REMODELLING OF GLYCOPROTEINS WITH SIALIC ACID ANALOGUES

Abstract

To complete their biosynthesis many soluble or membrane bound glycoconjugates require a final sialylation step which is catalyzed by distinct sialyltransferases differing in specificity with respect to the glycan acceptor structure and the linkage formed. Important biological functions of glycoconjugates depend on the degree of terminal sialylation. Replacement of naturally occurring sialic acids by synthetic analogues endowed with special properties can serve to influence certain biological functions of soluble or membrane-bound sialoglycoconjugates. In particular, those sialic acid analogues that deserve special interest are sialidase-resistant or O-acetylesteraseresistant, or they are radiolabeled, fluorescently labeled, or carry a photoreactive functional group, or they may differ in charge or hydrophobicity from the parent sialic acid. It has been shown that CMP sialate synthase has a low substrate specificity with respect to the C-5 and C-9 positions of sialic acid. As a consequence many CMPactivated analogues of sialic acid can be obtained on a preparative scale. Detailed studies have revealed that different sialyltransferases accept a variety of CMP-activated sialic acid analogues as donor substrates. This finding opens the way for preparation by enzymatic synthesis on a large scale of glycoproteins differently modified in the sialic acid moiety. Membrane-bound receptors can be modified and the biological consequences studied. With accumulating knowledge of the donor substrate specificity of CMP-sialic acid synthase and sialyltransferase, analogues can be synthesized that are tailored for special applications or for use as inhibitors of enzymes of sialoglycoconjugate biosynthesis. Furthermore, glycoproteins can be prepared that resist biological degradation by sialidases.