Carbohydrate metabolism: glycogenin primes glycogen synthesis: 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.

Try it

Biochemistry · Carbohydrate metabolism: glycogenin primes glycogen synthesis

Sample item

Glycogen synthase can extend an existing chain but cannot start a new one. Which protein provides the starting primer for a new glycogen molecule?

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

The answer and the reasoning

A. Glycogenin, which attaches glucose to one of its own tyrosine residues

Glycogenin is a protein that autoglycosylates, using UDP-glucose to attach glucose to one of its tyrosine residues and extending the chain to about eight units. Glycogen synthase then lengthens this primer, and branching enzyme adds the branches. Each glycogen particle therefore has a glycogenin core.

Option by option

  • A. Glycogenin, which attaches glucose to one of its own tyrosine residues (correct)

    Correct. Glycogenin builds the first short chain on its own tyrosine, which glycogen synthase then extends.

  • B. Debranching enzyme, which removes branch points

    Debranching enzyme acts in glycogen breakdown, not in synthesis.

  • C. Branching enzyme, which creates alpha-1,6 links

    Branching enzyme acts only after chains have been lengthened and does not initiate synthesis.

  • D. UDP-glucose pyrophosphorylase, which makes UDP-glucose

    UDP-glucose pyrophosphorylase supplies the activated glucose donor and does not form a primer.

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.

More Biochemistry questions

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, Glycogenin, which attaches glucose to one of its own tyrosine residues. Glycogenin is a protein that autoglycosylates, using UDP-glucose to attach glucose to one of its tyrosine residues and extending the chain to about eight units.

Why is "Debranching enzyme, which removes branch points" not the answer?

Debranching enzyme acts in glycogen breakdown, not in synthesis.

Why is "Branching enzyme, which creates alpha-1,6 links" not the answer?

Branching enzyme acts only after chains have been lengthened and does not initiate synthesis.

Why is "UDP-glucose pyrophosphorylase, which makes UDP-glucose" not the answer?

UDP-glucose pyrophosphorylase supplies the activated glucose donor and does not form a primer.

What concept does this question test?

It tests Carbohydrate metabolism: glycogenin primes glycogen synthesis in Biochemistry. Glycogen synthase then lengthens this primer, and branching enzyme adds the branches. Each glycogen particle therefore has a glycogenin core.

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 Carbohydrate metabolism: glycogenin primes glycogen synthesis?

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.