Aspartylglucosaminidase (1-aspartamido-P-acetylglucosamineamidohydrolase, EC 220.127.116.11)(AGA) is a lysosomal enzyme that hydrolyzes N-acetylglucosamine-asparagine linkages which occur in mammalian glycoproteins. It catalyzes one of the final steps in the ordered breakdown of glycoproteins.
The enzyme cleaves the amide bond between Asn and N-acetylglucosamine. The enzyme requires a free a-carboxy and a-amino group on the Asn for catalysis to occur and fucose linked to the 6-position of N-acetylglucosamine must be removed before cleavage by AGA, but it allows substantial variation of the carbohydrate chain in the substrate.
The reaction catalysed by AGA results in two products, an aspartic acid and a carbohydrate chain with core 1-amino-N-acetylglucosamine, from which the 1-amino group is non-enzymatically degraded in the acidic milieu of lysosomes.
Loss-of function mutations in the human AGA gene lead to aspartylglucosaminuria. The majority of the mutations produce either a truncated polypeptide or cause defective folding and phosphorylation of the polypeptides.
PROCESSING AND ACTIVATION
Human AGA is translated as a polypeptide of 346 amino acids containing two potential glycosylation sites. AGA is synthesized as an enzymatically inactive precursor polypeptide that is activated proteolytically by a cleavage in the endoplasmic reticulum (ER) into two subunits, 27 kDa pro-α and 17 kDa β subunit. The active enzyme complex is transported into lysosomes where the pro-a subunit is trimmed from the C-terminus, resulting in mature 24 kDa a subunit. AGA is activated in the lumen of the endoplasmic reticulum (ER) from a single chain precursor by a cleavage that creates the 27-kDa pro-α- and 17-kDa β-subunits and exposes the active site N-terminal threonine in the beginning of the b-subunit . The active enzyme is a heterotetramer consisting of two a- and two b-subunits, and we have earlier suggested that dimerization of two precursor polypeptides actually precedes the proteolytic activation step. The maturation of AGA is completed in lysosomes where a small peptide is cleaved from the C terminus of both subunits. The intracellular events allow easy monitoring for the lysosomal entry of the AGA polypeptides. AGA is transported into lysosomes via the mannose 6-phosphate receptor pathway, which is based on the recognition of the mannose 6-phosphate marker in the oligosaccharide chains of soluble lysosomal hydrolases by a specific receptor in the Golgi and consequent transport of the complex into a prelysosomal compartment. The mannose 6-phosphate marker is generated by a Golgi-resident enzyme, UDP-N-acetylglucosamine phosphotransferase, which recognizes lysosomal enzymes on the basis of a three-dimensional determinant in the protein. The phosphotransferase rec ognizes several distinct regions of native AGA that are located far apart at the surface of the molecule, and the phosphorylation of the oligosaccharides of AGA requires correct spatial position of these lysine-containing structures
The AGA genes are expressed in diverse tissues, consistent with the housekeeping role of the enzyme. The household character of AGA is at both the DNA and the mRNA level. the human aspartylglucosaminidase gene appears to be regulated by a core promoter consisting of two functionally important Sp1 binding sites and, possibly, an additional contributing AP-2 site. Moreover, a more distantly located region exhibiting inhibitory control on gene expression was detected.
AGA is expressed ubiquitously, but both the specific activity and the protein level of the enzyme have been reported to be low in brain as compared with other tissues.
Thera are no differences in the amounts of the AGA transcripts in various tissues. Consequently. regulation of the expression of the AGA gene. Unlike that of most human genes, does not appear to take place primarily at the transcriptional level. This information is of essential importance when the future therapy of this disease is considered.
Finland where it is the most common lysosomal storage disorder , recently, patients from other ethnic backgrounds have been reported .In order to facilitate biochemical and immunological studies of the defective enzyme in these patients, we have undertaken the purification and characterization of the normal human enzyme.
Pollitt and colleagues in England first described aspartylglucosaminuria in two siblings who had severe mental retardation. There was an abnormal urinary excretion of aspartylglucosamine, 2-acetamido-l-(/LL-aspartamido)-l,2-dideoxyglucose, which could be explained on the basis of a deficiency of the enzyme aspartylglycosaminidase.
More than 100 patients have been described so far, most of them in Finland. The clinical course is fairly uniform in most patients . After normal development during the first months of life, there may be frequent upper respiratory tract infections and the appearance of herniae. After about one year of age, clumsiness,muscular hypotonia and slight mental retardation may develop. By about 10 years, there are coarse facial features, moderate to severe mental retardation, increasedclumsiness, excitable behaviour and skeletal dysplasia. The disease progresses with impairment of speech and of motor functions and often leads to death in the thirdor fourth decade.
Electron microscopic investigations of biopsy or autopsy material showed the presence of enlarged lysosomes in all organs studied. The storage lysosomes usually contained amorphous fibrillogranular material on a clear background. Storage phenomena were also observed upon light microscopy, e.g. there was abnormal vacuolation of lymphocytes in blood smears.
Biochemical studies on the nature of the storage material in tissues showed that it consisted mainly of aspartylglucosamine, with minor contributions from other glycoasparagines . Such compounds are formed in the course of glycoprotein catabolism and are normally cleaved to asparagine and N-acetylglucosamine before being released from the lysosome. As uncleaved aspartylglucosamine is unable to pass through the tysosomal membrane, it accumulates in the absence of sufficient aspartylglycosaminidase activity.
The biochemical diagnosis of aspartylglucosaminuria is made by the demonstration of an increased excretion of aspartylglucosamine in the urine, e.g. on a thin layer chromatogram, and of a deficient aspartylglycosaminidase activity in plasma, leukocytes, lymphocytes or cultured fibroblasts; prenatal diagnosis and carrier detection are also possible by use of the enzyme assay .
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