Functional groups of sialic acids involved in binding to siglecs (sialoadhesins) deduced from interactions with synthetic analogues

The siglecs, formerly called sialoadhesins, are a family of I‐type lectins binding to sialic acids on the cell surface. Five members of this family have been identified : sialoadhesin, myelin‐associated glycoprotein (MAG), Schwann cell myelin protein (SMP), CD22 and CD33. We have investigated the relevance of substituents at position C‐9 and in the N‐acetyl group of N‐acetylneuraminic acid, using a series of synthetic sialic‐acid analogues either on resialylated human erythrocytes or as free α‐glycosides in hapten inhibition. All five siglecs require the hydroxy group at C‐9 for binding, suggesting hydrogen bonding of this substituent with the binding site. Remarkable differences were found among the proteins in their specificity for modifications of the N‐acetyl group. Whereas sialoadhesin, MAG and SMP do not tolerate a hydroxy group as in N‐glycolylneuraminic acid, they bind to halogenated acetyl residues. In the case of MAG, N‐fluoroacetylneuraminic acid is bound about 17‐fold better than N‐acetylneuraminic acid. In contrast, human and murine CD22 both show good affinity for N‐glycolylneuraminic acid, but only human CD22 bound the halogenated compounds. In conclusion, our data indicate that interactions of the hydroxy group at position 9 and the N‐acyl substituent contribute significantly to the binding strength.