J Biol Chem, Vol. 274, Issue 15, 10014-10018, April 9, 1999

Structural Basis for the Resistance of Tay-Sachs Ganglioside GM2 to Enzymatic Degradation

Yu-Teh Li, Su-Chen Li, Akira Hasegawa^¶, Hideharu Ishida^¶, Makoto Kiso^¶, Anna Bernardi, Paola Brocca^**, Laura Raimondi, and Sandro Sonnino^**

From the  Department of Biochemistry, Tulane University School of Medicine, New Orleans, Louisiana 70112, the ^¶ Department of Bio-Organic Chemistry, Gifu University, Gifu 501-11, Japan, the  Department of Organic and Industrial Chemistry, University of Milan, Via Venezian 21, 20133 Milano, Italy, and the ^** Department of Medical Chemistry and Biochemistry, University of Milan, Via Fratelli Cervi 93, 20090 Segrate (Mi), Italy

To understand the reason why, in the absence of GM2 activator protein, the GalNAc and the NeuAc in GM2 (GalNAc14(NeuAc23)Gal14Glc1-1'Cer) are refractory to -hexosaminidase A and sialidase, respectively, we have recently synthesized a linkage analogue of GM2 named 6'GM2 (GalNAc16(NeuAc23)Gal14Glc1-1'Cer). While GM2 has GalNAc14Gal linkage, 6'-GM2 has GalNAc16Gal linkage (Ishida, H., Ito, Y., Tanahashi, E., Li, Y.-T., Kiso, M., and Hasegawa, A. (1997) Carbohydr. Res. 302, 223-227). We have studied the enzymatic susceptibilities of GM2 and 6'GM2, as well as that of the oligosaccharides derived from GM2, asialo-GM2 (GalNAc14Gal1 4Glc1-1'Cer) and 6'GM2. In addition, the conformational properties of both GM2 and 6'GM2 were analyzed using NMR spectroscopy and molecular mechanics computation. In sharp contrast to GM2, the GalNAc and the Neu5Ac of 6'GM2 were readily hydrolyzed by -hexosaminidase A and sialidase, respectively, without GM2 activator. Among the oligosaccharides derived from GM2, asialo-GM2, and 6'GM2, only the oligosaccharide from GM2 was resistant to -hexosaminidase A. Conformational analyses revealed that while GM2 has a compact and rigid oligosaccharide head group, 6'GM2 has an open spatial arrangement of the sugar units, with the GalNAc and the Neu5Ac freely accessible to external interactions. These results strongly indicate that the resistance of GM2 to enzymatic hydrolysis is because of the specific rigid conformation of the GM2 oligosaccharide.