Proteoglycans are proteins associated with polysaccharide chains known as glycosaminoglycans (GAGs), with heparan sulfates (HS) and chondroitin sulfates (CS) being the most representative. The chain synthesis is carried out by glycosyltransferases in the Golgi apparatus and begins with the formation of a tetrasaccharide composed of a xylose, two galactoses, and a glucuronic acid. The decision-making step occurs during the addition of the fifth sugar : the transfer of a GlcNAc residue by the enzyme Extl3 leads to the generation of a HS chain, while the addition of a GalNAc residue by CsGalNAcT1/2 results in CS formation. The regulation of this branching step remains poorly understood. It is hypothesized that the amino acid sequence of the protein or its three-dimensional structure may dictate the addition of either HS or CS.
To explore this hypothesis, researchers from the SAGAG group established a method to generate peptides carrying the GAG common tetrasaccharide, using the recombinantly produced enzymes Xylt1, B4GalT7, B3GalT6, and GlcAT-1. They demonstrated that Extl3 preferentially acts on peptides derived from heparan sulfate proteoglycans. Surprisingly, Extl3 also converts, to a lesser extent, peptides derived from chondroitin sulfate proteoglycans.
This new strategy for producing native glycosylated peptides will allow researchers to further explore the specificity of glycosyltransferases involved in the branching step of GAG biosynthesis.
Chemo-enzymatic synthesis of tetrasaccharide linker peptides to study the divergent step in glycosaminoglycan biosynthesis. Bourgeais M, Fouladkar F, Weber M, Boeri-Erba E, Wild R. Glycobiology 2024 ; 34(5) : cwae016.
Contact : Rebekka Wild (IBS/Structure and Activity of Glycosaminoglycans Group)