Glycosylation PDF

Glycosylation Location of glycosylation activity Universal in organisms relative hydrophilicity, flexibility, mobility in aqueous environments and their extreme diversity Prokaryotes, fungi: structural roles in the cell wall and in resisting large differences in osmolarity between cytoplasm...

Glycosylation Location of glycosylation activity Universal in organisms relative hydrophilicity, flexibility, mobility in aqueous environments and their extreme diversity Prokaryotes, fungi: structural roles in the cell wall and in resisting large differences in osmolarity between cytoplasm and environment Eukaryotes: glycosylation of secreted proteins may provide solubility, hydrophilicity, and negative charge, thus reducing unwanted intermolecular interactions and protecting against proteolysis. Membrane proteins (receptors, adhesion molecules, channels): promote proper folding, ensure stability and function.  Secreted proteins can be N-glycosylated, O-glycosylated, and/or modified with glycosylphosphatidylinositol (GPI) anchors, and proteoglycans are modified with attached glycosaminoglycan chains. Specific enzymes involved in each modification. N-linked glycans and GPI anchors are preassembled before being transferred to proteins and then further modified in the ER–Golgi pathway. O-linked glycans, glycosaminoglycans and the glycosylation of lipids involve reactions in both the ER and Golgi.  Initiation and Maturation of the Major Types of Eukaryotic Glycoconjugates Chapter 4, Figure 1. Essentials of Glycobiology, Fourth Edition Buy the Book ©2022 The Consortium of Glycobiology Editors, La Jolla, California  Glycan are added to the growing chain on the inside of the ER or the Golgi. The portion of a molecule that faces the inside of the lumen of the ER or Golgi will ultimately face the outside of the cell or the inside of a secretory granule or lysosome (non-cytoplasmic site). Some glycan chains are made on the cytoplasmic site of intracellular membranes and flipped across to the other side. Some “leaderless secretory proteins” destined for the extracellular space never enter the ER lumen but are instead transferred directly through the plasma membrane. E.g. galectin Different glycotransferases → the types of glycans found on the two sides of the membrane appear to be distinct from each other.  Glycosyltransferases use activated monosaccharides nucleotide sugars lipid-phosphate-linked sugars(e.g. dolicholphosphate mannose) Glycan modifications Sulfotransferases (e.g. 3′-phosphoadenyl-5′-phosphosulfate) Acetyltransferases (e.g. acetyl-CoA) Methyltransferases (e.g. S-adenosylmethionine) Almost all donors for glycosylation reactions and glycan modifications are synthesized within the cytosolic compartment → actively transported into ER/ Golgi. Glycotransferase Type II membrane proteins → an amino-terminal cytoplasmic tail, a transmembrane (TM) region, and a carboxy-terminal region with the catalytic domain. The catalytic domain is Golgi lumen → synthesize the glycan chains on proteins and lipids during their transit through the secretory pathway. The soluble, secreted enzymes are derived from the cleaved integral enzymes by proteases in the trans regions of the Golgi and in post-Golgi compartments. Some are synthesized as soluble enzyme. Possible of secreted glycosyltransferase to add sugars on the cell surface  Topology and Localization of Golgi Glycosylation Enzymes Chapter 4, Figure 2. Essentials of Glycobiology, Fourth Edition Buy the Book ©2022 The Consortium of Glycobiology Editors, La Jolla, California Localization of the Enzymes in Golgi Compartments The enzymes, their glycan substrates (protein/lipid to be glycosylated), and the appropriate nucleotide sugar donor must be located in the same compartment. Highly ordered and sequential processes. Glycosyltransferases segregate into distinct overlapping compartments within the secretory pathway. Enzymes acting early in the biosynthetic pathway localize to cis- and medial-Golgi compartments, whereas those acting later in the pathway tend to localize in the trans-Golgi cisternae and the TGN.  Have mechanisms to retain in specific location Heteromeric complex formation is observed between enzymes that catalyze sequential reactions in the same pathway and that are localized in the same cisternae. https://febs.onlinelibrary.wiley.com/doi/10.1002/1873-3468.13553