New type of linkage between a carbohydrate and a protein: C-glycosylation of a specific tryptophan residue in human RNase Us
1994
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. 'H 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 0-glycosidic bonds.
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