Carbohydrate metabolism: L-gulonolactone oxidase and ascorbate: 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: L-gulonolactone oxidase and ascorbate

Sample item

Most mammals make ascorbic acid from glucose through the uronic acid pathway, but humans and guinea pigs cannot and need it in the diet. Which enzyme is non-functional in humans?

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

The answer and the reasoning

B. L-gulonolactone oxidase

The last enzyme of ascorbate synthesis, L-gulonolactone oxidase, is encoded by a pseudogene in humans, other primates and guinea pigs. Humans therefore depend on dietary vitamin C. The rest of the uronic acid pathway, which gives glucuronate for conjugation, still functions.

Option by option

  • A. Glucose-6-phosphate dehydrogenase

    Glucose-6-phosphate dehydrogenase is the first enzyme of the pentose phosphate pathway and is not involved in ascorbate synthesis.

  • B. L-gulonolactone oxidase (correct)

    Correct. The gene for L-gulonolactone oxidase is inactive in humans, so ascorbate cannot be made.

  • C. Aldose reductase

    Aldose reductase forms sorbitol from glucose in the polyol pathway and is active in humans.

  • D. UDP-glucose dehydrogenase

    UDP-glucose dehydrogenase works normally in humans and produces UDP-glucuronate used for conjugation.

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 B, L-gulonolactone oxidase. The last enzyme of ascorbate synthesis, L-gulonolactone oxidase, is encoded by a pseudogene in humans, other primates and guinea pigs.

Why is "Glucose-6-phosphate dehydrogenase" not the answer?

Glucose-6-phosphate dehydrogenase is the first enzyme of the pentose phosphate pathway and is not involved in ascorbate synthesis.

Why is "Aldose reductase" not the answer?

Aldose reductase forms sorbitol from glucose in the polyol pathway and is active in humans.

Why is "UDP-glucose dehydrogenase" not the answer?

UDP-glucose dehydrogenase works normally in humans and produces UDP-glucuronate used for conjugation.

What concept does this question test?

It tests Carbohydrate metabolism: L-gulonolactone oxidase and ascorbate in Biochemistry. Humans therefore depend on dietary vitamin C. The rest of the uronic acid pathway, which gives glucuronate for conjugation, still functions.

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: L-gulonolactone oxidase and ascorbate?

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.