# Aspartylglucosaminidase
**Aspartylglucosaminidase** (AGA; also called glycosylasparaginase, EC 3.5.1.26) is a lysosomal enzyme that cleaves the N-glycosidic bond between an asparagine residue and an N-acetylglucosamine sugar, one of the final steps in the breakdown of N-linked glycoproteins inside lysosomes[1](https://medlineplus.gov/genetics/gene/aga/) • [2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5134220/). Deficiency of the enzyme causes [aspartylglucosaminuria](https://www.edgepedia.org/aspartylglucosaminuria) (AGU), a recessively inherited lysosomal storage disease characterized by progressive intellectual disability and premature death, usually before the age of 50 years[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5134220/).
| Key fact | Detail |
|---|---|
| Enzyme class | Lysosomal amidase, N4-(β-N-acetylglucosaminyl)-L-asparaginase, EC 3.5.1.26[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC5412292/) |
| Reaction | Hydrolyzes the bond between L-asparagine and N-acetylglucosamine (GlcNAc-Asn) in glycoasparagines[1](https://medlineplus.gov/genetics/gene/aga/) |
| Protein family | N-terminal nucleophile (Ntn) hydrolase superfamily; catalytic residue is N-terminal threonine Thr206 of the β-subunit[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC5412292/) • [4](https://www.mdpi.com/2073-4409/11/10/1592) |
| Maturation | Synthesized as a 346-amino-acid precursor that autocleaves N-terminally to Thr206 into pro-α (27 kDa) and β (17 kDa) subunits, forming an (αβ)2 heterotetramer[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC5412292/) |
| Gene | AGA, 9 exons, chromosome 4q34.3[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5134220/) |
| Major disease mutation | C163S (Cys163Ser), responsible for 98% of AGU cases in Finland[1](https://medlineplus.gov/genetics/gene/aga/) |
| Structure solved | High-resolution crystal structure published in 1995 (PDB 1APY)[5](https://www.nature.com/articles/nsb1295-1102) • [4](https://www.mdpi.com/2073-4409/11/10/1592) |
## Function in glycoprotein degradation
Many glycoproteins carry carbohydrate chains attached to the protein backbone through an N-glycosidic bond between the amino acid L-asparagine and the monosaccharide N-acetylglucosamine. Within lysosomes, aspartylglucosaminidase cuts this bond, releasing L-aspartic acid, ammonia, and either free N-acetylglucosamine or a carbohydrate chain[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5134220/). The cut is one of the last steps in lysosomal glycoprotein breakdown[1](https://medlineplus.gov/genetics/gene/aga/).
The enzyme acts only after complete proteolysis of the polypeptide around the glycosylated asparagine, because both the β-amino and β-carboxyl groups of the L-asparagine moiety must be free for the substrate to anchor in the active site[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5134220/). Glycosylasparaginase also acts as an L-asparaginase, hydrolyzing free L-asparagine to L-aspartic acid and ammonia[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5134220/).
## Structure and activation
AGA is a member of the N-terminal nucleophile (Ntn) hydrolase superfamily, a group of enzymes that share an αββα-fold, very little sequence similarity, and a catalytic nucleophile (serine, threonine, or cysteine) generated at the [N-terminus](https://www.edgepedia.org/n-terminus) of the β-chain by autocatalytic cleavage[4](https://www.mdpi.com/2073-4409/11/10/1592) • [6](https://pmc.ncbi.nlm.nih.gov/articles/PMC2144523/). The enzyme is synthesized in the endoplasmic reticulum as a single inactive 346-amino-acid precursor polypeptide; after removal of a 23-residue signal peptide, two precursors homodimerize and each undergoes autocatalytic cleavage N-terminal to Thr206, producing a 27 kDa pro-α and a 17 kDa β subunit[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC5412292/). Two αβ dimers then pack together into the final heterotetramer[5](https://www.nature.com/articles/nsb1295-1102). Further lysosomal trimming of both subunits does not affect enzyme activity[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC5412292/).
The same threonine serves a dual role: its side-chain hydroxyl acts as the nucleophile in substrate cleavage, and its free α-amino group acts as the base, and it also drives its own activation by attacking the preceding peptide bond[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC5412292/) • [7](https://pmc.ncbi.nlm.nih.gov/articles/PMC1223969/). The catalytic residue sits in a deep pocket of a funnel-shaped active site, and the enzyme has an exceptionally high pH optimum for a lysosomal enzyme[5](https://www.nature.com/articles/nsb1295-1102). Mutating the active-site threonine to alanine abolishes both self-activation and catalytic activity[8](https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/aspartylglucosaminidase).
## The AGA gene and disease mutations
The AGA gene comprises 9 exons and is located on chromosome 4q34.3[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5134220/). Biallelic pathogenic variants cause aspartylglucosaminuria, and most known variants change a single amino acid in the enzyme[1](https://medlineplus.gov/genetics/gene/aga/). In Finnish patients, three research groups independently identified homozygosity for two linked point mutations, G482A and G488C, causing the substitutions R161Q and C163S; expression studies showed that only C163S abolishes enzyme activity, by destroying a disulfide bond and preventing autocleavage of the precursor into active subunits[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5134220/).
This AGUFIN major mutation is a founder mutation (variant inherited from a small ancestral population) responsible for about 98% of Finnish cases, which simplifies carrier detection and prenatal diagnosis in that population[1](https://medlineplus.gov/genetics/gene/aga/) • [2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5134220/). A 2-base-pair deletion in exon 2, the AGUFIN minor mutation, accounts for roughly 1.5% of Finnish patients. More than 30 disease-causing AGA mutations are cataloged worldwide, about half of them missense mutations, and no genotype-phenotype correlations have been described[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5134220/) • [9](https://www.ncbi.nlm.nih.gov/sites/books/NBK599378/).
## Role in aspartylglucosaminuria
When AGA activity is deficient, undegraded aspartylglucosamine (GlcNAc-Asn) and related glycoasparagines accumulate in tissues and body fluids, and hypertrophied storage lysosomes appear in all tissues[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5134220/). The resulting disease, aspartylglucosaminuria, is the most common disorder of glycoprotein degradation and belongs to the Finnish disease heritage: approximately 260 Finnish patients have been reported, with a prevalence of 1.5 to 5 per 100,000 live births in Finland and an estimated carrier rate of 1 in 50 to 60 among Finns[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5134220/) • [9](https://www.ncbi.nlm.nih.gov/sites/books/NBK599378/). Clinically, affected children show developmental delay, recurrent infections, and coarsening facial features, with progressive intellectual and psychomotor decline through adulthood[9](https://www.ncbi.nlm.nih.gov/sites/books/NBK599378/).
Diagnosis rests on finding elevated aspartylglucosamine on urinary oligosaccharide analysis together with reduced aspartylglucosaminidase activity in serum, leukocytes, or fibroblasts, or biallelic pathogenic variants in AGA by molecular testing[9](https://www.ncbi.nlm.nih.gov/sites/books/NBK599378/). An Aga-knockout mouse replicates the human disease and has been the main model for therapy research[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5134220/).
## Therapeutic development
No disease-modifying treatment is established. Early enzyme replacement therapy in AGU mice reduced brain aspartylglucosamine accumulation by up to 40% depending on age and protocol, but clinical development is complicated by the need for high early doses of recombinant enzyme to cross the blood-brain barrier, and no human enzyme replacement trials have been reported[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5134220/). Allogenic stem cell transplantation has not proved effective[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5134220/).
[Gene therapy](https://www.edgepedia.org/gene-therapy) is the main active approach. In October 2020 the European Commission granted orphan designation EU/3/20/2334 to an adeno-associated viral vector serotype 9 encoding a codon-optimized human aspartylglucosaminidase transgene (NGN-AGA), intended to make the patient's own cells produce the enzyme[10](https://www.ema.europa.eu/ga/medicines/human/orphan-designations/eu-3-20-2334). A first-in-human phase 1 study of scAAV9/AGA gene therapy in children and adults with aspartylglucosaminuria has also been registered[11](https://trialscreen.org/trials/phase-1-2-safety-efficacy-scaav9-aga-gene-therapy-participants-aspartylglucosaminuria-trial-nct07530796).
## References
1. [AGA gene, MedlinePlus Genetics](https://medlineplus.gov/genetics/gene/aga/)
2. [Arvio M, Mononen I (2016). Aspartylglycosaminuria: a review. Orphanet Journal of Rare Diseases 11:162](https://pmc.ncbi.nlm.nih.gov/articles/PMC5134220/)
3. [Peltola M, et al. Functional Analysis of the Ser149/Thr149 Variants of Human Aspartylglucosaminidase. PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC5412292/)
4. [The Human Ntn-Hydrolase Superfamily: Structure, Functions and Perspectives. Cells 2022;11:1592](https://www.mdpi.com/2073-4409/11/10/1592)
5. [Oinonen C, Tikkanen R, Rouvinen J, Peltonen L (1995). Three-dimensional structure of human lysosomal aspartylglucosaminidase. Nature Structural Biology 2:1102-1108](https://www.nature.com/articles/nsb1295-1102)
6. [Oinonen C, Rouvinen J (2000). Structural comparison of Ntn-hydrolases. Protein Science 9:2329-2337](https://pmc.ncbi.nlm.nih.gov/articles/PMC2144523/)
7. [Autoproteolytic activation of human aspartylglucosaminidase. PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC1223969/)
8. [Aspartylglucosaminidase, Handbook of Proteolytic Enzymes (ScienceDirect Topics)](https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/aspartylglucosaminidase)
9. [Goodspeed K, Chen X, Tchan M. Aspartylglucosaminuria. GeneReviews, NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/sites/books/NBK599378/)
10. [EU/3/20/2334 - orphan designation for treatment of aspartylglucosaminuria, European Medicines Agency](https://www.ema.europa.eu/ga/medicines/human/orphan-designations/eu-3-20-2334)
11. [A Phase 1 gene therapy study of scAAV9/AGA in Aspartylglucosaminuria, TrialScreen](https://trialscreen.org/trials/phase-1-2-safety-efficacy-scaav9-aga-gene-therapy-participants-aspartylglucosaminuria-trial-nct07530796)
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*Topic: Encyclopedia › Life and health › Biological foundations › Biochemistry and metabolism › Enzyme classes and activities › Proteolytic and peptidase enzymes › Proteases by catalytic mechanism › Threonine proteases and the proteasome › Non-proteasomal Ntn threonine hydrolases*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Sep 30, 2026; Oct 11, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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