Amino acid metabolism: propionyl-CoA precursor amino acids: 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 · Amino acid metabolism: propionyl-CoA precursor amino acids

Sample item

Propionyl-CoA is converted via methylmalonyl-CoA to succinyl-CoA. Which set of amino acids is catabolised partly through this route?

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

The answer and the reasoning

A. Methionine, valine, isoleucine and threonine

Isoleucine, valine, methionine and threonine yield propionyl-CoA (valine and isoleucine through separate routes), which is carboxylated by a biotin enzyme and isomerised by a B12-dependent mutase. Defects here give propionic and methylmalonic acidaemias.

Option by option

  • A. Methionine, valine, isoleucine and threonine (correct)

    Correct. These four amino acids, along with odd-chain fatty acids, feed propionyl-CoA metabolism.

  • B. Tyrosine, aspartate, asparagine and glutamate

    Tyrosine forms fumarate and acetoacetate, while aspartate and asparagine enter via oxaloacetate and glutamate via alpha-ketoglutarate.

  • C. Alanine, glycine, serine and cysteine

    These are mostly glucogenic and enter as pyruvate or related intermediates.

  • D. Leucine, lysine, tryptophan and phenylalanine

    Leucine and lysine are purely ketogenic and form acetyl-CoA or acetoacetate.

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 A, Methionine, valine, isoleucine and threonine. Isoleucine, valine, methionine and threonine yield propionyl-CoA (valine and isoleucine through separate routes), which is carboxylated by a biotin enzyme and isomerised by a B12-dependent mutase.

Why is "Tyrosine, aspartate, asparagine and glutamate" not the answer?

Tyrosine forms fumarate and acetoacetate, while aspartate and asparagine enter via oxaloacetate and glutamate via alpha-ketoglutarate.

Why is "Alanine, glycine, serine and cysteine" not the answer?

These are mostly glucogenic and enter as pyruvate or related intermediates.

Why is "Leucine, lysine, tryptophan and phenylalanine" not the answer?

Leucine and lysine are purely ketogenic and form acetyl-CoA or acetoacetate.

What concept does this question test?

It tests Amino acid metabolism: propionyl-CoA precursor amino acids in Biochemistry. Defects here give propionic and methylmalonic acidaemias.

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 Amino acid metabolism: propionyl-CoA precursor amino acids?

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.