Semisynthesis of N-glycoprotein libraries

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The function of glycoproteins is frequently regulated by their carbohydrate moieties. These sugar moieties show a natural microheterogeneity. The consequences of microheterogeneity are not well understood since homogenous glycoproteins (glycoforms) cannot be obtained by purification in sufficient quantities. Thus, convergent synthetic approaches were developed providing homogenous glycoproteins via native chemical ligation and subsequent refolding. The demanding synthetic routes towards glycoforms could be significantly shortened in the first funding period leading to the first libraries of natural glycoforms of IL-6 and saposin D. All synthetic glycoforms were biologically active. For the lysosomal adapter protein saposin D several bioactivities related to membrane remodelling were found to be carbohydrate-dependent.In the second funding period we aim to further improve the synthesis of glycoforms of IL-6 and saposins by using chemoenzymatic methods in the synthesis of N-glycans and N-glycopeptides. The pimary goal is the assembly of a representative library of glycoforms of all four human sapsins. Due to their structural and functional homology it can be assumed that also the saposins A-C have bioactivities with carbohydrate-dependency, which should be revealed by established assays systems. The in vivo functions of saposins are correlated with the formation of saposin oligomers complexed with lipids. Since these complexes have been investigated only with non-glycosylated saposins, the glycosylated complexes will be targeted specifically and characterized subsequently.