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Bioconjugation is a chemical strategy. It is used to connect two molecules, at least one of which is a biomolecule. This connection is a stable covalent link. Biomolecular conjugation methods are used in medicine, diagnostics, biocatalysis, and material science. Synthetically modified biomolecules can achieve a variety of functions. These functions include tracking biological events in cells. They can also reveal the functional mechanism of enzymes. In addition, the distribution of proteins in the organism can be determined. It can also image specific biomarkers. And deliver drugs to target cells. However, we know that bioconjugation is a vast field with many facets. From a variety of chemical principles to a wide range of applications, we design cutting-edge bioconjugation strategies to help the pharmaceutical, diagnostic and research industries keep pace with this rapidly evolving area of research and development.
What is Protein-Polysaccharide Bioconjugation?
Protein-polysaccharide bioconjugation creates stable covalent bonds between biomolecules. These bioconjugates perform multiple functions. Functions include tracking cellular events, revealing enzyme activity, determining protein biodistribution, imaging biomarkers, and delivering drugs to target cells. Lysine residues are frequent modification sites in protein bioconjugation. N-hydroxysuccinimide (NHS) esters commonly modify amines on lysines. Achieving optimal deprotonated lysines requires aqueous solution pH below the lysine ammonium group pKa (approximately 10.5). NHS-ester is a standard conjugation reagent. It acylates lysines through nucleophilic reaction. Similar reagents include isocyanates and isothiocyanates. These reagents also modify protein lysines under mild conditions. Mild conditions mean low temperature and physiological pH.
Chemical Strategies for Protein-Polysaccharide Bioconjugation
Chemical Strategies: Chemical conjugation employs several key strategies. Periodate oxidation targets vicinal diols in polysaccharides. Sodium periodate oxidizes these diols. This reaction generates reactive aldehyde groups. These aldehydes then react with primary amines on proteins. They form Schiff bases. Subsequent reduction using sodium cyanoborohydride or sodium borohydride produces stable secondary amine linkages.
Maleimide-Thiol Chemistry: Maleimide-thiol chemistry offers high stability and selectivity. Maleimide groups are typically attached to proteins. These groups react specifically with free sulfhydryl groups. Sulfhydryl groups occur naturally in protein cysteine residues. Alternatively, they can be added to polysaccharides. The reaction forms stable thioether bonds.
Enzymatic Ligation: Enzymatic ligation uses enzymes like transglutaminase and glycosyltransferases. These enzymes create very specific covalent bonds. Enzymatic methods typically preserve protein function. They also produce homogeneous conjugates.
Reductive Amination: Reductive amination involves aldehyde groups on polysaccharides. These aldehydes can be naturally present or chemically introduced. The aldehydes react directly with protein primary amines. This forms unstable Schiff bases. Reduction then yields stable secondary amine linkages.
Isothiocyanate Chemistry: Isothiocyanate chemistry utilizes isothiocyanate groups. These groups react with protein primary amines. The reaction forms thiourea linkages. This strategy is frequently used for labeling proteins. Polysaccharide-derived fluorescent probes or other tags are attached this way.