Final answer:
The first sugar residue that becomes part of the core oligosaccharide in glycoprotein synthesis is N-acetylglucosamine (GlcNAc), which is attached to a dolichol phosphate carrier in the RER before further monosaccharides are added as the structure is processed in the Golgi apparatus and ultimately presented on the cell surface.
Step-by-step explanation:
The Core Oligosaccharide in Glycoprotein Synthesis
The first sugar residue to become part of the core oligosaccharide in glycoprotein synthesis is N-acetylglucosamine (GlcNAc). This step occurs during the assembly of the core glycoside in the rough endoplasmic reticulum (RER). The core oligosaccharide synthesis begins with the transfer of GlcNAc onto a dolichol phosphate carrier, followed by the addition of more monosaccharides such as mannose and other GlcNAc units to extend the core structure. The nascent glycoproteins, with their partial glycans attached, are then transported to the Golgi apparatus, where terminal glycosylation further elaborates the oligosaccharide chains.
In the Golgi vesicles, additional monosaccharides such as galactose and fucose may be added, giving rise to complex branched structures that play critical roles in cell recognition and cell adhesion. Upon completion, these glycoproteins are transported to the cell surface in vesicles that bud from the trans-Golgi network. Upon fusion with the plasma membrane, the glycoproteins, with their carbohydrate moieties, are presented on the exterior surface of the cell, where they perform a myriad of functions including cell-to-cell communication and immune response modulation.