Regulation of substrate specificity of plant α-mannosidase by cobalt ion: In vitro hydrolysis of high-mannose type N-glycans by Co ²⁺-activated Ginkgo α-mannosidase

In our previous study (Woo, K. K., et al., Biosci. Biotechnol. Biochem., 68, 2547-2556 (2004), we purified an α-mannosidase from Ginkgo biloba seeds; it was activated by cobalt ions and highly active towards high-mannose type free N-glycans occurring in plant cells. In the present study, we have found that the substrate specificity of Ginkgo α-mannosidase is significantly regulated by cobalt ions. When pyridylamino derivative of Man9GlcNAc2 (M9A) was incubated with Ginkgo α-mannosidase in the absence of cobalt ions, Man5-GlcNAc2-PA (M5A) having no α1-2 mannosyl residue was obtained as a major product. On the other hand, when Man9GlcNAc2-PA was incubated with α-mannosidase in the presence of Co²⁺ (1 mM), Man3-1GlcNAc2-PA were obtained as major products releasing α1-3/6 mannosyl residues in addition to α1-2 mannosyl residues. The structures of the products (Man8-5GlcNAc2-PA) derived from M9A by enzyme digestion in the absence of cobalt ions were the same as those in the presence of cobalt ions. These results clearly suggest that the trimming pathway from M9A to M5A is not affected by the addition of cobalt ions, but that hydrolytic activity towards α1-3/6 mannosyl linkages is stimulated by Co²⁺. Structural analysis of the products also showed clearly that Ginkgo α-mannosidase can produce truncated high-mannose type N-glycans, found in developing or growing plant cells, suggesting that α-mannosidase might be involved in the degradation of high-mannose type free N-glycans.