We developed a new method to site-specifically modify the proximity of the protein active site, so-called “Post-Photoaffinity Labeling Modification (P-PALM)”. By applying this method to a sugar binding protein (lectin), the various functionalizations of lectin were successfully carried out. For example, we constructed a fluorescent sugar sensor which can read out the binding of sugar by the fluorescence change by introducing a fluorescent dye to the proximity of the sugar binding pocket of lectin. By the site-selective introduction of another fluorophore to this sensor, the ratiometric sugar detection with higher accuracy and intracellular sugar sensing were achieved. In addition, we succeeded not only in the construction of biosensors but also in the modulation of the sugar selectivities of lectin by tethering the sub-binding site. It is clear that, P-PALM enables extension of the function of lectin at the molecular level, and the functionalized lectin is expected to contribute to glycoscience research as new sugar detection tools.
Glycoconjugate-binding molecules can contribute a great deal to carbohydrate research and carbohydrate-related diseases. However, it is difficult to obtain carbohydrate-specific antibodies due to low immunogenicity of glycoconjugates. Recent phage-display technology serves a new methodology to identify glycoconjugate-specific molecules effectively from large repertory of candidates. Many libraries such as peptide library, antibody library, and mutation lectin library have been applied for this purpose. By using this system, we will be able to design carbohydrate-binding molecules having the affinity and specificity we need.
This review describes theoretical and experimental aspects of vibrational circular dichroism (VCD) spectroscopy, and its application to structure analysis of carbohydrates targeting on their chiral property.
Enzymatic glycosylation reactions by combined use of activated donor substrates and deactivated enzymes have been reviewed. Glycosidase-catalyzed transglycosylation reaction of sugar oxazolines to various glycosyl acceptors have successfully been achieved for the formation of poly- and oligo- saccharides such as chitin, glycosaminoglycans, and the core trisaccharide of N-linked oligosaccharide. These reactions proceeded efficiently using appropriate glycosidases under basic conditions, where the enzymes showed lower hydrolytic activity toward their natural substrates. In order to realize the transglycosylation reactions under neutral conditions, several mutants chitinaseA1 from Bacillus circulans WL-12 were constructed. Some of mutants were found to be efficient for transglycosylation under neutral conditions. These reactions could be applied to the stepwise synthesis of oligosaccharides and oligosaccharide-containing macromonomers. Novel thermosensitive polymers carrying oligosaccharides have successfully been synthesized by the radical copolymerization of the resulting macromonomers with N-isopropylacrylamide. One-pot chemo-enzymatic synthesis of oligosaccharides starting from a free sugar has been demonstrated via a sugar oxazoline intermediate formed in situ and the subsequent enzymatic transglycosylation. This novel technique of transferring an oligosaccharide moiety having an N-acetylhexosamine unit is a promising tool to pave the way to the new generation of oligosaccharide synthesis.