Diversity in sialic and polysialic acid residues and related enzymes
The occurrence of extensive diversity in the family of sialic acids is now well recognized but biological significance of this diversity has been clarified only partially. In 1986 we showed natural occurrence of deaminated form of neuraminic acid, 2-keto-3-deoxy-D- glycero-D-galacto-nononic acid (KDN) as a capping residue of polysialyl chain of rainbow trout egg polysialoglycoprotein. Subsequent studies of our own and by others showed ubiquitous occurrence of KDN from bacteria to human although expression of this residue in large amounts was limited to certain species of animal. KDN was different from other 'families' of sialic acids usually denoted as modified N-acylneuraminic acids of which O-acetyl substitution is most frequently found. The definition of sialic acid has to be changed by the finding of KDN. Prior to the finding of poly N-acetylneuraminic acid group in mammals, in 1978 we discovered poly N-glycolylneuraminic acid in rainbow trout eggs. This was the first report of the occurrence of polysialic acid structure in animal glycoproteins. We also found the occurrence of polyKDN chains. All these findings made significant contribution to show that diversity of sialic acid family and especially diversity in polysialic acid structure are much more extensive and more ubiquitous than generally believed before. We showed that diversity of polysialic acid is not only originated from the diversity in building block but also the type of interresidue linkage. The diversity of sialic acid and polysialic acid is reflected in diversities in molecules that recognize these structures. We studied some of these molecules that specifically recognize different types of sialic acids and polysialic acids, and biosynthetic mechanism of polysialic acids.