How many ATP molecules are consumed in one turn of the Urea cycle?

Appeared in: AIIMS CRE, SNO-2024

Explanation

The Urea cycle consumes a total of 3 ATP molecules to synthesize one molecule of urea.
In the first step, Carbamoyl Phosphate Synthetase I (CPS-I) utilizes 2 ATP molecules.
In the third step, Argininosuccinate Synthetase utilizes 1 ATP molecule.
The sum of ATP molecules consumed is 2 + 1 = 3.

Option A: This is an incomplete count. While 2 ATP are used in the first step of the cycle (formation of carbamoyl phosphate), another ATP is consumed later.
Option C: This number refers to the total count of high-energy phosphate bonds that are broken, not the number of ATP molecules consumed. The conversion of one ATP to AMP and pyrophosphate (PPi) is energetically equivalent to breaking two bonds.
Option D: This significantly underestimates the energy required. The urea cycle is an anabolic pathway that requires a substantial energy investment to convert toxic ammonia into non-toxic urea.

Visual 1: Diagram - A diagram of the Urea Cycle highlighting the two ATP-consuming steps. The first step (CPS-I) should show 2 ATP converting to 2 ADP. The third step (Argininosuccinate Synthetase) should show 1 ATP converting to AMP and PPi.

Clinical Relevance

Nursing practice connection: Knowing Energetics of the Urea Cycle helps nurses interpret findings accurately and avoid errors in routine assessment, medication administration, and patient teaching.
The urea cycle is the primary way the body disposes of toxic ammonia, and it is a highly energy-dependent process primarily occurring in the liver.
In liver disease (e.g., cirrhosis), the liver's ability to produce ATP is compromised, impairing the urea cycle. This leads to hyperammonemia (high blood ammonia), which is toxic to the brain and causes hepatic encephalopathy.
What if? If a patient has a genetic deficiency in Argininosuccinate Synthetase, the cycle stops after consuming 2 ATP. Ammonia levels would rise sharply, and citrulline would accumulate in the blood (Citrullinemia), but the full 3 ATP would not be consumed per attempted cycle.

How to Approach the Question

Identify the core subject: The question is about the biochemistry of the Urea Cycle.
Pinpoint the specific detail required: The question asks for the number of 'ATP molecules', not 'energy equivalents' or 'high-energy bonds'. This distinction is critical.
Recall the steps of the Urea Cycle that require energy. Remember that there are two key enzymatic steps involving ATP.
Sum the ATP molecules used in each step: Carbamoyl Phosphate Synthetase I uses 2 ATP, and Argininosuccinate Synthetase uses 1 ATP.
Calculate the total: 2 + 1 = 3 ATP molecules.
Evaluate the options based on this calculation, being careful to distinguish the correct answer (3) from the common distractor (4), which represents high-energy bonds.

Concept Tested & Keywords

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