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Mannosidase is an enzyme which hydrolyses mannose. A family of mannosidases are also responsible for processing newly formed glycoproteins in the endoplasmic reticulum into mature glycoproteins containing highly heterogeneous complex-type glycans. Protein N-glycosylation is regulated by a variety of enzymes. Different types of glycosidases and glycosyltransferases catalyze the sugar modification of protein N-glycosylation. α-Mannosidase is a key glycosidase, composed of many members. It plays an important role in the conversion of glycans from high mannose type to complex structure by cutting mannose residues.
Figure 1. Protein structure of mannosidase.
The mannosidases involved in the N-glycosylation process can be divided into α-mannosidase and β-mannosidase according to the difference in the hydrolysis bond positions they catalyze, and they are involved in mannose α-1, 2. The hydrolysis process of α-1,3, α-1,6 and β1-4 glycosidic bonds. So far, there are more than twenty kinds of α-mannosidase cDNA cloned, of which six are from humans. α-Mannosidase is mainly present in the endoplasmic reticulum, Golgi apparatus, lysosomes and other organelles in the cytoplasm. Its function is to splice the differently connected mannose residues at the ends of the oligosaccharide structure to form high mannose, complex, and heterogeneous organelles. Synthetic N-oligosaccharides. Based on substrate specificity, enzyme molecular weight, conserved sequence domains and sensitivity to alkaloid inhibition, Moremen divided the discovered α-mannosidase into two major categories, I and II.
N-glycosylation is a process in which oligosaccharides are bound to the amino groups of aspartyl residues of endocrine proteins and membrane-bound proteins by glycosyltransferase catalyzed by the endoplasmic reticulum, that is, the ribosome of the rough endoplasmic reticulum At the same time as the protein peptide chain is synthesized, once the aspartic acid-Xaa-tryptophan-serine (Asn-Xaa-Ser Thr, Xaa is all amino acid residues except proline) triplet sequence code, that is, sugar It is possible to start glycosylation at the base of the site. As the peptide chain enters the endoplasmic reticulum cavity, the N-sugar chain can be trimmed and processed into a high mannose type by processing enzymes located on the endoplasmic reticulum membrane structure. Enter the Golgi apparatus and continue to be processed and synthesized by enzymes on the membrane structure. In a few cases, the Asn-Xaa-Cys sequence is also used as a glycosylation site. The process of N-glycosylation involves the participation of a variety of enzymes, mainly glycoacyltransferase and glycosidase. The former is responsible for transferring the monosaccharide part of the active donor (usually NDP-sugar) to the acceptor substance. Such as sugars, proteins and lipids; the latter's catalytic activity is to trim various sugar groups on the sugar chain of glycoproteins, and the two together complete N-glycosylation. The following mainly summarizes some characteristics of α-mannosidase in glycosidases. The function of different types of α-mannosidase is to trim the differently connected mannosyl groups in the sugar chain of glycoprotein. It is not only necessary for the maturation of N-glycans, but also participates in its degradation process.
So far, humans have discovered nine diseases related to the glycosylation process, and they are all autosomal recessive diseases. The pruning of each glycosylation in the process of N-glycosylation maturation and degradation is catalyzed by specific enzymes. Lack or insufficient enzymes in any link can lead to incomplete glycosylation of intracellular and secreted glycoproteins. Produce pathological symptoms of congenital glycosylation dysfunction. The diseases that are currently found to be caused by the disorder of α-mannosidase are congenital dysplasia anemia type II and α-Mannosidosis (α-Mannosidosis).
Figure 2. Alpha-mannosidosis is a lysosomal storage disorder.
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