Glycosylation

Glycosylation (see also chemical glycosylation) is the reaction in which a carbohydrate, i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule (a glycosyl acceptor). In biology glycosylation mainly refers in particular to the enzymatic process that attaches glycans to proteins, lipids, or other organic molecules. This enzymatic process produces one of the fundamental biopolymers found in cells (along with DNA, RNA, and proteins). Glycosylation is a form of co-translational and post-translational modification. Glycans serve a variety of structural and functional roles in membrane and secreted proteins. [1] The majority of proteins synthesized in the rough endoplasmic reticulum undergo glycosylation. It is an enzyme-directed site-specific process, as opposed to the non-enzymatic chemical reaction of glycation. Glycosylation is also present in the cytoplasm and nucleus as the O-GlcNAc modification. Aglycosylation is a feature of engineered antibodies to bypass glycosylation. [2] [3] Five classes of glycans are produced:

Purpose

Glycosylation is the process by which a carbohydrate is covalently attached to a target macromolecule, typically proteins and lipids. This modification serves various functions. [4] For instance, some proteins do not fold correctly unless they are glycosylated. [1] In other cases, proteins are not stable unless they contain oligosaccharides linked at the amide nitrogen of certain asparagine residues. The influence of glycosylation on the folding and stability of glycoprotein is twofold. Firstly, the highly soluble glycans may have a direct physicochemical stabilisation effect. Secondly, N-linked glycans mediate a critical quality control check point in glycoprotein folding in the endoplasmic reticulum. [5] Glycosylation also plays a role in cell-to-cell adhesion (a mechanism employed by cells of the immune system) via sugar-binding proteins called lectins, which recognize specific carbohydrate moieties. [1] Glycosylation is an important parameter in the optimization of many glycoprotein-based drugs such as monoclonal antibodies. [5] Glycosylation also underpins the ABO blood group system. It is the presence or absence of glycosyltransferases which dictates which blood group antigens are presented and hence what antibody specificities are exhibited. This immunological role may well have driven the diversification of glycan heterogeneity and creates a barrier to zoonotic transmission of viruses. [6] In addition, glycosylation is often used by viruses to shield the underlying viral protein from immune recognition. A significant example is the dense glycan shield of the envelope spike of the human immunodeficiency virus. [7]

Overall, glycosylation needs to be understood by the likely evolutionary selection pressures that have shaped it. In one model, diversification can be considered purely as a result of endogenous functionality (such as cell trafficking). However, it is more likely that diversification is driven by evasion of pathogen infection mechanism (e.g. Helicobacter attachment to terminal saccharide residues) and that diversity within the multicellular organism is then exploited endogenously.

Other Languages
català: Glucosilació
español: Glicosilación
français: Glycosylation
한국어: 당화
italiano: Glicosilazione
Bahasa Melayu: Pengglikosilan
Nederlands: Glycosylering
norsk nynorsk: Glykosylering
polski: Glikozylacja
português: Glicosilação
slovenščina: Glikozilacija
српски / srpski: Glikozilacija
srpskohrvatski / српскохрватски: Glikozilacija
svenska: Glykosylering
Türkçe: Glikozilasyon
українська: Глікозилювання
中文: 醣基化