Gluconeogenesis: glucogenic 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.

Try it

Biochemistry · Gluconeogenesis: glucogenic substrates

Sample item

A researcher incubates hepatocytes from a fasted rat with different labelled substrates and measures net synthesis of glucose. Which substrate cannot give a net gain of glucose carbon in animals?

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

The answer and the reasoning

C. Palmitate, through acetyl-CoA from beta-oxidation

Acetyl-CoA from even-chain fatty acids cannot be converted to a net gain of oxaloacetate, because the two carbons entering the TCA cycle are lost as CO2 in the following turns, and pyruvate dehydrogenase is irreversible. Propionyl-CoA from odd-chain fatty acids enters as succinyl-CoA and is glucogenic. Glycerol and alanine feed gluconeogenesis directly.

Option by option

  • A. Propionate, through propionyl-CoA to succinyl-CoA

    Propionyl-CoA is carboxylated and isomerised to succinyl-CoA, which can form oxaloacetate and glucose.

  • B. Glycerol, through dihydroxyacetone phosphate

    Glycerol is phosphorylated and oxidised to dihydroxyacetone phosphate, an intermediate of gluconeogenesis.

  • C. Palmitate, through acetyl-CoA from beta-oxidation (correct)

    Correct. Acetyl-CoA from an even-chain fatty acid cannot yield net glucose.

  • D. Alanine, through pyruvate

    Alanine is transaminated to pyruvate and is the main glucogenic amino acid of the glucose-alanine cycle.

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 C, Palmitate, through acetyl-CoA from beta-oxidation. Acetyl-CoA from even-chain fatty acids cannot be converted to a net gain of oxaloacetate, because the two carbons entering the TCA cycle are lost as CO2 in the following turns, and pyruvate dehydrogenase is irreversible.

Why is "Propionate, through propionyl-CoA to succinyl-CoA" not the answer?

Propionyl-CoA is carboxylated and isomerised to succinyl-CoA, which can form oxaloacetate and glucose.

Why is "Glycerol, through dihydroxyacetone phosphate" not the answer?

Glycerol is phosphorylated and oxidised to dihydroxyacetone phosphate, an intermediate of gluconeogenesis.

Why is "Alanine, through pyruvate" not the answer?

Alanine is transaminated to pyruvate and is the main glucogenic amino acid of the glucose-alanine cycle.

What concept does this question test?

It tests Gluconeogenesis: glucogenic substrates in Biochemistry. Propionyl-CoA from odd-chain fatty acids enters as succinyl-CoA and is glucogenic. Glycerol and alanine feed gluconeogenesis directly.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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: glucogenic 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.