For the past fifteen years, it has appeared increasingly evident that the N-glycosylation process was accompanied by the release of oligomannoside type oligosaccharides. This material is constituted of oligosaccharide-phosphates and of neutral oligosaccharides possessing one GlcNAc (OS-Gn1) or two GlcNAc (OS-Gn2) at the reducing end. It has been demonstrated that oligosaccharide-phosphates originated from the cleavage by a specific pyrophosphatase, of non-glucosylated cytosolic faced oligosaccharide-PP-Dol and chiefly the Man5GlcNAc2-PP-Dol. The Man5GlcNAc2-P, as the main product, is recovered in the cytosolic compartment and is further degraded to Man5GlcNAc1 by not-yet depicted enzymes. In contrast, OS-Gn2 produced from hydrolysis of oligosaccharide-PP-Dol (presumably as a transfer reaction onto water) when the amount of protein acceptor is limiting, are generated into the lumen of rough endoplasmic reticulum (ER). They are further submitted to processing α-glucosidases and rough ER mannosidase and are (mainly as Man8GlcNAc2) exported into the cytosolic compartment. This material is further degraded into a single component, the Man5GlcNAc1: Man α1→2 Manα1→2Manα1→3(Manα1→6)Manβ1→4GlcNAc by the sequential action of a cytosolic neutral chitobiase followed by cytosolic mannosidase. Furthermore, OS-Gn1 could have a dual origin: in one hand, they originate from OS-Gn2 by the cytosolic degradation pathway indicated above, on the other hand, we will discuss a possible origin from the degradation of newly synthesized glycoproteins. Considered first as a minor phenomenon, these observations have lead to the concept of intracellular oligomannoside trafficking, a process which results from more fundamental phenomena such as the control of the dolichol cycle, and the so-called quality control of glycoproteins. In this review, we would like to describe the evolution of ideas on the origin, intracellular trafficking and putative roles of these oligomannosiders released during the N-glycosylation process.
Membranes of many cell types or cell secretions contain sulfated glycoconjugates with a variety of different structures. The sulfate groups are terminal structures on oligosaccharides and may endow these molecules with special biological effects. Sulfates may mask antigenic or lectin binding sites, they may protect physiologically important compounds from premature degradation, or they may regulate the biosynthesis and biological role of glycoproteins and proteoglycans. Sulfate has been suggested to be heavily involved in binding events which influence diverse functions, such as root nodulation in legumes, lymphocyte homing, cell-cell adhesion and viral replication. Many different sulfotransferases exist in nature; these enzymes synthesize various sulfate esters with great specificity, similar to that of glycosyltransferases. A better understanding of the role of sulfation will follow upon further characterization and cloning of sulfotransferases, together with studies which define their regulation and gene expression.
HPLC chiral stationary phases based on glycoproteins have included α1-acid glycoprotein from human serum, cellobiohydrolase I from the fungus Trichoderma reesei, ovoglycoprotein, avidin and ovotransferrin from egg whites, and flavoprotein, riboflavin binding protein, from egg whites and yolks. This review article deals with the preparation of HPLC stationary phases based on glycoprotein, their chiral recognition properties, and chiral recognition mechanism on these stationary phases.