Emerging biological significance of carbohydrates has increased the importance of carbohydrate-binding molecules (CBMs) as diagnostic and therapeutic agents as well as research tools in glycobiology. Especially, there are rapidly growing interests in small molecules with a carbohydrate-binding property due to the ease of preparation and chemical modification. This minireview focuses on small-size CBMs that bind carbohydrates through non-covalent interactions. Small-size CBMs of synthetic origin have been progressively developed in the field of supramolecular chemistry. The representative architectures of the synthetic CBMs are briefly reviewed mainly focusing on recent examples. We also highlight naturally-occurring CBMs, pradimicins and its congeners. They are the only class of natural products with a lectin-like ability to recognize D-mannosides (Man) in the presence of Ca²⁺ ion. Our recent findings on molecular basis of their Man recognition are also described.

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L-Arabinofuranosyl (Araf) residues are a quantifiably important constituent of plant cell walls. The results of earlier studies have led to the hypothesis that UDP-L-arabinopyranose (UDP-Arap) is the sugar donor and that the conversion to Araf occurs during the glycosyl transfer reaction. However, this mechanism is unlikely because UDP-L-arabinofuranose (UDP-Araf) has been shown to be the sugar donor in the conversion of Araf-containing oligosaccharides in plant extracts. We speculated that UDP-Arap reacts with a mutase to form UDP-Araf and identified and partially characterized a rice UDP-L-arabinopyranose mutase (UAM) that catalyzes the interconversion of UDP-Arap and UDP-Araf. To investigate the effects of depleting Araf residues on cell wall structure and on rice growth and development, we used RNAi to suppress UAM expression in rice plants. Several transgenic plants had reduced proportions of Araf in their cell walls together with a decrease in the extent of substitution of the xylan backbone and a reduction of between 25% and 80% in ferulic acid and p-coumaric acid content. Transgenic plants with over 25% reduction in Araf residues were dwarfed and infertile. These results suggested that Araf residues are required for normal plant growth, development, and reproduction.

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In the fission yeast Schizosaccharomyces pombe, galactose residues are transferred to N- and O-linked oligosaccharides of glycoproteins by galactosyltransferases in the lumen of the Golgi apparatus. Although galactose residues are not essential for growth of S. pombe, the galactosylation of protein is required for maintenance of normal cell shape, sexual agglutination, and tolerance toward various drugs. Cell surface glycoproteins play a key role in flocculation and filamentous invasive growth in yeasts. We identified fission yeast gsf2⁺, encoding a flocculin that binds to galactose residues located on cell surface glycoconjugates. Flocculation and invasive growth of S. pombe is tightly controlled by gsf2⁺ expression. Furthermore, pyruvylation of galactose residues negatively regulates flocculation by capping galactose residues in N-linked galactomannan. S. pombe appears to have a unique galactose-specific recognition system in which Gsf2p/flocculin plays an essential role in mediating cell-cell interactions.

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