New type of linkage between a carbohydrate and a protein: C-glycosylation of a specific tryptophan residue in human RNase Us.
We report a new type of linkage between a carbohydrate and a protein, involving the rarely modified side chain of a tryptophan residue. An aldohexopyranosyl residue was found to be linked via a C-C bond to the indole ring of the tryptophan residue at position 7 of human RNase Us. Mass spectrometric analysis of peptides containing this residue showed a molecular mass 162 Da higher than that expected for tryptophan. The fragmentation pattern of the modified amino acid side chain was reminiscent of that of aromatic C-glycosides, suggesting a direct attachment of a hexose residue to a C-position of the tryptophan indole moiety. 1H and 13C NMR spectroscopic data confirmed this inference and unequivocally demonstrated the substituent to be an aldohexopyranosyl residue, C-glycosidically linked to the C2 atom of the indole. This mode of attachment differs from the ones known so far, in which carbohydrates are linked to an amino acid side chain by N- or O-glycosidic bonds.