Amino acid metabolism: energy cost of urea synthesis: 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: energy cost of urea synthesis

Sample item

Synthesis of one molecule of urea in the liver consumes ATP at two steps: carbamoyl phosphate formation and argininosuccinate formation. In terms of high-energy phosphate bonds, what is the total cost of one urea?

A2B4C6D3

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

The answer and the reasoning

B. 4

Carbamoyl phosphate synthetase I uses two ATP (two high-energy bonds). Argininosuccinate synthetase cleaves one ATP to AMP and pyrophosphate, which is then hydrolysed, costing two more high-energy bonds. The total is four, even though only three ATP molecules are used.

Option by option

A. 2
Two counts only the carbamoyl phosphate step.
B. 4 (correct)
Correct. Two bonds at the first step and two at argininosuccinate synthesis give four.
C. 6
Six would double count the cost; only four bond equivalents are spent.
D. 3
Three is the number of ATP molecules used, but one is cleaved to AMP and so costs two bonds.

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.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, 4. Carbamoyl phosphate synthetase I uses two ATP (two high-energy bonds).

Why is "2" not the answer?

Two counts only the carbamoyl phosphate step.

Why is "6" not the answer?

Six would double count the cost; only four bond equivalents are spent.

Why is "3" not the answer?

Three is the number of ATP molecules used, but one is cleaved to AMP and so costs two bonds.

What concept does this question test?

It tests Amino acid metabolism: energy cost of urea synthesis in Biochemistry. Argininosuccinate synthetase cleaves one ATP to AMP and pyrophosphate, which is then hydrolysed, costing two more high-energy bonds. The total is four, even though only three ATP molecules are used.

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: energy cost of urea synthesis?

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.