Glycosylation is one of the most ancient enzymatic solutions to increase the functional repertoire of proteins beyond the capabilities of the genome. Through the covalent addition of monosaccharides and oligosaccharides to the side-chains of particular amino acids, glycosylation provides a stable annotation of the polypeptide backbone with a non-templated chemical information code. Glycosylation is deeply involved in numerous biological processes including protein folding, quality control, signaling and immune recognition.
Glycosylation is the most abundant PTM in eukaryotes, with >50% of the proteome predicted to be modified. Nascent polypeptides destined for transport into the ER lumen via the Sec61 translocon are modified by oligosaccharyltransferase through the transfer of a pre-assembled tetradecasaccharide onto the side chains of an asparagine residue in a strict consensus sequence. Glycosylation is the point of initiation of a quality-control pathway that distinguishes it from most other PTMs: the carbohydrate acts both as a ticket into the folding pathway, a timer for folding and a label on misfolded clients to be degraded. If the protein folds correctly it is shuttled to the Golgi where a series of enzymes (mannosidases, GlcNAc-transferases, galactosyltransferases, sialyltransferases, fucosyltransferases) will process the N-glycan into one of myriad possible mature structures. A second, more distributed glycosylation system also takes place in the Golgi. In O-glycosylation sugars are added one by one onto serine or threonine hydroxyl groups, and do not utilize a lipid-linked oligosaccharide precursor.
Glycosylation is an enzymatic process that attaches sugars to proteins, lipids or other organic molecules. Glycosyltransferases catalyze a reaction between an activated nucleotide-donor substrate and a hydroxyl, carboxyl or amino group on an acceptor residue of the target molecule. Protein glycosylation (the most prevalent PTM in eukaryotes) occurs on a ribosome-synthesized polypeptide chain in the lumen of the ER or in the cis-, medial-, or trans-cisternae of the Golgi, or in special cases, in the cytoplasm or nucleus. As a consequence, a polypeptide backbone may exist as many different glycoforms and the glycoform of a glycoprotein may be dynamic. A single sugar may be added directly to the protein backbone, as in the case of monosaccharide O-linked N-acetylglucosamine added to proteins in the nucleus or through an indirect mechanism that uses a lipid carrier to transport a pre-assembled oligosaccharide across the ER membrane to be transferred en bloc to the polypeptide. The structure of proteins is altered by attached glycans which create physical limitations when folding the backbone of the protein. They also affect its solubility by introducing charge and hydration repulsion effects, and they create a set of epitopes on the protein surface that can be used for binding by lectins, antibodies, and pathogens. Glycosylation therefore mediates between the primary sequence.
The two largest protein glycosylation pathways, N- and O-linked, are chemically dissimilar and have highly different biosynthetic logistics and architectures. The dolichol-pyrophosphate-anchored tetradecasaccharide Glc3Man9GlcNAc2 precursor forms stepwise on the cytoplasmic ER side before transfer en bloc to Asn's amide nitrogen within Asn-X-Ser/Thr sequons (X excludes Pro). Glycoproteins begin their biosynthesis with glycosylation which happens during translation inside the ER. The transferred oligosaccharide acts as a recognition signal by the chaperones calnexin and calreticulin to direct the nascent polypeptide chain toward folding. Subsequent stepwise trimming of the precursor oligosaccharide by glucosidases and mannosidases to a Man5GlcNAc2 core is then modified in the Golgi into three major families: high-mannose glycans, which are found primarily in ER resident proteins; complex glycans with GlcNAc-branched antennae modified with galactose, sialic acid and fucose; and hybrid glycans that have features of both. The spatial and temporal constraints of this pathway results in a relatively homogeneous product that is variably processed. In contrast, O-glycosylation is post-folding, Golgi-exclusive and the linkage has no consensus motif. O-glycosylation starts with the transfer of a single monosaccharide GalNAc to the hydroxyl oxygen of Ser or Thr residues by a tissue-specific family of polypeptide GalNAcTs. This initial GalNAc molecule is then gradually modified to generate eight core structures (linear Core 1 and branched Core 2 and Core 3) that can be further elaborated on epithelial cell surfaces to form mucin-type glycans by the attachment of sialic acid, fucose and sulfate. The initial GalNAc molecule is progressively modified to form eight core structures including linear Core 1 and branched Core 2 which then evolve into mucin-type glycans through the attachment of sialic acid, fucose, and sulfate occurring on epithelial cell surfaces. A parallel O-GlcNAc pathway has also been found in the cytoplasm and the nucleus. In this pathway, a single GlcNAc residue is reversibly added to nuclear pore proteins and transcription factors and can thus regulate chromatin structure and stress signaling. As O-glycosylation is performed one sugar at a time, without the use of a lipid carrier, the entire process can be up- or down-regulated more rapidly to allow cells to reconfigure their surface charge and hydration in response to changing conditions. Thus, N-glycosylation may provide a stable, quality-controlled base that is used for communication across the organism, while O-glycosylation may be a more flexible, context-dependent code that controls local adhesion, barrier, and signaling functions.
N-linked and O-linked glycans are two common types of covalent modifications on proteins.1,5
Glycosylation is a spatially distributed process, ranging from the ER lumen to the trans-Golgi network (TGN), it is directly connected with the process of protein synthesis, as well as the structure of the extracellular surface. The biosynthesis of N-linked glycans starts on the ER membrane cytosolic side before they attach to asparagine via a lipid-linked oligosaccharide precursor assembled on a dolichol carrier and flipped into the ER lumen where they are co-translationally added to new proteins. This means the polypeptide chain is arrested in a folding-competent state by capture by the glycan, which acts as a marker for recruitment of chaperones. When the protein has folded sufficiently to be released from the calnexin cycle by trimming of the glucose residues, it becomes subject to stepwise removal of mannose residues by mannosidases as it buds from one Golgi cisterna to the next. The final trimming of the oligosaccharide precursor to a biantennary or triantennary complex-type structure is both a processing event and a quality-control check: if the protein is misfolded, it is not allowed to exit the Golgi. O-linked glycosylation starts on the Golgi side after protein folding. The first sugar (GalNAc) is added to a serine or threonine residue in the protein backbone. Extension of the O-glycan is a stochastic process, does not have a pre-fabricated oligosaccharide core and is specific to tissue types, forming patchy micro-domains on the surface. O-Glycans can be elongated to mucin-like polymers, or capped with sulfated sialyl-Lewis (CA blood group) epitopes. O-linked glycosylation, not being lipid-linked, can also be rapidly upregulated by both substrate supply and enzyme induction. The glycosylation process for both pathways is dependent on the efficiency of vesicle transport. The timing of events between the two types of glycosylation is very different. N-glycans co-translate with protein folding and maturation, while O-glycans remodel surfaces after folding is complete.
The general chemistry of glycosidic bond formation is controlled by the family of enzymes glycosyltransferases. The process of nucleophilic attack is highly stereocontrolled, and is often characterized by the anomeric configuration of the donor substrate. Inverting glycosyltransferases, so named for their inversion of anomeric configuration, use a concerted SN2-like reaction mechanism where the acceptor hydroxyl attacks the anomeric carbon, and is activated by a general base. At the transition state, the reactants are fully ordered, the sugar ring adopts a distorted half-chair conformation and the phosphate group is displaced. For inverting enzymes, the active site of the enzyme positions and activates the donor and acceptor, and can be broadly classified according to two types of fold: GT-A fold enzymes utilize a divalent metal ion to stabilize the leaving group nucleoside diphosphate, while GT-B fold enzymes instead use a pair of arginine residues and helix dipoles to neutralize the negative charge. Retaining glycosyltransferases on the other hand keep the anomeric configuration of the donor, and are proposed to use a double displacement reaction mechanism involving a covalent glycosyl-enzyme intermediate, yet a structural analysis of the GT-B fold has shown a lack of conserved nucleophile, instead supporting a dissociative mechanism with a short-lived oxocarbenium ion that is intercepted by the acceptor before it collapses. For both inverting and retaining glycosyltransferases, the large activation barrier of glycosidic bond formation is facilitated by active site orientation of both donor and acceptor, pre-organization of water networks, and substrate-assisted catalysis from neighboring donor hydroxyls. An elaborate recognition surface: the acceptor-binding cleft, controls the specificity of the reaction by recognizing a substrate peptide sequence, peptide secondary structure, and pre-existing glycan patterns, ensuring the right glycosyltransferase transfers the right sugar at the right position. The change in conformation of the donor sugar from a free-chair in the Michaelis complex to a distorted half-chair conformation at the transition state can also be a requirement for catalysis, and acts as a kinetic barrier against spontaneous hydrolysis.
Transferases (glycosyltransferases) and hydrolases (glycosidases) form a yin-yang system of enzymes that add, edit and remove glycans in the secretory pathway and at the cell surface. Transferases are the synthetic workhorse, encoded by several hundred genes in the mammalian genome whose expression profiles determine the tissue-specific glycome: individual isoforms have different donor specificities, acceptor preferences and linkage regiochemistries, and can be combined in a combinatorial manner to generate complex glycans. Their action is modulated by Golgi localization – sequential activation in the compartments as cargo moves from cis to trans – and by different pools of competing substrates that direct flux through branching biosynthetic pathways. Glycosidases are the erasers and sculptors of glycans. Glucosidases and mannosidases in the ER process the N-glycan precursor to reveal or mask epitopes that are bound by lectins or that label misfolded proteins for ERAD. Glycosidases in the Golgi further trim antennae, making space for new additions and to reach the correct valency for lectin recognition. Extension and trimming also happen at the cell surface and in the extracellular milieu, where exoglycosidases nibble away glycan termini to control receptor lifetime and abort signalling cascades. Defects in either cause human disease: N-acetylglucosaminyltransferases are the cause of congenital disorders of glycosylation with multi-system organ failure and hypomorphic mutations in sialyltransferases in cancer result in a hypersialylated phenotype with immune evasion. Lysosomal glycosidases are the target of inherited metabolic diseases characterised by storage of undegraded glycans, and pathogens in the host mucosa use surface-anchored glycosidases to degrade host mucins. Targeting of the yin-yang system also has therapeutic potential: inhibitors of Golgi mannosidases prevent maturation of viral glycoproteins and engineered glycosyltransferases produce homogeneous glycoforms of recombinant biologics with more desirable effector functions.
Thus glycosylation effectively puts a dynamic chemical garnish on proteins that can significantly affect their folding, localization and interaction profiles. The addition of oligosaccharides with unique size properties to protein side chains can block protease cleavage sites and build up hydrophilic patches that change tertiary structure and expose binding sites for lectins that direct sorting routes. In addition to shielding, the attached glycans themselves can function as allosteric modifiers that change backbone dynamics, stabilize otherwise unstable domains and adjust the kinetic thresholds between conformational states. Functionally, N-glycans can act as molecular ratchets in the crowded milieu of the secretory pathway to bias folding toward productive intermediates, while O-glycans can tune the adhesive properties and stiffness of a protein at its final destination. The net result of these activities is that a single polypeptide can give rise to multiple glycoforms with very different half-lives, signalling efficacies and receptor affinities, in effect expanding the functional output of a single gene. The regulatory flexibility this implies means that glycosylation can act as a post-translational rheostat fine-tuning protein behavior in response to metabolic conditions, developmental cues and pathological stress, without the need for sequence change.
Folding is influenced by glycosylation right from the point where the nascent chain first enters the ER lumen. Here, co-translational attachment of the tetradecasaccharide to asparagine residues provides a hydrophilic, and rigid anchor to help prevent aggregation and to guide the polypeptide into the calnexin-calreticulin cycle. The glycan-based chaperone machinery therefore functions as a molecular clock. The protein undergoes continuous cycles of glucose trimming and re-addition until the hydrophobic patches become buried at the correct conformation which triggers the final removal of glucose before releasing the glycoprotein into forward trafficking. The oligosaccharide therefore acts as a folding sensor, uncovering mannose residues to signal up ERAD in the event of folding failure. In addition to this quality-control role, the glycan itself can also stabilize tertiary structure, for example by buttressing flexible loops, and by hydrogen bonding to nearby amino-acid side-chains, which reduces conformational entropy and shields the backbone from solvent dynamics. The O-glycans added later in the Golgi further stabilize the protein after folding by forming dense, hydrated brushes that are protease resistant, and by sterically occluding the access of denaturants to the peptide interior. The dual function of glycans as folding catalysts and stability mechanisms explains why defects in glycosylation cause various conformational diseases and why therapeutic glycoproteins need specific glycan profiles to meet pharmaceutical requirements for shelf-life and bioactivity.
Glycans also regulate subcellular targeting and residence times. Glycans contain sorting signals that are recognized by specific lectins, which are part of sorting machinery in organelles. For example, mannose-6-phosphate residues on lysosomal hydrolases are recognized by cation-dependent mannose-6-phosphate receptors in the TGN, which divert these proteins from the default secretory pathway to acidic compartments. Mislocalization of these proteins can cause lysosomal storage diseases if, for example, the phosphorylation or trimming efficiency is impaired. For cell surface receptors, the degree of sialylation and fucosylation of N-glycans can determine if they are recycled back to the plasma membrane or if they are sorted into multivesicular bodies and targeted for degradation. Changing the pattern of terminal sugars can shift the balance and alter the duration of signaling. In the case of O-glycans, clusters of sialylated Core 2 glycans can form a sialyl-GAG rich, highly negatively charged glycocalyx. This can inhibit nonspecific interactions while displaying specific ligands for selectins, which can control adhesion of leukocytes versus rolling. In the cytoplasm, O-GlcNAc on transcription factors can be dynamically added and removed, affecting nuclear import by changing the charge and providing binding sites for importins. Secreted glycoproteins can have their enzymatic activity regulated by glycans; sterics of the glycan can physically block active sites until trimming occurs, providing a latency mechanism that can restrict function to particular tissues or developmental times. In this way glycosylation can choreograph the localization and activity of a protein in response to metabolic or signaling cues.
Multivalent structures, such as glycans, can mediate protein-protein interactions by sterically guiding, charge shielding, and bridging through lectin interactions. N-glycans, frequently located at inter-domain linkers, can extend away from the protein surface to sterically exclude undesired interactions and simultaneously display terminal residues that will interact with a matching receptor. The size and branching of the glycans will affect the avidity of the interactions and thus the interactions can be fine-tuned to modulate binding. O-glycans can be concentrated in mucin domains to form long sugar brushes, which can mediate homotypic galectin binding, cross-linking extracellular structures and mediating cell-cell interactions. The glycocalyx is a programmable space, with sialylation of a protein surface preventing low-affinity interactions through electrostatic repulsion, while fucosylation may unmask hidden epitopes, selectively interacting with selectins. Intracellularly, O-GlcNAc modifications to transcription factors or nuclear pore proteins can alter protein-protein interactions by binding serine/threonine residues that could have been phosphorylated. This allows an O-GlcNAc modification to exchange a fast, kinase-dependent phosphorylation signal for a slower, metabolic state dependent modification, without changing the protein sequence. Glycosylation of viral envelope proteins can simultaneously mask neutralising epitopes and create new protein interaction surfaces. For example, the O-glycan of the SARS-CoV-2 RBD at residue 494 can contribute to ACE2 binding by stabilising the interaction through polar interactions with the protein-protein interface that do not contact the peptide backbone directly. Sugars can function to bridge, block, or allosterically modulate protein-protein interactions, tuning their strength and specificity.
In summary, glycosylation is an informational post-translational code that carries both structural and regulatory messages. This results in an informational and dynamic glycalyx that can modulate signaling and immune responses. The attached glycans can act as both ligands and receptors, and often the same glycan structure can either enhance or inhibit signaling depending on its context. For signaling, the glycans can modify the conformation of receptors by sterically limiting the flexibility of their extracellular domains, shifting the relative balance between the active and inactive conformations, and they can also serve as binding sites for endogenous lectins that can then serve as platforms for the assembly of higher order signaling complexes. For immunity, the glycan code serves as a biomarker to distinguish self from non-self. Immune cells are equipped with an array of glycan pattern recognition receptors that monitor the glycan profiles of potential pathogens and host tissues. If these receptors detect unusual glycan structures, they can initiate immune activation. The glycan code can be tuned to precisely control a range of innate and adaptive immune responses, from neutrophil rolling on inflamed endothelium to T cell activation thresholds, without the need for new protein synthesis. In addition, glycan remodeling takes place on timescales that are well suited to the needs of a fast-adapting immune system, so that the same receptor can be toggled from an adhesive to an anti-adhesive state within minutes of cytokine stimulation. The cross-talk between signaling modulation and immune activation suggests that glycans are not just passive structural decorations, but active participants in cellular decision-making, and that they can translate metabolic and environmental information into specific physiological responses. Abnormalities in these glyco-codes are now also being appreciated as a hallmark of cancer and autoimmune diseases, where changes in glycosylation patterns can subvert normal immune surveillance and allow pathogenic cells to gain an edge in evading immune clearance.
Glycans regulate signal transduction by controlling receptor topology, receptor clustering and ligand-binding affinity in a context-specific manner. For example, in Notch signalling, O-fucose glycans added to epidermal growth factor-like repeats of the receptor itself directly regulate ligand binding in a context-specific manner with different fucosylations either sending a strong signal or having no signal. N-glycan antennae on growth-factor receptors are of differing sizes in their glycan branching and this structural difference can create steric clashes that either enable or prevent dimerization of receptors. A bulky highly branched glycan antennae can create a physical steric hindrance to phosphatases from binding to the receptor and prolonging kinase phosphorylation, hyperactivating downstream pathways. On the other hand, a smaller, less branched glycan antennae will not sterically clash with phosphatase binding and can then prevent downstream signalling. In this way, one receptor isoform can have a range of intensities in signalling allowing a more fine grained response from the cell to an external signal. Glycosylation and phosphorylation are linked via the O-GlcNAc modification cycle where the addition of a single N-acetylglucosamine (GlcNAc) to a serine or threonine residue directly competes with phosphorylation at that residue. This creates a direct competition between a relatively fast, kinase driven all-or-none switch and a slower, metabolic sensor dependent rheostat. Glycan-phosphorylation crosstalk in this way provides an integration of nutrient status and signalling to downregulate pathways such as insulin signalling in times of metabolic stress. Erroneous glycosylation therefore can be a hijacking of these tightly controlled processes as is observed in oncogenesis where an increase in N-glycan branching creates a constitutively active receptor state or in diabetes where O-GlcNAc hypermodification results in a prolonged insulin signal.
The glycosylation of immune molecules and cells regulates all aspects of immune cell activation. In the innate immune system, P-selectin glycoprotein ligand-1 on neutrophils requires extended and sialylated O-glycans to mediate tethering and rolling on the inflamed endothelium before extravasating towards the site of infection. Complement proteins are a major family of glycoproteins, which control complement activation and host protection. Complement receptor 1 (C1q) globular heads have numerous N-glycans that orientate their binding domains towards microbial surfaces to reduce self-binding while CD59, a glycoprotein anchored to the membrane by a glycosylphosphatidylinositol (GPI) anchor, provides a carbohydrate shield that protects host cells from the membrane attack complex. The glycosylation of the Toll-like receptor family of pattern recognition receptors affects their own membrane localisation, accessibility to ligands and therefore the threshold for inflammatory activation. N-glycans modulate T cell receptor (TCR) activation thresholds; the absence of β-1,6-branching of N-glycans on T cells reduces the content of the N-acetyllactosamine motif, increases TCR clustering and reduces activation threshold predisposing to autoimmunity, while an increase in N-glycan branching has the opposite effect. CD28 costimulation is negatively regulated by N-glycosylation and deglycosylation of CD28 increases binding to CD80/CD86 ligands and downstream signalling. In B cells, the pre-BCR requires N-glycosylation for assembly and surface expression during development and the density of N-glycans on the mature BCR modulates clustering and internalisation, which affects BCR signalling. The activation of natural killer cells via NKp30 is N-glycosylation-dependent for oligomerisation and binding to tumour-expressed ligand B7-H6, and N-glycosylation of the receptor directly regulates cytotoxicity. The sialic acid–binding immunoglobulin-like lectins (Siglecs) are a family of inhibitory receptors that recognise sialylated glycans as self-associated molecular patterns and transmit inhibitory signals to prevent inflammation and tissue damage.
Alterations in glycosylation are involved in a wide range of diseases, from cancer to inborn errors of metabolism. It is important to note that disease-associated changes in glycosylation are not simply epiphenomena, they also contribute to altered protein folding, cell adhesion, and tissue homeostasis. In cancer, for instance, glycosylation is frequently reprogrammed in tumor progression, and inborn errors of metabolism, a wide range of genetic defects in glycosylation cause multi-system diseases. Glycans are increasingly recognized as being biomarkers for diseases, as they are frequently altered in different types of disease, can predict disease onset and course and, in some cases, are responsive to treatment. In many cases, these changes are also the result of signaling abnormalities, and as such may be used as a 'canary in a coal mine' in disease detection. Glycosylation can be targeted for therapeutic purposes in a number of different ways, including targeting glycosylation directly or targeting altered glycoconjugates.
A vast remodelling of the cellular glycome occurs during malignant transformation, which is evidenced by two major phenomena: under-expression of complex-type glycans and the aberrant expression of neo-antigens. The first phenomenon is due to the truncation of complex glycan structures, leading to the expression of incomplete precursor antigens such as the Tn and sialyl-Tn antigens. Tn antigens are not expressed in normal epithelia but are aberrantly expressed on tumor cells. Tn antigens can also act as decoy ligands that can bind inhibitory lectins on the surface of natural killer cells and macrophages. This protects the malignant clone from attack by the innate immune system. In parallel, the neo-synthetic programme installs new epitopes such as sialyl-Lewis structures that allow circulating tumor cells to bind to selectin-expressing endothelium and colonize other sites. In addition to these qualitative changes, quantitative modifications to branching and terminal sugars result in a glycocalyx that is both electrostatically charged and sterically bulky, which can alter the mobility of receptors and inhibit apoptosis signaling. In particular, receptor tyrosine kinases, which are often heavily modified by extended N-glycans, are sequestered by galectin networks, which extend their dwell time on the plasma membrane and hyper-activate proliferation signaling. In hepatocellular carcinoma, N-glycosylation of MerTK is aberrant and this stabilizes the protein by inhibiting its ubiquitin-mediated degradation, resulting in hyper-activation of Akt metabolic reprogramming.
Fig. 2 Altered glycans and related pathophysiological events involved in NB progression.2,5
Congenital Disorders of Glycosylation (CDG) are a genetically and clinically diverse group of disorders in which an alteration of glycan synthesis or processing leads to multi-organ disease. Type I CDGs are caused by defects in the synthesis or transport of the lipid-linked oligosaccharide (LLO) precursor, whereas Type II CDGs are caused by defects in Golgi glycosidases, glycosyltransferases or nucleotide-sugar transporters. Patients have multi-system disease, because glycosylation is a ubiquitous post-translational modification required for the normal function of most proteins. Common features include neurological dysfunction, liver fibrosis, bleeding diatheses, and developmental delay. Facial dysmorphia in patients with CDG is a strong indication that glycosylation is required for normal embryonic development. Many of these disorders are lysosomal storage disorders, as in α-mannosidosis, where the unprocessed high mannose glycans accumulate in endosomes. This leads to autophagic defects that cause neurodegeneration and skeletal defects. In other diseases, hypoglycosylation of secreted proteins leads to impaired protein folding and clearance, causing ER stress and chronic inflammation. The lack of fucosylated selectin ligands in leukocyte adhesion deficiency type II results in the inability of neutrophils to migrate to sites of inflammation, causing recurrent bacterial infections. Abnormal O-glycosylation of mucins in inflammatory bowel disease (IBD) is characterized by an increase in sialylation and a decrease in sulfation of the O-glycans. This results in a decrease in the barrier function of the intestinal mucus layer and contributes to the chronic intestinal inflammation. These examples emphasize the importance of proper glycosylation for protein function. The clinical variability, even among patients with the same enzyme deficiency, demonstrates that the heterogeneity of glycans is also a factor in disease presentation, since residual enzyme activity or a bypass of the glycan processing step can mitigate the disease severity.
Glycosylation is an important post-translational modification that can control protein function at both structural and regulatory levels, making it a key link between genotype and phenotype. Glycosylation involves the covalent attachment of sugar moieties to proteins, which can affect folding, localization and protein activity. These modifications can affect protein folding by sterically affecting the available conformational space, contribute to protein localization by recognition by lectins or directly affecting receptor activity and the regulation of signaling thresholds by affecting protein stability, localization and availability for phosphorylation. The addition of these sugar moieties to create glycoforms can also convey information that is independent of the protein-coding genes in a way that is metabolically regulated and reversible, allowing the cell to respond without altering gene expression. Dysregulation of glycosylation is a common hallmark of cancer cells, for example, which they can use to escape immune detection or increase their metastatic potential. On the other hand, deficiencies in glycosylation enzymes can lead to widespread protein misfolding, for example, in congenital disorders of glycosylation. Glyco-engineering or altering glycosyltransferases therefore represents a promising route for therapeutics, with glycomics at the forefront of personalized medicine. Moving forward, linking glycomics to other omics data, such as proteomics, will be vital to understand the integrated regulation of N- and O-glycan pathways, in order to harness this information in a clinical setting and act directly on the glycan.
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