Glycoprotein Peptides
* Please kindly note that our products and services can only be used to support research purposes (Not for clinical use).
Online Inquiry
What are glycoproteins?
Glycoproteins (GPs) are proteins that have sugar groups covalently affixed to their polypeptide chains. Proteins and carbohydrates are indispensable to numerous biological processes worldwide, as they collaborate to perform numerous functions. GP undergo two distinct forms of glycosylation: O-linked and N-linked.
N-glycosylation
Covalent attachment of N-glycans to the side chains of aspartic residues of GPs by N-acetylglucosamine (GlcNAc) occurs during the translation of target proteins. A core structure with two GlcNAc and mannose remnants is shared by all three types of N-glycans: complex, heterogeneous, and high mannose. The process of N-linked glycosylation begins in the endoplasmic reticulum (ER), where oligosaccharide transferase (OST) attaches oligosaccharides to the appropriate asparagine residues of the growing polypeptide. GPs go through major changes to produce high mannose once they have been appropriately folded and modified by glycogen in the Golgi apparatus. The addition of fucose, galactose, or GlcNAc to structures, together with sialylation, results in structures that are incredibly complex and varied.
O-glycosylation
Glycosylation is sensitive to changes in sugar content and methods for sugar attachment; the O-linkage is created by serine and threonine connecting the glycan hydroxyl groups. Beginning with glycosylation to various amino acid side chain glycans (e.g., threonine and serine), O-glycosylation proceeds by the sequential enzymatic transfer of a single disaccharide onto proteins. The endoplasmic reticulum is responsible for O-glycosylation of proteoglycans including O-fucose, O-glucose, O-galactose, O-GlcNAc, O-mannose, and O-xylose. Mammals primarily create O-GlcNAC, a kind of O-glycosylation, by activating the peptide GalNAc transferase, due to the availability of mucin. In their own special way, prokaryotes and eukaryotic creatures both engage in glycosylation. Glycosylation encompasses the attachment of sugar modules to proteins both within and outside of cells, in contrast to O- and N-glycosylation. Another dynamic post-translational change that has similarities with this is protein phosphorylation.
GPs with N-glycosylation and O-glycosylation. (Yue Z., et al., 2023)
Biological roles of GPs in cells. (Yue Z., et al., 2023)
Where are glycoproteins found?
Protein glycosylation in eukaryotic cells can take place in several sites, including the ER, Golgi apophysis, nucleus, cytoplasm, or mitochondria. Most secreted glycosylation processes start in the ER and end at the Golgi apparatus. Glycosylation of proteins occurs often in the cytoplasm and on the cell surface in bacteria and archaea. There is a clear correlation between glycosylation and protein folding, solubility, stability, behavior, protease conservation, and cellular targeting. Proper protein glycosylation is believed to be essential for the proper assembly of protein compounds, control of protein-protein interactions, and proper assembly of higher protein structures. GPs abound both within and outside of cells, in various locations across the cell membrane. In addition to serving as receptors for cell signals, they facilitate communication between cells and serve as antigens (such as markers for blood types on RBCs). The extracellular matrix has several structural GPs, including collagen and fibronectin. These help with structural support, cell adhesion, wound healing, and tissue formation. Organs with mucosal surfaces—that is, the digestive, reproductive, and respiratory systems—secrete GPs known as mucins. Acting as a protective mucus covering and lubricant on surfaces, they serve to keep germs out. Blood carries antibodies (immunoglobulins), hormones, clotting factors, various GPs, and Hormone signaling, blood coagulation, and immune system defense all depend on this collection of GPs—absolutely vital for life. GPs are necessary for the cells that make up every organ and tissue in the body to fulfill their particular roles. The liver synthesizes GPs with roles in detoxification and metabolism. To control reproduction, endocrine glands release GP hormones like follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
Glycoprotein structure
In numerous cell components, such as the cytoplasm, cellular organelles, and cell surface, sugar chains are frequently observed to be associated with proteins. The carbohydrate moiety in GPs that are coupled to cell surfaces is composed of large hetero- and homo-polysaccharides and disaccharides. The carbohydrate units galNAc, xylose, glucuronic acid, D-mannose, N-acetylneuraminic acid, L-fucose, GlcNAc, and D-galactose are frequently observed in GPs that bind to cell surfaces. Glycans and proteins are classified as either N-glycans or O-glycans based on the numerous mechanisms by which they are linked. N-glycans connect to asn residues, whereas O-glycans link to serine, threonine, or tyrosine residues. As part of a quality-control procedure, glucose is either added to or withdrawn from the ER, resulting in the formation of a carbohydrate-protein conjugate. Subsequently, the Golgi apparatus receives the structure to endure glycosylation that is specific to each cell type. The mucin-type or non-mucin-type of O-glycans is determined by the type of monosaccharide that is bonded to the amino acid. Despite the fact that cells contain both types of GPs, the latter are commonly found in the nucleus and mitochondria of cells, whereas the former are more frequently found outside of cells. The site of GP glycosylation is determined by the secondary and tertiary structures of a polypeptide chain. The quantity of N- and O-linked glycan units and the locations of glycosylation, which binds glycan units to the polypeptide chain, can vary among GPs.
Structure of GPs. (Guo W., et al., 2021)
What are the roles of glycoproteins and glycolipids?
GPs and glycolipids are macromolecules mostly found on cell surfaces, where they are crucial for cell signaling, recognition, and structural integrity. They consist of carbohydrate (sugar) chains attached to proteins or lipids, respectively. Both are situated on the extracellular surface of the cell membrane, with GPs often acting as receptors and adhesion molecules, while glycolipids aid in membrane stability and operate as signaling platforms. Aberrations in GPs and glycolipids can contribute to diseases, including autoimmune disorders, infections, and cancer, as pathogens sometimes use these molecules to enter cells or evade immune detection.
GPs and glycolipids. (Cummings R D., 2019)
What are the functions of glycoproteins?
Structure and regulatory role
The cell surface is enveloped by a dense layer of glycans, comprising GPs, saccharides, and polysaccharides attached to the cytosolic membrane, which has a crucial influence on several cellular biological functions. The glycocalyx is essential for several cellular functions. Glucose or N-acetylglucosamine may confer stability and resilience to structures such as plant and eubacterial cell walls and the exoskeletons of arthropods. Moreover, glycans have crucial physical protective roles. GPs are crucial for gut stability, and the extensive array of glycans on the intestinal epithelium can inhibit microbial invasion and facilitate barrier protection.
Cell signaling
GPs attach to particular substances (such as hormones) and then send signals into the cell. Attaching glycosyl groups to proteins may be done in a site-specific manner, meaning that various portions of the same protein can be glycosylated at different places. Glycosylation is an intricate and varied process that enables proteins to carry out extra biological functions. On N-linked proteins, glycosylation takes place most often. Incorporating β-linked GlcNAc residues from uridine diphosphate (UDP)-GlcNAc is the main factor that decides the amount of branching on N-linked glycans on cell surface GPs. There are a number of biological processes that these branched glycans interact with; for instance, N-glycan branching quantities and levels can influence cell proliferation and differentiation in a synergistic approach, in addition to affecting protein structure. The Notch signaling system, which typically regulates the differentiation of different types of communities during embryonic and adult development, is aberrantly activated in many cancer processes. Notch proteins are essential for the Notch signaling pathway. The extracellular structural region of the Notch receptor can be changed by glycans of different types, such as O-glucose, N-glycan, and O-fucose. Thus, the structure and type of glycans on Notch proteins may activate or deactivate the Notch signaling pathway; glycosylation further regulates cell development by modulating Notch signaling.
Cell recognition
Cells are able to recognize and communicate with one another through GPs found on their surface. GPs include blood type antigens. There are three types of macromolecules called selectins that have a role in adhesion: The GP family includes the three lectins P-selectin, E-selectin, and L-selectin. Some of the many biological processes that they impact include the migration and collection of leukocytes to lymphoid organs, as well as the actions of neutrophils, eukaryotic organisms, and blood platelets. At the outset, throughout, and after the inflammatory phase, protein glycosylation is an essential step. An example of this is the regulation of innate cell transit and recruitment to sites of injury or infection by glycosylation-mediated recognition between endothelium and leukocytes. Cell surface glycosylation constitutes an immune cell trafficking and recruitment control system that endothelial cells exhibit in response to illness or tissue injury. Alterations to this glycosylation affect the binding of selectin. The adherence of endothelial cells and leukocytes is dependent on PSGL-1, a highly glycosylated protein. According to studies conducted by Pushpankur Ghoshal and colleagues, clathrin (a glycosylation suppressor) helps sickle erythrocytes and endothelial cells identify each other and promote adhesion by actively inhibiting PSGL-1's function. GPs may also be involved in the processes of antigen recognition, absorption, and processing.
Chemical Glycoprotein Synthesis
GP synthesis usually follows one of two paths. The initial step involves the early creation of the hypothesized glycan-protein connection, which allows for the synthesis of glycopeptide building blocks that may subsequently be assembled in a linear fashion. In the second, known as convergent assembly, the connection is built towards the end of a synthesis process, after the protein framework necessary for its presentation has already been established. Considering the potential instability linked with the connection and the protective needs of glycosylated building blocks, the second strategy stands out as the preferable choice. A significant obstacle is the fact that convergent glycosylation of oligopeptides is not always effective when applied to proteins due to a lack of chemo-and/or regioselectivity. Chemoselective, site specific, and site selective are the three main classifications used to describe convergent protein glycosylation techniques. This differentiation is arbitrary and dependent on the overall glycosylation technique; it has nothing to do with the selectivity or specificity of any one ligation. When one uses the same chemoselective (e.g., thiol reactive) reaction to glycosylate a single naturally occurring cysteine thiol, it will undergo indiscriminate glycosylation with any available cysteine thiol; when one uses the same reaction to modify a single cysteine thiol, it will undergo site selectivity when introduced to a preselected position; and so on.
Convergent protein glycosylation. (Gamblin D P., et al., 2019)
Site-specific protein glycosylation methods. (Gamblin D P., et al., 2009)
Enzymatic Glycoprotein and Glycopeptide Synthesis
In particular, enzymatic techniques have been effective in developing preexisting carbohydrate displays on peptide/protein scaffolds. Early instances show that by using a sialyltransferase and CMP-N-Ac-neuraminic acid, Paulson and colleagues were able to desialylate 95% of the sialic acids in the protein. The role of UDP-Glc: GP glucosyltransferase (UGGT) in the monoglucosylation of a synthetic Man9GlcNAc2 MTX complex (see section) was investigated more recently by Ito and colleagues. This complex is an enzyme/substrate system that is essential for the calnexin/calreticulin (CNX/CRT) protein folding "quality control" cycle. The intrinsic specificity of glycosyltransferases makes them useful tools for elaborating glycopeptide structures before or after linear assembly. A library of O-linked lactosaminoglycans was formed by combining β1,3-N-acetylglucosaminyltransferase (LgtA) with β-1,4-galactosyltransferase, as shown in a remarkable example by Nishimura and colleagues. In addition, this study was expanded to encompass two distinct α-2,3-sialyltransferases. These enzymes, when coupled with β1,4-GalT, could insert a maximum of five sialylated hexasaccharides into a MUC1 glycopeptide. The use of glycosyltransferases to the synthesis of oligosaccharides is highly relevant to much of this research.
Harnessing the enzymes that are in charge of creating the sugar-protein bond is a promising strategy for enzymatic GP production. The enzyme oligosaccharyltransferase (OST) is involved in N-linked GP biosynthesis. It transfers a high-mannose core oligosaccharide from a fatty acid pyrophosphate carrier to the side-chain amide of an Asn residue in the consensus sequence Asn-X-Thr/Ser of the developing GP. Other sequences, such as Asn-Ala-Cys, are also rarely glycosylated. The results of using this enzyme alone in in vitro GP production have been disappointing, nevertheless. Despite the feasibility of carbohydrate transfer to a 17-residue peptide with a unique Asn-Asn-Thr-Ser sequence, direct glycosyl transfer to RNase-A proved unsuccessful. Also, it is inefficient to transfer to sequences where X = Trp, Asp, Glu, or Leu, or when X = Pro is not conceivable.
Synthesis of a glycosylated fragment of MUC1. (Gamblin D P., et al., 2009)
TGase-catalyzed glycosylation of insulin. (Gamblin D P., et al., 2009)
Glycoprotein examples
- Immunoglobulins (Antibodies) are GPs that are essential to the immune system, since they identify and neutralize invaders, including bacteria and viruses. The glycosylation of antibodies influences their stability, solubility, and immunological signaling capabilities.
- Erythropoietin (EPO) is a GP hormone crucial for the formation of red blood cells. The glycosylation of EPO is essential for its stability in the circulation and its efficacy as a signaling protein. EPO is utilized therapeutically for the treatment of anemia.
- Mucins are extensively GPs present in mucus that safeguard the epithelial cells of the respiratory, digestive, and reproductive systems. Mucins serve as a physical barrier against infections and contribute to cellular signaling.
- Integrins are transmembrane GPs that enable cell adhesion and signaling. They improve cellular contact with the extracellular matrix and enhance cell interaction, migration, and immune responses.
- LH is a GP hormone that regulates reproductive functions. The carbohydrate components of LH help maintain its structure and are essential for binding to its receptor.
- Transferrin is responsible for iron transport in the bloodstream. Its glycan structures help determine its half-life in circulation and influence iron uptake in cells.
- Glycophorin, which is located in the red blood cell membrane, is responsible for the maintenance of cell structure and the formation of a hydrophilic coat that prevents red blood cells from clustering together. Additionally, its carbohydrate groups function as receptors for specific viruses, including the influenza virus.
No suitable glycoprotein peptides was found. We can provide customized services, please contact us.
References
- Yue Z., et al., Advances in protein glycosylation and its role in tissue repair and regeneration, Glycoconjugate Journal, 2023, 40(3): 355-373.
- Cummings R D. Stuck on sugars–how carbohydrates regulate cell adhesion, recognition, and signaling, Glycoconjugate Journal, 2019, 36: 241-257.
- Guo W., et al., Glycan nanostructures of human coronaviruses, International Journal of Nanomedicine, 2021: 4813-4830.
- Gamblin D P., et al., Glycoprotein synthesis: an update, Chemical Reviews, 2009, 109(1): 131-163.
- Davis B G. Synthesis of glycoproteins, Chemical reviews, 2002, 102(2): 579-602.
