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Oligosaccharide assembly begins in the endoplasmic reticulum (ER) lumen, where an N-linked oligosaccharide precursor is added to proteins. Glucosidases then remove glucose units, and an ER mannosidase trims a specific mannose before the protein transfers to the Golgi apparatus for further modifications.

Glucosidase I removes one glucose unit from the N-linked oligosaccharide precursor, while glucosidase II removes two additional glucose units. These sequential trimming steps occur in the ER and prepare the oligosaccharide for further modifications in the Golgi. This processing is essential for proper protein folding and quality control.

Mannosidase I removes three mannoses from different parts of the precursor in the Golgi lumen. Later, mannosidase II removes two additional mannoses, yielding a final core of three mannoses. These trimming steps create the structural foundation for adding other sugar units like N-acetylglucosamine.

After the mannose core is established, additional sugar units such as N-acetylglucosamine, galactose, and sialic acid are added to the precursor. A nucleotide sugar transporter delivers these sugars bound to nucleotide phosphates like UDP-N-acetylglucosamine. This process forms the complex oligosaccharide side chains on proteins.

Nucleotide phosphates released during sugar addition are recycled back to the cytosol through the nucleotide sugar transporter. This recycling mechanism allows the transporter to bring additional incoming sugars into the Golgi lumen, maintaining efficient synthesis of complex oligosaccharides on glycoproteins.

High-mannose oligosaccharides contain only mannose and N-acetylglucosamine, while complex oligosaccharides include additional sugars like galactose and sialic acid. High-mannose cores remain tightly bound and inaccessible to enzymes, whereas complex glycan cores are more accessible for further sugar addition and modification.

Endo H sensitivity indicates whether an oligosaccharide has undergone sugar addition in the Golgi. Since sugar addition is unidirectional, Endo H resistance develops as glycans mature. This characteristic allows researchers to trace the forward transport of newly synthesized glycoproteins through the secretory pathway.