Silyl protection in the solid-phase synthesis of N-linked glycopeptides. Preparation of glycosylated fluorogenic substrates for subtilisins.

The trimethylsilyl (TMS) group was used for protection of the hydroxy groups of three disaccharide 1-amino-alditols and of the glycosylamines of glucose, maltotriose and maltoheptose. The per-O-trimethylsilylated derivatives were coupled with N alpha-Fmoc-Asp(Cl)-OPfp 7 to give six glycosylated building blocks for the solid-phase synthesis of N-linked glycopeptides. Building block 8 was used in the synthesis of five internally quenched fluorescent substrates which were studied by enzymatic hydrolysis with savinase, a subtilisin-type enzyme.

Attachment of oligosaccharides to peptide antigen profoundly affects binding to major histocompatibility complex class II molecules and peptide immunogenicity.

To investigate the immunogenicity of glycopeptides, a peptide fragment from hen egg lysozyme, HEL(81-96)-Y (here named 1) which is immunogenic in H-2k mice and known to bind to the murine major histocompatibility complex (MHC) class II molecule Ek, was synthesized in five different glycosylated forms. The N-terminal serine of HEL(81-96)-Y was derivatized with D-glucose (2), maltotriose (3), and a branched D-glucose pentasaccharide (4). Furthermore, 1 was prepared with a central serine or asparagine derivatized with the branched D-glucose pentasaccharide (5) and GlcNAc (6), respectively. The ability of the five glycopeptides and the non-glycosylated peptide, labeled with 125I, to bind to the two MHC class II molecules, Ak and Ek, was studied using a gel filtration assay. None of them could bind to Ak. Neither 5 nor 6 were able to bind to Ek. Surprisingly 2, 3 and 4 bound better to Ek than did the non-glycosylated peptide 1. The increased binding varied depending on the type of oligosaccharide attached to the N terminus of the peptide. The better binding to Ek of glycopeptide 4 was found to be due to an increased association rate. The binding of 1 as well as 4 was optimal at pH 5.0. Functional studies showed that 4 was able to elicit a heteroclitic proliferative response from T cells of mice immunized with the native non-glycosylated peptide. Circular dichroism studies of 1 and 4 indicated a more unordered structure of 4 and a predominant alpha-helical conformation of 1, suggesting that the MHC class II molecule may bind to peptides which are in a non-alpha-helical conformation. These results demonstrate that glycosylation has considerable influence on peptide immunogenicity for T lymphocytes.