetic enzymes, including GARS, that assembles in the cytoplasm to channel substrates efficiently.
How can I study phosphoribosylamine-glycine ligase activity in the lab?
Common methods include enzymatic assays, fluorescence microscopy for purinosome detection, mass spectrometry for PTMs, and CRISPR-based knockout or knock-in models.
What is the reaction catalyzed by GARS?
5-phospho-D-ribosylamine + ATP + glycine = N(1)-(5-phospho-D-ribosyl)glycinamide + ADP + 2 H+ + phosphate.
Is GARS a potential drug target?
Yes, GARS and the purine synthesis pathway are targets for antimetabolite drugs like methotrexate and pemetrexed. Inhibitors of GARS could be developed for cancer therapy.
What model systems are used to study GARS?
Cell lines (e.g., HeLa, HEK293T, HCT116), Drosophila, and mouse models are used. CRISPR knockout and knock-in in human cells are particularly powerful.
Conclusion
Phosphoribosylamine-glycine ligase activity (GO:0004637) is a fundamental enzymatic function in de novo purine biosynthesis, catalyzed by GARS within the trifunctional GART protein. Its regulation, structural features, and role in the purinosome are critical for understanding cellular nucleotide homeostasis. Dysregulation of GARS is linked to cancer and metabolic disorders, making it a promising target for therapeutic intervention. CRISPR-based models offer unprecedented opportunities to dissect its mechanism and develop targeted therapies.
References
- 1. Rudolph J et al.. 1995. Investigation of the mechanism of phosphoribosylamine transfer from glutamine phosphoribosylpyrophosphate amidotransferase to glycinamide ribonucleotide synthetase.. Biochemistry 34(7):2241-50 PMID: 7532005
- 2. Pedley AM et al.. 2018. Detecting Purinosome Metabolon Formation with Fluorescence Microscopy.. Methods Mol Biol 1764:279-289 PMID: 29605921
- 3. Cheng YS et al.. 1987. Chemical characterization of phosphoribosylamine, a substrate for newly discovered trifunctional protein containing glycineamide ribonucleotide synthetase activity.. Adv Enzyme Regul 26:319-33 PMID: 3673708
- 4. Welin M et al.. 2010. Structural studies of tri-functional human GART.. Nucleic Acids Res 38(20):7308-19 PMID: 20631005
- 5. Antle VD et al.. 1996. Substrate specificity of glycinamide ribonucleotide synthetase from chicken liver.. J Biol Chem 271(14):8192-5 PMID: 8626510
- 6. Henikoff S et al.. 1986. Multiple purine pathway enzyme activities are encoded at a single genetic locus in Drosophila.. Proc Natl Acad Sci U S A 83(3):720-4 PMID: 3080748
- 7. Tang C et al.. 2023. GART Functions as a Novel Methyltransferase in the RUVBL1/β-Catenin Signaling Pathway to Promote Tumor Stemness in Colorectal Cancer.. Adv Sci (Weinh) 10(25):e2301264 PMID: 37439412
- 8. Liu C et al.. 2019. Mapping Post-Translational Modifications of de Novo Purine Biosynthetic Enzymes: Implications for Pathway Regulation.. J Proteome Res 18(5):2078-2087 PMID: 30964683
