Endoplasmic Reticulum

ID #1276

Can you explain the process of N-linked glycosylation? 1) Why is it attached to an asparagine? 2) What is a residue? 3) Will there always be an asparagine in the beginning? 4) How does the dolichol get into the membrane? 5) Why are there two phosphates on the dolichol molecule? 6) Why is the dolichol molecule bent as it is?

1) Because it needs to be covalently linked to a nitrogen, hence the designation "N-linked", and there aren't that many amino acids with nitrogen in their side chains.

2) This generic term can be used to describe individual amino acids in a protein chain (each AA is a "residue").

3) No, glycosylation doesn't have to occur right near the N-terminus of the protein. It will occur anywhere you encounter Asn-X-Ser or Asn-X-Thr, where X is any amino acid but proline. (That recognition sequence isn't something you need to know for the exam) Since this sequence isn't all that uncommon, it suggests that proteins might be glycosylated a number of times, and in fact they are.

4) It's a lipid, so it gets there the same way any other lipid gets into a membrane.

5) To give the enzymes that build your carbohydrate tree a reactive atom to covalently link the sugars to. Why 2 instead of just 1 I don't know.

6) You've now reached the extent of my knowledge on this subject. I have no idea why it has adopted this particular conformation.

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