Question

What is true of protein glycosylation in the ER?

A. Only proteins phosphorylated on an asparagine residue become glycosylated.
B. Sugar residues are added one at a time by a series of enzymes attached to the ER membrane. C. A block of sugar residues is added to the N-terminal signal sequence, creating a common, N-linked oligosaccharide.
D. Only proteins bearing a dolichol residue become glycosylated.
E. Oligosaccharides are added by an enzyme that has its active site on the lumenal side of the ER membrane.

Answer 1

Protein glycosylation in the ER involves attaching sugars to proteins in the RER, which is the first step in the synthesis of glycoproteins. Oligosaccharides are covalently linked to proteins, and their addition plays a role in membrane protein function. Proteins are synthesized in the ER and undergo modifications before being transported to the Golgi apparatus.

Step-by-step explanation:

Protein glycosylation in the ER involves attaching sugars to proteins in the rough endoplasmic reticulum (RER). This process begins with the creation of a core glycoside, to which partial glycans are linked. As the proteins move through the Golgi vesicles, more sugars are added to complete glycoprotein synthesis.

Oligosaccharides, which are branched glycoside-linked sugars, are often attached to proteins via the hydroxyl group on serine or threonine, or to the amide nitrogen on asparagine. The oligosaccharide domains of glycoproteins play a major role in membrane protein function.

In the ER, proteins are synthesized by ribosomes attached to the ER, and undergo modifications like folding or addition of sugars before being transported to the Golgi apparatus for further processing.