These enzymes are critical for glycan processing in the ER and Golgi, and each enzyme shows specificity for removing a particular sugar (e.g., mannosidase). Glycosidases catalyze the hydrolysis of glycosidic bonds to remove sugars from proteins. These enzymes are broad in scope, as glycosidic bonds have been detected on almost every protein functional group, and glycosylation has been shown to incorporate most of the commonly occurring monosaccharides to some extent (6). Each Gtf has specificity for linking a particular sugar from a donor (sugar nucleotide or dolichol) to a substrate and acts independent of other Gtfs.

Because enzyme activity varies by cell type and intracellular compartment, cells can synthesize glycoproteins that differ from other cells in glycan structure (5).Įnzymes that transfer mono- or oligosaccharides from donor molecules to growing oligosaccharide chains or proteins are called glycosyltransferases (Gtfs). While it may seem chaotic because of all of the enzymes involved, the different mechanisms of glycosylation are highly-ordered, step-wise reactions in which individual enzyme activity is dependent upon the completion of the previous enzymatic reaction. Instead of using templates, cells rely on a host of enzymes that add or remove sugars from one molecule to another to generate the diverse glycoproteins seen in a given cell.

Unlike other cell processes such as transcription or translation, glycosylation is non-templated, and thus, all of these steps do not necessarily occur during every glycosylation event. The molecular events of glycosylation include linking monosaccharides together, transferring sugars from one substrate to another and trimming sugars from the glycan structure. Glycosylation is thought to be the most complex post-translational modification because of the large number of enzymatic steps involved (5).

Glycan length-short- or long-chain oligosaccharides.

Glycan structure-branched or unbranched chains.

Glycan composition-the types of sugars that are linked to a particular protein.

Glycosidic linkage-the site of glycan (oligosaccharide) binding.

The cell is able to facilitate this diversity, because almost every aspect of glycosylation can be modified, including: Glycosylation increases the diversity of the proteome to a level unmatched by any other post-translational modification. Eukaryotes have the greatest range of organisms that express glycoproteins, from single-celled to complex multicellular organisms. Glycosylated proteins (glycoproteins) are found in almost all living organisms that have been studied, including eukaryotes, eubacteria and archae (3,4). Because they are hydrophilic, they can also alter the solubility of a protein (2).

Because they can be very large and bulky, oligosaccharides can affect protein–protein interactions by either facilitating or preventing proteins from binding to cognate interaction domains. These sugars can also act as ligands for receptors on the cell surface to mediate cell attachment or stimulate signal transduction pathways (1). Sugar moieties on soluble proteins can be bound by specific receptors in the trans Golgi network to facilitate their delivery to the correct destination. In the ER, glycosylation is used to monitor the status of protein folding, acting as a quality control mechanism to ensure that only properly folded proteins are trafficked to the Golgi. Protein glycosylation has multiple functions in the cell. Lipids and proteoglycans can also be glycosylated, significantly increasing the number of substrates for this type of modification. Additionally, some proteins that are trafficked from the Golgi to the cytoplasm are also glycosylated. Most soluble and membrane-bound proteins expressed in the endoplasmic reticulum are glycosylated to some extent, including secreted proteins, surface receptors and ligands, and organelle-resident proteins. Approximately half of all proteins typically expressed in a cell undergo this modification, which entails the covalent addition of sugar moieties to specific amino acids. Glycosylation is a critical function of the biosynthetic-secretory pathway in the endoplasmic reticulum (ER) and Golgi apparatus.

Cystol had higher ph than er lumen