Glycoproteins: N-linked glycosylation: practice question

A Biochemistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Biochemistry · Glycoproteins: N-linked glycosylation

Sample item

A drug blocks the first step of N-linked glycosylation by inhibiting transfer of N-acetylglucosamine-1-phosphate to dolichol phosphate. Which statement correctly describes the normal process it disrupts?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

C. A preformed oligosaccharide on dolichol phosphate is transferred en bloc to asparagine in the ER

N-glycans are assembled on the lipid carrier dolichol phosphate in the ER membrane and then transferred as a block of 14 sugars to asparagine in an Asn-X-Ser/Thr sequon, then trimmed and modified in the ER and Golgi. Tunicamycin blocks the first step. O-linked sugars, in contrast, are added singly in the Golgi to serine or threonine.

Option by option

  • A. Sugar chains are attached to lysine residues of the nascent protein in the cytosol

    N-linked sugars attach to asparagine, not lysine, and they are added in the ER lumen.

  • B. Single sugars are added one by one directly to serine residues of the protein in the Golgi

    Stepwise addition to serine or threonine in the Golgi describes O-linked glycosylation.

  • C. A preformed oligosaccharide on dolichol phosphate is transferred en bloc to asparagine in the ER (correct)

    En bloc transfer from dolichol phosphate to asparagine in the ER is the defining step of N-glycosylation.

  • D. Mannose residues are phosphorylated in the cytosol to target proteins to lysosomes

    Mannose-6-phosphate tagging occurs in the Golgi by phosphotransferase and is a later modification.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is C, A preformed oligosaccharide on dolichol phosphate is transferred en bloc to asparagine in the ER. N-glycans are assembled on the lipid carrier dolichol phosphate in the ER membrane and then transferred as a block of 14 sugars to asparagine in an Asn-X-Ser/Thr sequon, then trimmed and modified in the ER and Golgi.

Why is "Sugar chains are attached to lysine residues of the nascent protein i…" not the answer?

N-linked sugars attach to asparagine, not lysine, and they are added in the ER lumen.

Why is "Single sugars are added one by one directly to serine residues of the…" not the answer?

Stepwise addition to serine or threonine in the Golgi describes O-linked glycosylation.

Why is "Mannose residues are phosphorylated in the cytosol to target proteins…" not the answer?

Mannose-6-phosphate tagging occurs in the Golgi by phosphotransferase and is a later modification.

What concept does this question test?

It tests Glycoproteins: N-linked glycosylation in Biochemistry. Tunicamycin blocks the first step. O-linked sugars, in contrast, are added singly in the Golgi to serine or threonine.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Glycoproteins: N-linked glycosylation?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.