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. 2023 Oct 11;28(20):7033. doi: 10.3390/molecules28207033

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© 2023 by the authors.

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

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Schematic diagram of O-GalNAc glycans and N-glycans. (A) The first step in the synthesis of O-GalNAc glycans is the attachment of UDP-GalNAc to Ser/Thr residues of the protein in the presence of ppGalNAcTs to form the initial structure GalNAcα1-O-Ser/Thr (Tn antigen). Tn antigen is synthesized through the core 1 structure (T antigen) by C1GALT1. C1GALT1 needs to be rendered active by COSMC in the ER, and active C1GALT1 exists as a dimer. Tn antigens can also synthesize core 3 structures via β3GnT6, and core 1 and core 3 structures can be further synthesized into core 2 or core 4 structures. STn antigens are produced by the addition of sialic acid to the α1-6 linkage of the GalNAc residue of Tn via ST6GalNAc-I. These core structures are usually further extended and terminated with sialic and fucose residues. Finally, terminal Lewis antigens include Le^a, Le^b, Sle^a, Le^x, Le^y, and Sle^x, etc.; (B) N-glycosylation is initiated by transferring GlcNAc to Dol-P at the ER membrane to form Dol-P-P-GlcNAc, a process mediated by GPT. The 5 GDP-Man residues are then sequentially ligated on by specific glycosyltransferases to generate the Man₅GlcNAc₂-P-P-Dol structure. This structure is flipped into the ER lumen and the addition of 4 Man and 3 Glc continues to generate the 14-glycan structure, the precursor of the N-glycan. The 14-glycan structure is transferred by OST to the Asn residue of the protein. The 14-glycan structure is then vesicle-wrapped into the Golgi for terminal glycosylation, which involves the removal of mannose and the addition of other sugar groups. After terminal glycosylation, N-glycosylated proteins are divided into three main types, high mannose, hybrid, and complex. The glycan structure is represented according to the SNFG format [46].

Schematic diagram of O-GalNAc glycans and N-glycans. (A) The first step in the synthesis of O-GalNAc glycans is the attachment of UDP-GalNAc to Ser/Thr residues of the protein in the presence of ppGalNAcTs to form the initial structure GalNAcα1-O-Ser/Thr (Tn antigen). Tn antigen is synthesized through the core 1 structure (T antigen) by C1GALT1. C1GALT1 needs to be rendered active by COSMC in the ER, and active C1GALT1 exists as a dimer. Tn antigens can also synthesize core 3 structures via β3GnT6, and core 1 and core 3 structures can be further synthesized into core 2 or core 4 structures. STn antigens are produced by the addition of sialic acid to the α1-6 linkage of the GalNAc residue of Tn via ST6GalNAc-I. These core structures are usually further extended and terminated with sialic and fucose residues. Finally, terminal Lewis antigens include Le^a, Le^b, Sle^a, Le^x, Le^y, and Sle^x, etc.; (B) N-glycosylation is initiated by transferring GlcNAc to Dol-P at the ER membrane to form Dol-P-P-GlcNAc, a process mediated by GPT. The 5 GDP-Man residues are then sequentially ligated on by specific glycosyltransferases to generate the Man₅GlcNAc₂-P-P-Dol structure. This structure is flipped into the ER lumen and the addition of 4 Man and 3 Glc continues to generate the 14-glycan structure, the precursor of the N-glycan. The 14-glycan structure is transferred by OST to the Asn residue of the protein. The 14-glycan structure is then vesicle-wrapped into the Golgi for terminal glycosylation, which involves the removal of mannose and the addition of other sugar groups. After terminal glycosylation, N-glycosylated proteins are divided into three main types, high mannose, hybrid, and complex. The glycan structure is represented according to the SNFG format [46].