An introduction to the sugar code.

Carbohydrates have physiological importance far beyond their roles as source of energy (glycolysis) and activated hydrogen for synthesis (pentosephosphate pathway) or as constituent of the backbone of nucleic acids and of cell wall polysaccharides. The extent of compositional and structural variability of their oligomers (glycans) is unsurpassed in Nature due to the unique property of independently combining the following parameters with sequence: anomeric status, linkage positions, ring size, addition of branches and site-specific introduction of substitutions. The monosaccharides (letters of the third alphabet of life) thus generate 'words' (signals) of high-density coding capacity. These 'words' are part of the glycans on proteins and lipids, and the glycome represented by these 'words' in their entirety has cell type-dependent features. The often limited intramolecular flexibility of oligosaccharides along with an abundance of contact points for intermolecular interactions is ideal for binding processes. Glycan-based 'words' can thus be 'read,' and their message translated into cellular effects by receptors called lectins. This journal's special issue covers central aspects of the concept of the sugar code.