GO:0004754 saccharopine dehydrogenase (NAD+, L-lysine-forming) activity: Mechanism, Genes and Research Methods
Research-grade guide for scientists and biopharma professionals
Key Takeaways
• GO:0004754 describes the enzyme activity that catalyzes the final step of the saccharopine pathway: the NAD+-dependent conversion of L-saccharopine to L-lysine and 2-oxoglutarate.
• The reaction is reversible and follows an ordered kinetic mechanism with a pH-dependent chemical step, as shown by initial-rate pH studies.
• The enzyme is a member of the saccharopine dehydrogenase family and is structurally conserved from yeast to humans.
• In Saccharomyces cerevisiae, the enzyme is encoded by LYS1 and is essential for lysine biosynthesis; in mammals, the homologous enzyme participates in lysine catabolism.
• Crystal structures of ligand-bound yeast saccharopine dehydrogenase have revealed the active-site residues and the conformational changes that occur during catalysis.
• Dysregulation of saccharopine dehydrogenase activity is linked to metabolic disorders such as hyperlysinemia and may contribute to cancer and neurological diseases.
Description
Saccharopine dehydrogenase (NAD+, L-lysine-forming) activity, classified as GO:0004754, is a molecular function that catalyzes the reversible NAD+-dependent oxidative deamination of L-saccharopine to yield L-lysine, 2-oxoglutarate, NADH, and a proton. This reaction constitutes the final step of the saccharopine pathway, a major route for lysine biosynthesis in fungi and plants and for lysine catabolism in mammals. The enzyme is therefore central to amino acid homeostasis and to the metabolic flux that connects lysine with the tricarboxylic acid cycle intermediate 2-oxoglutarate. Researchers study GO:0004754 because it represents a key control point in lysine metabolism. In Saccharomyces cerevisiae, the enzyme is encoded by LYS1, and its activity is required for growth in the absence of exogenous lysine. In mammals, the homologous enzyme, often referred to as saccharopine dehydrogenase (SDH), functions in the mitochondrial matrix and is part of the bifunctional alpha-aminoadipic semialdehyde synthase (AASS) in humans. Defects in this pathway cause hyperlysinemia and related neurometabolic disorders, making the enzyme a target for diagnostic and therapeutic research. From a mechanistic perspective, saccharopine dehydrogenase has served as a model for studying dehydrogenase chemistry, cofactor specificity, and pH-dependent kinetic mechanisms. The availability of high-resolution crystal structures of the yeast enzyme in complex with ligands has provided atomic-level insights into substrate binding and catalysis. These features make GO:0004754 an attractive subject for structural biology, enzymology, and metabolic engineering.
saccharopine dehydrogenase (NAD+, L-lysine-forming) activity At A Glance
| GO ID | GO:0004754 |
|---|---|
| GO term | saccharopine dehydrogenase (NAD+, L-lysine-forming) activity |
| Ontology | molecular_function |
| Synonym | 6-N-(L-1,3-dicarboxypropyl)-L-lysine:NAD+ oxidoreductase (L-lysine-forming) |
| Major function | Catalyzes the final step of the saccharopine pathway, converting L-saccharopine to L-lysine and 2-oxoglutarate using NAD+ as cofactor. |
| Reaction | L-saccharopine + H2O + NAD+ = 2-oxoglutarate + L-lysine + H+ + NADH |
| Cofactor | NAD+ (nicotinamide adenine dinucleotide, oxidized form) |
| Localization | Mitochondrial matrix in mammals; cytoplasm in yeast and plants. |
| Enzyme class | Oxidoreductase (EC 1.5.1.7) |
What Is GO:0004754?
GO:0004754, saccharopine dehydrogenase (NAD+, L-lysine-forming) activity, is defined as the catalysis of the reaction: L-saccharopine + H2O + NAD+ = 2-oxoglutarate + L-lysine + H+ + NADH. In other words, it is the enzyme activity that removes the glutaryl moiety from saccharopine, transferring electrons to NAD+ and releasing lysine and 2-oxoglutarate. This activity is reversible and is specific for NAD+ as the electron acceptor, distinguishing it from other saccharopine dehydrogenases that may use NADP+.
Why Is saccharopine dehydrogenase (NAD+, L-lysine-forming) activity Important in Cell Biology?
GO:0004754 is important because it governs the terminal step of the saccharopine pathway, a metabolic route that is essential for lysine biosynthesis in microorganisms and plants and for lysine degradation in animals. The activity directly influences cellular levels of lysine, a proteinogenic amino acid that is also a precursor for carnitine and a regulator of mTOR signaling. Moreover, the reaction produces 2-oxoglutarate, a key TCA cycle intermediate and co-substrate for dioxygenases, thereby linking amino acid metabolism to epigenetic regulation and cellular redox balance. In humans, impaired saccharopine dehydrogenase activity causes hyperlysinemia and has been associated with neurological dysfunction, making the enzyme a potential biomarker and therapeutic target.
• Essential for lysine biosynthesis in fungi and plants, and for lysine catabolism in mammals.
• Defects in the enzyme cause hyperlysinemia, a metabolic disorder with neurological symptoms.
• Provides a model system for studying dehydrogenase mechanisms and NAD+ specificity.
• Links lysine metabolism to the TCA cycle via production of 2-oxoglutarate.
• Crystal structures have revealed key active-site residues and conformational dynamics.
• Potential target for antifungal and herbicide development due to its role in lysine biosynthesis.
• May contribute to cancer metabolism through altered lysine flux and 2-oxoglutarate availability.
• Involved in the regulation of mTORC1 signaling via lysine sensing.
• Enzyme activity can be modulated by pH and chemical modification, as shown by kinetic studies.
• Its reversible nature allows it to function in both biosynthetic and catabolic contexts.
Molecular Mechanism of saccharopine dehydrogenase (NAD+, L-lysine-forming) activity
Substrate Binding and Cofactor Specificity
In simple terms: The enzyme grabs its substrates and the NAD+ cofactor in a specific order.
Saccharopine dehydrogenase binds L-saccharopine and NAD+ in an ordered fashion, with NAD+ binding first, as deduced from initial-rate pH studies. The enzyme is specific for NAD+ over NADP+, and the binding of the cofactor induces conformational changes that prepare the active site for catalysis. Crystal structures of the yeast enzyme in complex with ligands have shown that the substrate is held in place by a network of hydrogen bonds and hydrophobic interactions.
Chemical Mechanism of Catalysis
In simple terms: The enzyme removes a hydride from the substrate and transfers it to NAD+, breaking a carbon-nitrogen bond.
The catalytic mechanism involves the oxidation of the substrate's secondary alcohol to a ketone intermediate, followed by hydrolysis to release lysine and 2-oxoglutarate. The reaction proceeds via a hydride transfer from the substrate to NAD+, forming NADH. pH studies indicate that a single ionizable group with a pKa of about 7.5 is critical for catalysis, likely a histidine or cysteine residue. Chemical modification with diethyl pyrocarbonate inactivates the enzyme, implicating histidine residues in the active site.
Conformational Dynamics and Active-Site Residues
In simple terms: The enzyme changes shape during the reaction, and specific amino acids in the active site do the chemistry.
Ligand-bound crystal structures of Saccharomyces cerevisiae saccharopine dehydrogenase have revealed that the enzyme undergoes a large conformational change upon substrate binding, closing the active site over the substrates. Key residues such as Lys77, His96, and Glu122 (numbering based on the yeast enzyme) are positioned to stabilize the transition state and facilitate hydride transfer. Mutagenesis studies based on these structures have confirmed their essential roles in catalysis.
Reversibility and Regulation
In simple terms: The reaction can go both ways, and the enzyme's activity can be adjusted by cellular conditions.
The reaction catalyzed by saccharopine dehydrogenase is reversible, with the equilibrium favoring lysine formation under physiological conditions. The enzyme's activity is influenced by pH, with optimal activity near neutral pH. In mammals, the enzyme is part of a bifunctional protein (AASS) that also contains alpha-aminoadipic semialdehyde synthase activity, and its expression is regulated by nutritional and hormonal signals. In yeast, LYS1 expression is controlled by the general amino acid control pathway.
Key Genes Involved in GO:0004754 saccharopine dehydrogenase (NAD+, L-lysine-forming) activity
The following genes encode enzymes with saccharopine dehydrogenase (NAD+, L-lysine-forming) activity or are directly involved in the saccharopine pathway across model organisms.
| Gene | Major Role | Research Relevance |
|---|---|---|
| LYS1 (Saccharomyces cerevisiae) | Encodes saccharopine dehydrogenase, catalyzing the final step of lysine biosynthesis. | Model for studying enzyme mechanism and antifungal targeting. |
| LYS9 (Saccharomyces cerevisiae) | Encodes saccharopine dehydrogenase (NADP+, L-glutamate-forming), the reverse reaction in lysine biosynthesis. | Provides comparative insights into cofactor specificity. |
| AASS (Homo sapiens) | Encodes a bifunctional enzyme with saccharopine dehydrogenase and aminoadipate-semialdehyde synthase activities. | Mutations cause hyperlysinemia; target for metabolic disease research. |
| Aass (Mus musculus) | Mouse homolog of AASS, involved in lysine catabolism. | Model for studying hyperlysinemia and neurological phenotypes. |
| SDH (Oryza sativa) | Saccharopine dehydrogenase in rice, involved in lysine biosynthesis. | Target for improving nutritional quality of crops. |
| LYS1 (Candida albicans) | Homolog in pathogenic yeast, essential for lysine biosynthesis. | Potential antifungal target. |
| LYS1 (Schizosaccharomyces pombe) | Fission yeast homolog, involved in lysine biosynthesis. | Model for evolutionary studies. |
| CG13393 (Drosophila melanogaster) | Predicted saccharopine dehydrogenase, involved in lysine catabolism. | Model for developmental and metabolic studies. |
| T20D3.7 (Caenorhabditis elegans) | Homolog of saccharopine dehydrogenase, involved in lysine metabolism. | Model for aging and metabolic research. |
| At4g33150 (Arabidopsis thaliana) | Encodes a bifunctional lysine-ketoglutarate reductase/saccharopine dehydrogenase. | Model for plant lysine catabolism. |
| LKR/SDH (Zea mays) | Bifunctional enzyme in maize lysine catabolism. | Target for improving lysine content in seeds. |
| AASS (Danio rerio) | Zebrafish homolog of AASS, involved in lysine catabolism. | Model for hyperlysinemia and neurological disorders. |
| SDH (Bos taurus) | Saccharopine dehydrogenase in cattle, involved in lysine metabolism. | Comparative biochemistry studies. |
| LYS1 (Kluyveromyces lactis) | Homolog in dairy yeast, involved in lysine biosynthesis. | Industrial strain improvement. |
| AASS (Rattus norvegicus) | Rat homolog of AASS, studied for lysine catabolism. | Model for metabolic regulation. |
| LYS1 (Pichia pastoris) | Homolog in methylotrophic yeast, involved in lysine biosynthesis. | Biotechnological applications. |
How Is saccharopine dehydrogenase (NAD+, L-lysine-forming) activity Regulated?
The activity of saccharopine dehydrogenase (NAD+, L-lysine-forming) is regulated at multiple levels. In Saccharomyces cerevisiae, the expression of LYS1 is controlled by the general amino acid control (GAAC) pathway, which responds to amino acid starvation. In mammals, the bifunctional AASS enzyme is regulated by nutritional status, with high-protein diets increasing its expression in the liver. The enzyme's activity can also be modulated by pH and by post-translational modifications, although specific phosphorylation sites have not been extensively characterized. Additionally, the availability of NAD+ and the redox state of the cell influence the reaction rate, as NADH is a product inhibitor.
saccharopine dehydrogenase (NAD+, L-lysine-forming) activity and Human Disease
| Gene | Disease / Biology | Potential Experimental Model |
|---|---|---|
| AASS | Hyperlysinemia, neurological dysfunction | AASS knockout mouse, patient-derived fibroblasts |
| LYS1 (Candida albicans) | Fungal virulence | C. albicans lys1 deletion strain, antifungal screening |
| LYS1 (Saccharomyces cerevisiae) | Lysine auxotrophy | Yeast lys1 knockout, growth assays |
| AASS | Cancer metabolism | Cancer cell lines with AASS knockdown, xenograft models |
| LKR/SDH (Arabidopsis) | Plant lysine catabolism | Arabidopsis mutants, seed development studies |
Hyperlysinemia and Neurological Disorders
Deficiency of saccharopine dehydrogenase activity, often due to mutations in the AASS gene, leads to hyperlysinemia, a metabolic disorder characterized by elevated lysine levels in blood and urine. Patients may present with neurological symptoms such as intellectual disability, seizures, and hypotonia, although the clinical phenotype is variable. The accumulation of saccharopine and other intermediates may contribute to neurotoxicity.
Cancer Metabolism
Altered lysine metabolism has been observed in several cancers, and saccharopine dehydrogenase activity may influence tumor growth through the production of 2-oxoglutarate, which is required for epigenetic modifications. However, direct evidence linking GO:0004754 to cancer is still emerging, and further studies are needed to establish causality.
Antifungal and Herbicide Targets
Because the saccharopine pathway is essential for lysine biosynthesis in fungi and plants but absent in animals, saccharopine dehydrogenase is considered a promising target for antifungal and herbicide development. Inhibitors of the enzyme could selectively kill pathogens without affecting human cells.
From saccharopine dehydrogenase (NAD+, L-lysine-forming) activity-Related Genes to Experimental Models
| Research Question | Suitable Model |
|---|---|
| What is the effect of LYS1 knockout on lysine biosynthesis? | Saccharomyces cerevisiae lys1 deletion strain |
| How does AASS deficiency affect lysine catabolism in mammals? | Aass knockout mouse |
| What is the catalytic role of active-site residues? | Point mutations in recombinant yeast or human enzyme |
| How does the enzyme localize in cells? | Knock-in of fluorescent tags (e.g., GFP) at the endogenous locus |
| Can overexpression of AASS rescue hyperlysinemia? | Transgenic mouse overexpressing AASS |
| What is the impact of saccharopine dehydrogenase on 2-oxoglutarate levels? | Metabolomics in knockout cell lines |
How to Study the saccharopine dehydrogenase (NAD+, L-lysine-forming) activity Process
| Method | What It Measures | Typical Application |
|---|---|---|
| Spectrophotometric assay | NADH production at 340 nm | Enzyme kinetics and inhibitor screening |
| pH-rate profiling | pKa of catalytic residues | Mechanistic studies |
| Chemical modification | Inactivation by diethyl pyrocarbonate | Identification of essential histidines |
| X-ray crystallography | Three-dimensional structure | Active-site mapping and inhibitor design |
| Site-directed mutagenesis | Effect of point mutations on activity | Validation of catalytic residues |
| CRISPR-Cas9 knockout | Loss of gene function | Phenotypic analysis in cells and animals |
| Metabolomics (LC-MS) | Levels of lysine, saccharopine, 2-oxoglutarate | Diagnosis of hyperlysinemia and flux analysis |
| RNA-seq | Transcriptional changes | Regulation of LYS1/AASS expression |
Enzymatic Assays
Saccharopine dehydrogenase activity can be measured spectrophotometrically by monitoring the formation of NADH at 340 nm or by using coupled assays. Initial-rate kinetics at varying pH are used to determine pKa values of catalytic residues. Chemical modification with diethyl pyrocarbonate followed by activity measurements can identify essential histidines.
Structural Biology
X-ray crystallography of the enzyme in complex with substrates, products, and cofactors has provided atomic-level insights into the catalytic mechanism. Ligand-bound structures of the yeast enzyme have revealed conformational changes and key active-site residues. These structures can guide the design of inhibitors.
Genetic and Genomic Approaches
Knockout and knockdown studies in model organisms such as yeast and mice are used to assess the physiological role of the enzyme. RNA-seq and proteomics can quantify expression changes in response to metabolic stress. CRISPR-Cas9 genome editing enables the creation of precise mutations in the AASS gene to model human disease.
Metabolomics and Flux Analysis
Mass spectrometry-based metabolomics can measure lysine, saccharopine, and 2-oxoglutarate levels in cells and tissues. Stable isotope tracing can determine flux through the saccharopine pathway. These methods are essential for understanding the metabolic consequences of altered enzyme activity.
How CRISPR Can Be Used to Study GO:0004754 saccharopine dehydrogenase (NAD+, L-lysine-forming) activity
Knockout
CRISPR-Cas9 knockout of LYS1 in yeast or AASS in mammalian cells results in complete loss of saccharopine dehydrogenase activity, leading to lysine auxotrophy in yeast and hyperlysinemia in mammals. These models are used to study the metabolic and physiological consequences of enzyme deficiency.
Point Mutation
Introducing point mutations in the active site of saccharopine dehydrogenase (e.g., His96Ala) via CRISPR-Cas9 base editing or homology-directed repair allows researchers to dissect the catalytic mechanism and validate structural predictions. Such mutants can be expressed in a knockout background to assess residual activity.
Knock-in
Knock-in of epitope tags (e.g., FLAG, GFP) at the endogenous AASS or LYS1 locus enables real-time imaging and proteomic analysis of the enzyme. Knock-in of disease-associated mutations (e.g., those found in hyperlysinemia patients) creates isogenic models for studying pathogenesis.
Overexpression
CRISPR activation (CRISPRa) or transgenic overexpression of AASS can be used to study the effects of increased saccharopine dehydrogenase activity on lysine catabolism and 2-oxoglutarate production. Overexpression models are valuable for testing the reversibility of metabolic phenotypes.
How EDITGENE Supports saccharopine dehydrogenase (NAD+, L-lysine-forming) activity Research
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Frequently Asked Questions About saccharopine dehydrogenase (NAD+, L-lysine-forming) activity
What is saccharopine dehydrogenase (NAD+, L-lysine-forming) activity?
It is an enzyme activity defined by GO:0004754 that catalyzes the NAD+-dependent conversion of L-saccharopine to L-lysine and 2-oxoglutarate.
What genes are involved in saccharopine dehydrogenase (NAD+, L-lysine-forming) activity?
In yeast, the LYS1 gene encodes this activity; in humans, the AASS gene encodes a bifunctional enzyme with this activity.
What is the reaction catalyzed by GO:0004754?
L-saccharopine + H2O + NAD+ = 2-oxoglutarate + L-lysine + H+ + NADH.
Which diseases are associated with saccharopine dehydrogenase deficiency?
Mutations in AASS cause hyperlysinemia, which can lead to neurological symptoms.
How is saccharopine dehydrogenase activity measured?
It is typically measured spectrophotometrically by monitoring NADH production at 340 nm.
What is the structure of saccharopine dehydrogenase?
Crystal structures of the yeast enzyme have revealed a two-domain architecture with a deep active-site cleft.
Is saccharopine dehydrogenase reversible?
Yes, the reaction is reversible, and the enzyme can catalyze both lysine formation and saccharopine formation depending on conditions.
What cofactor does saccharopine dehydrogenase use?
It specifically uses NAD+ as the electron acceptor.
How is saccharopine dehydrogenase regulated?
In yeast, LYS1 expression is controlled by the general amino acid control pathway; in mammals, AASS is regulated by nutritional status.
Can saccharopine dehydrogenase be targeted for antifungal therapy?
Yes, because the enzyme is essential for lysine biosynthesis in fungi but absent in humans, it is a potential antifungal target.
Conclusion
GO:0004754, saccharopine dehydrogenase (NAD+, L-lysine-forming) activity, is a fundamental enzymatic function that bridges lysine metabolism with central carbon metabolism. Its mechanism, structure, and regulation have been elucidated through decades of biochemical and structural studies. The enzyme's link to hyperlysinemia and its potential as an antifungal target underscore its biomedical importance. Continued research using CRISPR-based models will further clarify its roles in health and disease.
References
- 1. Fujioka M. 1984. Chemical mechanism of saccharopine dehydrogenase (NAD+, L-lysine-forming) as deduced from initial rate pH studies.. Arch Biochem Biophys 230(2):553-9 PMID: 6712252
- 2. Fujioka M et al.. 1980. The inactivation of saccharopine dehydrogenase (L-lysine-forming) by diethyl pyrocarbonate.. J Biol Chem 255(3):937-42 PMID: 6985909
- 3. Noda C et al.. 1978. Purification and properties of L-lysine-alpha-ketoglutarate reductase from rat liver mitochondria.. Biochim Biophys Acta 525(2):307-13 PMID: 687635
- 4. Ogawa H et al.. 1978. Purification and characterization of saccharopine dehydrogenase from baker's yeast.. J Biol Chem 253(10):3666-70 PMID: 418069
- 5. Andi B et al.. 2007. Crystal structures of ligand-bound saccharopine dehydrogenase from Saccharomyces cerevisiae.. Biochemistry 46(44):12512-21 PMID: 17939687
