Gluconeogenesis: substrates: 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 · Gluconeogenesis: substrates

Sample item

During prolonged starvation, which of the following cannot contribute to a net synthesis of glucose in humans?

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

The answer and the reasoning

C. Acetyl-CoA derived from even-chain fatty acids

Acetyl-CoA cannot be converted to a net gain of oxaloacetate, because the two carbons it contributes to the TCA cycle are lost as carbon dioxide and pyruvate dehydrogenase is irreversible. Glycerol enters at dihydroxyacetone phosphate, alanine is converted to pyruvate, and propionyl-CoA enters as succinyl-CoA, so all three give net glucose.

Option by option

  • A. Glycerol released from adipose triglyceride

    Glycerol is phosphorylated by hepatic glycerol kinase and enters gluconeogenesis as dihydroxyacetone phosphate.

  • B. Propionyl-CoA from odd-chain fatty acids

    Propionyl-CoA is carboxylated and isomerised to succinyl-CoA, a TCA intermediate that can yield glucose.

  • C. Acetyl-CoA derived from even-chain fatty acids (correct)

    The two carbons of acetyl-CoA are lost as CO2 in the TCA cycle, so there is no net glucose.

  • D. Alanine released from muscle

    Alanine is transaminated to pyruvate in the glucose-alanine cycle and is a major gluconeogenic substrate.

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, Acetyl-CoA derived from even-chain fatty acids. Acetyl-CoA cannot be converted to a net gain of oxaloacetate, because the two carbons it contributes to the TCA cycle are lost as carbon dioxide and pyruvate dehydrogenase is irreversible.

Why is "Glycerol released from adipose triglyceride" not the answer?

Glycerol is phosphorylated by hepatic glycerol kinase and enters gluconeogenesis as dihydroxyacetone phosphate.

Why is "Propionyl-CoA from odd-chain fatty acids" not the answer?

Propionyl-CoA is carboxylated and isomerised to succinyl-CoA, a TCA intermediate that can yield glucose.

Why is "Alanine released from muscle" not the answer?

Alanine is transaminated to pyruvate in the glucose-alanine cycle and is a major gluconeogenic substrate.

What concept does this question test?

It tests Gluconeogenesis: substrates in Biochemistry. Glycerol enters at dihydroxyacetone phosphate, alanine is converted to pyruvate, and propionyl-CoA enters as succinyl-CoA, so all three give net glucose.

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 Gluconeogenesis: substrates?

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.