In N-linked glycoproteins, to which of the following amino acids, oligosaccharides are covalently attached?
**Core Concept**
N-linked glycoproteins are formed through a covalent bond between an oligosaccharide and a specific amino acid residue. This process occurs in the endoplasmic reticulum and is crucial for proper protein folding and function. The amino acid involved in this linkage is key to understanding glycoprotein synthesis.
**Why the Correct Answer is Right**
The correct amino acid for N-linked glycosylation is asparagine. This occurs when an oligosaccharide is attached to the nitrogen atom of asparagine's side chain, forming a covalent bond. This process is mediated by the enzyme oligosaccharyltransferase and requires the presence of specific sequences, such as Asn-X-Ser/Thr, where X can be any amino acid except proline.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because serine is involved in O-linked glycosylation, not N-linked.
**Option B:** Incorrect as threonine is also involved in O-linked glycosylation.
**Option C:** Incorrect because while cysteine can form disulfide bonds important for protein structure, it is not directly involved in N-linked glycosylation.
**Clinical Pearl / High-Yield Fact**
Remembering that N-linked glycosylation involves asparagine is crucial, as defects in this process can lead to congenital disorders of glycosylation, which have significant clinical implications.
**Correct Answer:** D. Asparagine.