N-glycans protect proteins from protease digestion through their binding affinities for aromatic amino acid residues

It was previously revealed [Yamaguchi, H., Nishiyama, T., and Uchida, M, (1 999) J. Biochem. 126, 261-265] that N-glycans of both the high-mannose and complex types have binding affinity for aromatic amino acid residues. This study shows that free N-glycans protect proteins from protease digestion th rough their binding affinities for the aromatic amino acid residues exposed on protein molecules, Protease digestion of bovine pancreatic RNase A and bovine alpha-lactalbumin was depressed in solutions (1 mM or so) of free N- glycans of both the high-mannose and complex types. The increasing order of the protective effects of the N-glycans paralleled that of their affinitie s for aromatic amino acid residues; and the presence of aromatic amino acid s practically abolished the protective effects of the N-glycans, The N-glyc ans also depressed the protease digestion of metallothionein, an aromatic a mino acid-free protein, in agreement with the observation that the N-glycan s also interact with the solvent-exposed aromatic amino acid residues of th e proteases, Thus it seems probable that the N-glycans protect proteins fro m protease digestion by steric hindrance attributable to their binding affi nity for the solvent-exposed aromatic amino acid residues of both substrate proteins and proteases.