Fatty acid synthesis: source of NADPH: 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 · Fatty acid synthesis: source of NADPH

Sample item

Hepatic fatty acid synthesis requires large amounts of cytosolic NADPH. Which pair of reactions is the main source?

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

The answer and the reasoning

A. Pentose phosphate pathway dehydrogenases and cytosolic malic enzyme

Glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase of the pentose phosphate pathway produce NADPH, as does malic enzyme, which converts malate to pyruvate while reducing NADP+ and is part of the citrate shuttle. Both pathways are induced in the fed state with insulin.

Option by option

  • A. Pentose phosphate pathway dehydrogenases and cytosolic malic enzyme (correct)

    These reactions use NADP+ and supply the reducing power for fatty acid synthase.

  • B. Pyruvate dehydrogenase and malate dehydrogenase

    Pyruvate dehydrogenase generates NADH, and malate dehydrogenase is NAD-linked, so neither supplies NADPH.

  • C. Mitochondrial isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase

    The mitochondrial dehydrogenases listed produce NADH within the matrix, not cytosolic NADPH.

  • D. Glyceraldehyde-3-phosphate dehydrogenase and lactate dehydrogenase

    Both glycolytic enzymes named reduce or oxidise NAD, not NADP.

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, Pentose phosphate pathway dehydrogenases and cytosolic malic enzyme. Glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase of the pentose phosphate pathway produce NADPH, as does malic enzyme, which converts malate to pyruvate while reducing NADP+ and is part of the citrate shuttle.

Why is "Pyruvate dehydrogenase and malate dehydrogenase" not the answer?

Pyruvate dehydrogenase generates NADH, and malate dehydrogenase is NAD-linked, so neither supplies NADPH.

Why is "Mitochondrial isocitrate dehydrogenase and alpha-ketoglutarate dehydr…" not the answer?

The mitochondrial dehydrogenases listed produce NADH within the matrix, not cytosolic NADPH.

Why is "Glyceraldehyde-3-phosphate dehydrogenase and lactate dehydrogenase" not the answer?

Both glycolytic enzymes named reduce or oxidise NAD, not NADP.

What concept does this question test?

It tests Fatty acid synthesis: source of NADPH in Biochemistry. Both pathways are induced in the fed state with insulin.

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 Fatty acid synthesis: source of NADPH?

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.