Glycosylation is a commonly used method of protein modification. Sugars are transferred to proteins under the action of glycosyltransferases, and glycosidic bonds are formed with amino acid residues on proteins. Proteins undergo glycosylation modifications to form glycoproteins. Glycosylation of peptides is the process of linking monosaccharides (such as glucose, galactose) or polysaccharides to peptides through chemical bonds. The peptides obtained by glycosylation are called glycopeptides. Glycopeptides are important analogs of sugar modification on proteins, and are protein fragments modified with sugars.
The result of glycosylation modification makes the protein structure more complex, more perfect in function, more refined in regulation and more specific in action. Glycosylation modification has a very important effect on membrane protein function and mediates various biological functions. Glycosylation modification has many effects on cell protection, stability, organization and barrier. Glycopeptides can act as specific ligands for exogenous receptors. In addition, glycopeptides can also act as specific ligands for endogenous receptors and participate in mediating clearance, turnover, and intracellular travel.
There are two main types of glycosylation: N-linked glycosylation and O-linked glycosylation.
N-linked glycosylation (N-linked glycosylation) modification refers to the covalent attachment of the side chain amide group of asparagine (Asn) to β-configuration N-acetamidoglucose (Glc-Nac). The characteristic of N-glycopeptide is that the second amino acid residue after Asn in the peptide chain sequence is serine (Ser) or threonine (Thr). Commonly used carbohydrates for N-linked glycosylation modification include glucose, fructose, glucosamine, galactose, galactosamine, etc.
O-linked glycosylation modification refers to the covalent attachment of sugars to hydroxyl amino acids in peptide chains. These hydroxyamino acid residues are usually Ser or Thr, in addition to tyrosine, hydroxylysine and hydroxyproline, among others. The sites of linkage are the hydroxyl oxygen atoms on the side chains of these residues. Commonly used sugars for O-linked glycosylation modification are glucose, galactose, N-acetamidogalactose (GalNAc), N-acetamidoglucose (GlcNac), mannose (Man), ribose, etc.
Glycopeptides can be synthesized by direct condensation strategy or building block strategy.
The direct condensation strategy is to first synthesize peptides and oligosaccharides separately. The two are then condensed to form glycopeptides. The advantage of this strategy is that it avoids the effect of acidic deprotection conditions on glycosidic bonds during peptide chain elongation. In addition, since the glycopeptide bond is formed in the last step, the requirement for protecting groups is not high, and the loss of sugar is small. Therefore, this strategy is suitable for glycopeptide synthesis of expensive glycosyls.
The key to the building block strategy is the synthesis of suitable glycosylated amino acids. Glycosylated amino acids are gradually condensed into glycopeptides with other fragments in solution. Or use solid-phase synthesis technology to synthesize glycopeptides with glycosylated amino acids as building blocks. This strategy has the advantages of easier control of the attachment site and stereoselectivity, and is suitable for the synthesis of larger glycopeptides with simpler sugar chains.
Glycosylation modification affects the structure and function of immune molecules, affects the body's immune response to antigens, and is closely related to a variety of diseases. The research on the structure, biological activity, synthesis and function of glycopeptides has a great impetus for the diagnosis, treatment and prevention of diseases, drug screening, and vaccine development. Glycopeptides such as vancomycin and teicoplanin are important antibiotics for the treatment of resistant bacterial infections. Glycopeptide antibiotics are active against almost all Gram-positive bacteria, and are clinically used for the treatment of severe Gram-positive bacterial infections. Other glycopeptides are often used to stimulate the immune system.