Glycosylation is a process involving many functional proteins for e.
g. antibodies used in neutralising pathogenic bacteria, viruses and antigens, resulting in considerable diversity of carbohydrate-protein bonds and glycan structures. Glycosylation of proteins has a great impact on their structures and functioning; the interactions of these protein-linked oligosaccharides with carbohydrate specific proteins modulate many important biological processes. The Golgi contains an array of Glycosyltransferases, Glycosidases and nucleotide sugar transporters that function collectively to the complete the synthesis of previously attached glycan-proteins from the ER; this is accomplished in order from the cis-Golgi nework (CGN) to the medial-Golgi and finally trans-Golgi network (TGN) (Stanley, P . 2011). Proteins entering the CGN can either proceed into the TGN and be segregated in different vesicles and transported around/out of the cell, or be returned to the ER, to which the immunoglobin BiP protein retains and holds the protein within the ER. Glycoproteins, Glycosphingolipids and glycophosphatidylinositol (GPI) anchors are just a few molecules that obtain their final post-translation modifications and sugar associations upon passage through the Golgi. Glycoproteins that are N-glycosylated (see Figure 1) arrive in the cis-Golgi, carrying N-linked Oligosaccharides which were added in the ER mainly at the Asparagine side chain (Alberts et al, 2015), these may be further glycosylated or remain unchanged during passage through the Golgi; and be present on cell surface or secreted glycoproteins.
It has been researched that Congenital disorders of Glycosylation (CDGs) are caused by mutations in genes encoding glycosylation pathway proteins, there are believed to be two different types of CDG diseases both of which are linked with the formation or modification of N-linked glycans in the Golgi. It is thought that over 100 different CDGs are known causing defects like mental retardation and seizures, although researchers have no definitive reason to how defective glycosylation in the Golgi causes CDGs (Frappaolo, A. 2018). N- and O-linked oligosaccharide modifications are done by a series of membrane-bound proteins and enzymes such as Glycosidases, glycosyl-transferases and nucleotide sugar transporters in an organised sequence. This sequence of modification is done so to and from consecutive Golgi cisternae, the movement and transport of proteins is thought to be the result of two models; Vesicular transport and cisternal maturation, which also facilitates passage to the extracellular environment (Stanley, P.
2011).The glycan structures found on glycosphingolipids are often like those found on O-glycoproteins since several Golgi glycosyltransferases can modify both types, the biosynthesis to create glycosphingolipids takes place at the cytosolic side of the cis-Golgi membrane, where the glycosyltransferases convert the subunits into various intermediates, further elongation in the Golgi apparatus through the coordinated action of multiple glycosyltransferases, eventually forming the glycosphingolipid yielding close to 400 different glycosphingolipid structures. The structural complexity and specificity of some glycosphingolipids for some cell types has made them suitable cell markers, for example some stage-specific embryonic antigens are glycosphingolipids that are expressed on mouse cells at specific embryonic stages (University of Zurich, 2016). In humans, the ABO blood group antigens, are also glycosphingolipids.GPI anchor modification comprises of a phosphatidylethanolamine being added in the Golgi in mammals, additionally mannose oligosaccharides of the anchor may be added as well (Fujita and Kinoshita 2010); The GPI-anchored protein is finally located in the outer leaflet of the plasma membrane after passage out of the TGN.