Glycosylation Golgi complex, whereby mannosidase-1 is in the cis-Golgi,

Glycosylation is the most common post-translational modification of proteins. This involves a glycosyl donor (usually a carbohydrate) binding to the hydroxyl or another functional group on a second molecule. The regulation of the glycoprotein’s specificity, solubility and secretion is dependent on the glycosylation mechanism (Stanley P, 2011). Glycosylation can be split into N- and O-linked. These two versions differ in the type of atom that oligosaccharides attach to. For N-linked glycosylation, the oligosaccharide attaches to nitrogen atom of the asparagine’s side chain whereas it attaches to oxygen atom on the side chain or serine/threonine for O-linked. Both processes commence in the endoplasmic reticulum (ER) and ends in the Golgi apparatus (Alberts et al, 2014).  Enzymes located in the Golgi complex determines the type of glycan to be manufactured. These enzymes are concentrated in specific areas of the Golgi complex, whereby mannosidase-1 is in the cis-Golgi, GlcNAc transferase is in the medial-Golgi, while glycosyltransferase and sialyltransferase are in the trans-Golgi. The localization of Golgi enzymes directs the order of action and highlights the specificity of the enzyme to a substrate as the protein moves through the Golgi complex (Baum J, 2017).   In N-linked glycosylation, the N-glycan first undergoes trimming in the ER, which involves glycosidase removing three glucose molecules and mannosidase removing a specific mannose from the structure. This exposes a signal for protein translocation from the ER to cis-Golgi via a COPII-coated transport vesicle (Alberts et al, 2014) with a chaperone protein ensuring only properly assembled and folded proteins can leave (Kelleher and Gilmore, 2006).  Upon arrival at the cis-Golgi, mannosidase I removes three mannose residues, Nacetylglucosamine is added by N-acetylglucosamine transferase I (Stanley et al, 2009), and another two mannoses are removed by mannosidase II. This creates a common core region where the bond between N-acetylglucosamines becomes resistant to attack by endoglycosidase. When the protein crosses the medial-Golgi, it acts as a substrate for the three different glycosyltransferases to add N-acetylglucosamines, galactoses and sialic acids/ Nacetylneuraminic acid (NANA) and fucose sequentially, forming a branched oligosaccharide chains using sugar moieties from a sugar nucleotide (Alberts et al, 2014). As the protein reaches the trans-Golgi, glycosyltransferases and glycosidase catalyzes further addition and removal of sugar residues of the protein. Glycosyltransferase is responsible for the addition of sugar residues to the core glycan structure, while N-acetylglucosamine is usually added first before other additional sugars. The carbohydrates and tyrosines also undergo sulfation (Stanley P, 2011). For O-linked glycosylation, there is no agreed sequence for the process due to the O-linked process having several transferases that can attach N-acetylgalatosamine (GalNAc) to serine and threonine residues, unlike N-linked which only catalyzed by oligosaccharyltransferase. The enzymes all differ in their specificity for the primary structures on the glycosylation target, so regions of different glycosylated amino acid residues are formed. However, the localization of enzymes and involvement of nucleoside sugar donors is analogous to the terminal modification of the N-linked sugars. The production of glycosphingolipids that also occur in the Golgi complex is also similar, where it involves lipid rafts and distinct enzymes that modify the glycans of glycolipids (Drickamer et al, 2003). Glycosylation produces three glycan structures that can be classified into three groups: highmannose (many mannose residues), complex oligosaccharide (contains different sugar groups), and hybrid (has multiple mannose and complex oligosaccharides). Specialized structures such as glycosylphosphatidylinositol (GPI) anchors are produced through glycosylation. Once glycosylation is completed in the trans-Golgi, the glycoproteins are sorted and directed to their destinations. These include the secretory vesicles (to be exported), membrane bilayer or the lysosomes (Stanley P, 2011). As a conclusion, glycosylation acts as a quality control checkpoint for the protein folding. The process also emphasizes the need for enzymes, chaperones, and transporters to function together efficiently in a packed environment to maintain the quality production of glycoproteins. 

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