The main purpose of the urea cycle is to____________

Question

BlackTom AI · Accepted Answer

Converting highly toxic ammonia into the much less toxic compound urea for safe excretion is the urea cycle’s central function. Relevant concepts include nitrogen metabolism, hepatic detoxification, and nitrogen excretion.

Step-by-step reasoning and biochemical steps:
1. Ammonia arises from amino acid catabolism and is neurotoxic at elevated levels; the liver removes it by incorporating it into urea. This occurs partly in mitochondria and partly in the cytosol of hepatocytes.
2. Carbamoyl phosphate synthetase I (CPS I), activated by N-acetylglutamate, combines NH4+ (ammonium) with CO2 (as bicarbonate) using 2 ATP to form carbamoyl phosphate. This is the committed detoxification step.
3. Ornithine transcarbamylase (OTC) transfers the carbamoyl group to ornithine to form citrulline; citrulline is exported to the cytosol.
4. Citrulline condenses with aspartate (which supplies a second nitrogen) via argininosuccinate synthetase (ASS), consuming ATP (converted to AMP + PPi), to form argininosuccinate.
5. Argininosuccinate lyase (ASL) cleaves argininosuccinate into arginine and fumarate (fumarate can feed into the TCA cycle).
6. Arginase hydrolyzes arginine to yield urea and regenerate ornithine, completing the cycle. Urea is then transported to the kidneys and excreted in urine.

Why this is the correct answer:
- The pathway’s stoichiometry and enzyme regulation exist to remove and safely excrete nitrogen as urea, preventing ammonia accumulation and neurotoxicity (hyperammonemia).

Why the other choices are incorrect:
- Produce energy: The cycle consumes ATP (net cost), so it is not an energy-producing pathway.
- Synthesize proteins: Protein synthesis occurs on ribosomes using amino acids; the urea cycle disposes of excess nitrogen, not synthesize proteins.
- Store nitrogen: The cycle eliminates nitrogen rather than storing it.

Therefore, the correct answer is: Eliminate toxic ammonia.

The main purpose of the urea cycle is to____________单项选择题

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