Keratan sulfate (KS) classified into glycosaminoglycans consists of β(1→3)-linked 6-O- and/or 6,6′-di-O-sulfated N-acetyllactosamine [Galβ(1→4)GlcNAc] (su-LacNAc) repeats. Furthermore, the su-LacNAc chain is randomly modified with (1→3)-α-L-fucoside (α-L-Fuc) branches on a GlcNAc residue forming a sulfated Lewis X (su-LeX) unit and capped by some monosaccharides such as sialic acid at the non-reducing end. Thus, naturally occurring KS is structurally very diverse. The structural diversities have been believed to correlate with bioactivities of KS particularly in the diseases such as inflammation and Alzheimer’s disease. However, the structure-activity relationships (SAR) in KS have been unclear at a molecular level. In order to elucidate the SAR, development of a practical method to synthesize structurally well-defined KS samples has been urgently demanded. In this study, we demonstrated on-demand synthesis of sequence-regulated KS oligomers having both α-L-Fuc branches in the appropriate order and a non-reducing end capping monosaccharide via transglycosylation by the keratanase II (KSase II) enzyme (Fig. 1). The transglycosylation of the su-LeX oxazoline (1) gave the su-LeX dimer, which was converted to the corresponding oxazoline (3). Then 3 was reacted with the su-LeX derivative (5) to give the su-LeX trimer derivative (7). Similarly, the octasaccharide derivative having two su-LeX units at both the reducing- and non-reducing ends (8), the heptasaccharide derivative having a su-LeX unit at the non-reducing end (9) and the sialylated hexasaccharide derivative (10) were successfully prepared in combination with 4+5, 1+6, and 2+5, respectively. These results indicate that the on-demand synthesis of sequence-regulated KS oligosaccharides can be achieved by KSase II-catalyzed transglycosylations.

Chemo-enzymatic synthetic method of sequence-regulated keratan sulfate oligosaccharides catalyzed by keratanase II

256th National Meeting (2018)

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