Enzymatic properties of a Ginkgo biloba endo-beta-N-acetylglucosaminidase and N-glycan structures of storage glycoproteins in the seeds.

An endo-beta-N-acetylglucosaminidase has been purified to homogeneity from mature seeds of Ginkgo biloba. The molecular mass of the endo-beta-N-acetylglucosaminidase, named Endo-GB, was estimated to be around 63 kDa by SDS-PAGE and around 62 kDa by Hiprep S-200 chromatography, respectively. The substrate specificity has been explored with regard to the pyridylaminated N-glycans. Several high mannose-type sugar chains bearing alpha-1,2-mannosyl residue(s), Man9-6GlcNAc2-PA, were the most favored substrates followed by Man5GlcNAc2-PA and a typical hybrid-type structure (GlcNAc1Man5Glc NAc2-PA) which does not bear an alpha-1,2-mannosyl residue. On the contrary, endo-GB could hardly hydrolyze the common core pentasaccharide of N-glycan (Man3GlcNAc2-PA) and the xylose-containing sugar chains (Man4-3Xyl1Glc NAc2-PA, Man3Fuc1Xyl1GlcNAc2-PA) being widely distributed in plant glycoproteins. Furthermore, we analyzed the structures of N-glycans conjugated to storage glycoproteins in the mature Ginkgo seeds to see the occurrence of endogenous substrates for Endo-GB. The structural analysis showed, however, only xylose-containing type N-glycans (Man3Fuc1Xyl1GlcNAc2 (95%) and Man3Xyl1 GlcNAc2 (5%)), which can not be substrate for Endo-GB, predominantly occur in the storage glycoproteins.